The Urban Plantings Series TREES for Urban Landscapes A 1 ,' ^ e=^> . -j^^^EE— — I Published by The Massachusetts Horticultural Society with the City of Boston, Parks and Recreation Department Foreword The Gallery Pear, Pyrus calleryana, is very popular as a street tree because of its compact size and its profusion of flowers in early spring. This second booklet in the series on urban horticulture is part of a continuing effort by the Boston Parks and Recreation Department and the Massachusetts Horticultural Society to address the relevant environmental, horticultural, and aesthetics issues surrounding Bostons open spaces. "Trees for Urban Landscapes" brings together the most current methods, ideas, and techniques for planting and maintaining trees. It also reinforces the notion that plantings, especially trees, enhance the urban environment and help to mitigate the effects of pollution, noise, and solar gain. Trees contribute to the environmental integrity and balance of the city. The Boston Parks and Recreation Department is responsible for an estimated 240, 000 trees, roughly divided between city streets and parks. Since 1987 the Parks Department has enacted a comprehensive tree management program that Foreword addresses the wide range of environmental, com- munity, and aesthetic elements of urban trees. The tree management program established a system that prioritizes tree care and works effec- tively with other city and state agencies, in- cluding a coordinated tree planting program under Mayor Flynns five-year capital improve- ment program. In 1989 the Parks Department hired a tree warden to work closely with the Shade Tree Advisory Committee to address Dutch Elm disease, inventory the city's trees, and select species and sites for new plantings. All of these efforts take place within a broader coalition of individuals, community groups, environmental groups, and park partners that helps determine the priorities and needs of each neighborhood. As urban open space is increasingly cited in the larger context of environmental issues, this coalition must con- tinue to respond quickly and intelligently to the urban agenda in a way that protects the present and future interests of Boston residents. We are indebted to the Massachusetts Hor- ticulutural Society for their technical expertise and public outreach, and share with them the mutual goal of improving the green spaces of Boston. Lawrence A. Dwyer Commissioner, City of Boston Parks and Recreation Department From Boston s Tree Warden My job as Bostons tree warden is to ensure that routine tree maintenance, removal of diseased trees, and emergency treatment of the City of Bostons 240,000 trees are carried out in a timely, professional manner. Accomplishing this task depends upon citizen involvement, and Boston is fortunate in having a long history of support from open space groups and from individual residents. This booklet, which offers an overview of urban tree care techniques, is equally useful to citizens and to professionals interested in the well-being of our trees. I encourage all Boston community groups and open space groups interested in caring for or planting trees to work with the Boston Parks and Recreation Department. Contact the Constituent Services Unit at 1010 Massachusetts Avenue, 3rd floor, Boston, Massachusetts 02118, or call (617) 725-4505 Brian Gilbert Tree Warden, City of Boston Parks and Recreation Department Foreword The American Forestry Association has in- itiated Global ReLeaf, a public information and education campaign to encourage tree plan- ting, and better forest management nationally and internationally, with the initial goal of planting 100 million trees in cities by 1992 to capture the benefits for energy conservation. As part of an effort to "act locally" on issues of global concern, Massachusetts has launched a MASS ReLeaf program through which corporate and private donations are used to purchase trees and seedlings for planting in neighborhoods throughout the state. The program recognizes the value of trees as natures way of absorbing carbon dioxide, one of the most harmful of the global-warming gases. The program began with a commitment by the Commonwealth to raise $1.5 million for tree plantings statewide over the next 10 years. And it will grow through the solicitation of private contributions to the effort. The MASS RELEAF project already has attracted several major corporate donors and is expected to blossom fully as part of the larger ENVest program. MASS ReLeaf allows benefactors to contribute something tangible and alive — trees and seedlings — for the environment betterment of the environment. Mass ReLeaf •{»**** Trees in Urban Landscapes by Phyllis Andersen Well-integrated green spaces can be seen linking Bostons urban and residential areas. A Timely Issue T rees fulfill many purposes in the urban JL environment. Just as buildings form the architecture of our cities, so trees provide the architecture of our open spaces, parks and boulevards. But trees are more than mere architecture or adornment in our cities. To some degree they mitigate the effects of the city — the heat, the sound and urban pollution — and thereby make our cities more livable and healthful. We estimate that there are some quarter of a million trees in the City of Boston, and taken as a whole, they significantly improve the climate of the city; on a global scale, they are not inconsequential in reducing the greenhouse effect. More importantly, though, trees remind us of the natural, living world that provides us our resources and sustains us. They remind us of our very dependence on green plants for our survival and our well-being. And because trees are long-lived, requiring special care when being planted, and some care throughout their lives, the integrity of our tree plantings is a good barometer of how we are caring for the green spaces in our cities. This second booklet in our Urban Plantings Series is designed to provide basic informa- tion to those making decisions affecting our urban green spaces, and also to provide an overview of the latest technical knowledge for the practitioner. It is also useful for individual citizens. These booklets reflect our current knowledge, but they are not comprehensive treatments. The reader is urged to contact professionals for advice and use the resources suggested at the back of the booklet. Introduction 3 We are very grateful to the Boston Parks and Recreation Department, our partner in this series; to the Massachusetts Department of Environmental Management through the U.S. Forest Service for their financial support of this booklet; and to the Lyda Ebert Family Foundation. The author of this booklet, Phyllis Andersen, is extremely well qualified to draw together the diverse information available about trees in urban settings. Phyllis is a landscape designer with special interest in public spaces and a consul- tant to numerous public agencies; and she currently teaches at Radcliffe Seminars Land- scape Design Program. She is a past president of the Massachusetts Horticultural Society. We want to thank Fran Beatty, Mike Connor, and Brian Gilbert of the Boston Parks and Recreation Department for their help; Jim MacArthur, Chief of the Bureau of Shade Tree Management and Pest Control for his assistance; and Dennis Ryan, Pro- fessor of Arboriculture University of Massachusetts, for his review of the manuscript. Special thanks go to Margaret S. Connors, landscape designer and consultant to the Massachusetts Horticultural Society, for coordinating this publication and to Robin Jareaux for her outstanding design of the series. Richard H. Daley (^&cXc*e/ /7. Executive Director, Massachusetts Horticultural Society Trees in Cities By Phyllis Andersen Trees in Cities 5 T rees are an essential part of an ideal A city. Cities have never provided the most benevolent conditions for tree growth, yet we assume that tree-lined streets, parks and squares are part of our urban heritage. Trees give character and order to streets, linking neighborhoods and giving them a sense of continuity and history. They enhance our physical well-being by providing cooling shade and improving air quality, and they add texture and pattern to architectural settings. Today, the combined stresses of increased development, restricted planting areas and decreased maintenance have caused a crisis in Boston's famed Commonwealth Mall was laid out in the mid-1880's with rows of American Elms. Elms lost to Dutch Elm disease are being replaced with a variety of hardy native species including sweetgum, green ash, and red maple. urban tree management. Experts now com- pute that the average life of a newly-planted tree on city streets is ten years. Yet despite this dire prediction, city trees outwit statistics and neglect. The evidence is everywhere. Think of the elms, and their younger replace- ments, on Boston's Commonwealth Avenue, the two hundred year old lindens in London squares, and the plane trees of Parisian boulevards. Trees in cities foil even the most cynical observer of man's indifference to his physical environment. Trees create some of the most vivid images imprinted on our memory of places. City trees are not solely the responsibility of municipal government. Architects, land- scape architects and designers, planners and developers must understand how to provide optimum conditions for tree growth as part of all new urban projects. Arborists and hor- ticulturalists must be consulted to develop responsible standards for planting and maintenance. And most importantly, as municipal budgets for tree maintenance decline, city residents must understand they can assume a simple but essential caretaker role for individual trees. The relationship between the physical en- vironment of cities and the biological needs of trees has not been studied on a scientific level equal to that of rural forestry manage- ment or "natural" ecosystems. Grant money and academic sponsorship have only recently come to the field of urban forestry and urban horticulture. The technology of tree growth in cities is a relatively unexplored discipline, and needs greatly expanded support. How trees • Trees moderate the effects of climate. improve life Deciduous trees (those that lose their leaves in the dtV 'n winter) provide cooling shade in summer and allow warming sunlight to penetrate in winter months. Trees also cool by evap- orating moisture through their leaves. They Trees in Cities 7 mitigate and redirect wind to make our streets and public spaces more comfortable for human activities. The ability of trees to significantly moderate climate and therefore reduce energy needs is one of the most vital reasons for regreening cities. • Trees help reduce urban pollution by filter- ing out particulate matter such as dust, ash, pollen and smoke. They not only extract significant amounts of carbon monoxide from the air and return oxygen to it, but also remove limited amounts of sulphur dioxide and hydrogen fluoride. • Trees deflect and absorb some levels of sound disturbing to city residents. • Trees reduce glare from the sun and from automobile headlights, floodlights and adver- tising signs. • Trees provide food, nesting sites and shelter for birds and other wildlife. • Trees give city residents a unique oppor- tunity to experience the change of seasons and the subtle processes of natural growth and change. Honeylocust, Gleditsia triacanthos kermis, casts a filtered shade on pedestrians in Boston's Quincy Market. Allees of Little Leaf Lindens, Tilia cordata line the Christian Science Church Center. The Lindens are tightly pruned in the European tradition. Trees and Urban Design During the great period of urban growth in the nineteenth century, American city planners were inspired by European urban design. The newly-planted squares and residential street design of London and the tree-lined boulevards of Paris served as models for many eastern cities. These ideas were combined with the grid, the traditional American device for laying out new cities. Cities designed on a grid system Trees and Urban Design 9 — like Chicago, Washington, D.C., parts of Manhattan and Boston — now provide us with some of our best examples of mature urban tree planting. Long rows of trees planted at regular intervals at curbside intersect at right angles with other tree lined streets. Under the best possible circumstances, these curbside trees mature and form the dramatic green tunnel effect so desirable on city streets. American street and park trees are allowed to grow in a natural manner in contrast to the European tradition of using pruning techni- ques like pollarding or topping to keep trees uniform. Street trees now must be accom- modated within reduced sidewalk areas, and must compete with an increasing number of other street amenities — parking meters, trash barrels, newspaper vending machines, underground utilities, traffic signals and direc- tional signs. This competition has so drastically reduced the size of tree pits that new methods of tree planting on city streets have to be devised. Continuous tree planting strips, grove planting and trees set back to private lawn areas are techniques being used as alternatives to the individual tree pit. Trees in cities are subject to a wide array of stresses. The healthy growth of a plant depends on the functioning of all of its parts; continuous stress or severe trauma to one or more parts can limit a plant's growth, causing its ultimate decline and death. Management of multiple stress has become the challenge of urban tree professionals. Cities provide unique physical conditions that affect tree growth and survival. Building and paving materials absorb heat from the sun and from commercial and residential heating systems, storing it for long periods. This phenomenon, known as the urban heat island, is characteristic of all cities, and makes The Urban Environment them warmer than their surrounding suburbs and rural areas. The benefits of this warmer condition are that city trees flower and leaf out earlier in spring, then turn color and lose their leaves later in autumn than their suburban counter- parts. But the heat absorption and retention of building surfaces can cause serious damage to trees. Those that are planted immediately adjacent to heat-retaining or reflecting sur- faces absorb large amounts of this reflected heat which they cannot adequately release. Add to this a limited water supply, and you understand how trees literally cook if not adequately protected. Water related stress is one of the most serious threats to newly-planted trees. Too much or too little water can weaken trees already in shock from transplanting. Recent evaluation of drought-stressed trees indicates damage often shows up in the second year after the drought. A regular watering schedule for new planting is critical for future health. Trees also suffer from flooded or poorly drained tree pits. The root system's oxygen supply can become severely restricted, with resulting damage to leaves and branches. New tree pit designs incorporating plastic drainage pipe help to prevent this problem. Compacted soil is a threat to both street and park planting. Caused by excessive pedestrian and vehicular traffic over the tree root area, compaction closes down pore space in soils so oxygen and water cannot penetrate soil layers. Compaction of street tree plantings can be prevented by finishing the tree pit properly. Pavers set in sand, metal tree grates, and fencing can prevent compaction and allow water to percolate down to the roots. Compacted park soil can be remedied by mechanical aeration which punches small holes in the soil's surface. The only perma- nent solution for soil compaction that serious- ly affects tree health is to redesign circulation Trees and Urban Design 11 patterns around the threatened planting. An accumulation of road salt (sodium chloride) in urban soil can damage trees by drawing water away from the root area, leaving the tree dehydrated. Road salt can be leached away from tree roots by a long, slow watering of the tree in spring. Use of road salt is a difficult issue for public works agencies, for public safety must be weighed against plant health. Sand is a good substitute for paths and less travelled areas. Concerned cities are now beginning to use a sand-salt mix on major roads, and sand alone on side streets. The damaging effect of air pollution on both humans and plants is one of the best- documented aspects of the urban environ- ment's deterioration. Sulphur dioxide, released into the air by burning fossil fuels, and ozone, formed in the lower atmosphere by reaction between components of auto exhaust and ultraviolet light, are the most harmful pollutants to tree growth. Trees appear most susceptible to damage during early summer when new growth is still soft and expanding. Air pollution inhibits growth and leads to a general decline of a plant. Finally, trees are prone to injuries from all manner of human intervention — automobile and truck damage, vandalism, and careless disposal of water containing cleaning solvents on tree pit areas. Dog urine also causes considerable damage to tree roots and bark. Public education and encouragement of a protective attitude toward public planting is the key to healthy city trees. The creamy white flowers of the Kousa or Japanese dogwood, Cornus kousa, begin in late June and last more than a month. Species Selection Magnolia x soulangiana is one of the first signs of spring in the city. T rees for urban planting are one of the JL last plant groups to be examined by the new field of bio-technology. Individual trees showing enhanced resilence to urban conditions are being cloned to develop a new range of plants that have reliable tolerance for urban sites. In other attempts to expand the variety of suitable urban trees, investigators are searching out species that grow in stressed conditions in their natural habitats, and are experimenting with their adaptability to city streets. Some of our most successful urban trees, like the Red Maple and the Sweetgum, occur naturally in flood plain areas where they are subject to extremes of flooding and drought. Monoculture is the planting of a single species to the exclusion of all others. Because of its majestic form and adaptability to a variety of urban conditions, the American Elm was seriously overplanted throughout the U.S. American cities still reverberate from 7#1 Species Selection 13 tremendous losses of these trees to Dutch Elm disease. Dire warnings against single species planting that resulted from the loss must be seriously evaluated against the aesthetic value of a limited single species planting design. Planting of a single species on a city street for visual continuity is a responsible action. Allowing a total city tree population to become heavily weighted with a single species, however, can set up future replacement and maintenance problems, and is not a responsible use of public funds. Tree selection is not an absolute science. Great progress is being made through publica- tions, computer networks and other forms of information exchange to share practical ex- perience and field research, reducing the margin of error in species selection. 1. Define the architectural setting and visual needs of the proposed planting area. Evaluate the need to create space or to reinforce the existing design. Determine the effect of the mature height and spread of proposed species. Consider tree spacing and the effect of a closed or open canopy on the site. 2. Evaluate physical constraints of the site and constant stresses that affect plants. Determine location of underground utilities in the immediate area, and also make allowance for a trees mature height if there are overhead lines. Consider size of the planting area, the soil quality, and micro-climate conditions. 3. Predict as closely as possible the long term maintenance program for the tree plan- ting. Will a species need regular pruning? Is it susceptible to disease or insect infestations? If so, can a responsible maintenance plan be designed and implemented? The degree of maintenance a tree will receive is a determin- ing factor in the selection of a species. Criteria for Species Selection Linear Tree Planting Strips Grove Planting Individual Tree Pit Planting Methods M Jtrrps can conclude that many in- dividual trees planted in small sidewalk pits placed 20'-30' apart are not thriving. Like potbound house plants, tree roots cannot find their way out of the constricted planting areas and eventually the tree suffers a loss of vigor and declines. New tree planting configurations are being developed for urban streets based on the principle that shared root space and proper spacing produces healthier trees. A long planting bed which runs parallel to the curb can be constructed for deep sidewalk areas. The bed should be at least four feet wide with a minimum soil depth of three feet. Length of the bed depends on site restrictions. Trees are then planted at regular intervals so their crowns will meet at maturity. Based on the same principle that multiple planting in a shared soil bed produces a healthier micro-climate, grove planting can be achieved on plazas, on deep sidewalks, in parking areas and on traffic islands and rotaries. Design of the planting area must be integrated into the existing streetscape design. Shared planting spaces can only be ac- complished through new construction or through major reconstruction of existing areas. In many parts of the city, the traditional single tree pit will continue to be the most widely used method of tree planting. The Planting Methods 15 size of individual tree pits depends on the sidewalk width, the proximity of other street amenities, and on local ordinances. Generally speaking, a tree pit should not measure less than four foot square, with a proper relation- ship to the root ball. Soil should be excavated just to the depth of the new tree's root ball. It is important not to loosen soil just under the root ball as newly planted trees have a tendency to sink after planting. After the tree is planted and the planting area backfilled with soil, the tree pit area should be graded to form a saucer, sloping slightly toward the trunk. Three inches of organic mulch, such as shredded pine bark, helps conserve water in the root area. Finally, the tree pit should have protection to minimize compaction. In addition to metal grates, tree pits can be planted with evergreen groundcovers and small bulbs which are valuable ways to beautify the larger streetscape. Pre-cast concrete panels are used to construct underground vaults which contain the tree root area and drainage and watering features. Used mainly on major construction projects, tree vaults protect root systems from potentially harmful underground utilities. In smaller cities and in residential areas of larger ones, planting of public trees on private property is an accepted practice. In Massachusetts, for example, Chapter 87 of the General Laws permits this type of planting. Front lawns offer an expanded soil area and protection from compaction and other forms of damage. Front lawn planting is an excellent way to create tree-lined streets when sidewalk areas are too narrow to accommodate any type of tree. Urban soil is not easy to characterize because it is not uniform. It is often rubble-filled, devoid of nutrients, and may have been Tree Vaults Front Lawn Planting Urban Soil Planting Trees in Cities originally brought to the site as fill from a distant location. Recent analysis of failed tree plantings has determined that, despite the poor quality of most urban soil, it is better to work with it than to replace it entirely with rich loam. One of the most serious impediments to vigorous tree growth is the inability of roots to move through interfaces of different soil layers created within the planting area. Urban tree roots are often unable to move beyond the original nursery soil, which saps the vigor of the tree, contributing to its eventual decline. Many cities are not back- filling their tree planting areas with rich loam, but are loosening and amending existing soil with compost or other organic material to enable freer movement of roots. Tree Wrap Traditional planting methods have until recently included use of burlap or specially- treated paper to wrap the tree trunk im- mediately after planting, to protect it from sunscald and water loss through the trunk. Except in the case of species with extremely Planting Methods 17 thin bark this practice is no longer recom- mended. Experts now believe that young trees must adapt as quickly as possible to urban conditions without special protection. It is very difficult to solve drainage problems once a tree is planted. Since we cannot see what is going on underground, drainage pro- blems are difficult to diagnose. New plantings include use of perforated plastic pipe under the root mass to provide continuous drainage. Traditional tree planting techniques have also always included staking newly-planted trees to support them during the first year of growth after transplanting. It is now accepted practice to stake only those trees particularly vulnerable to high winds. Large specimen trees that have been moved with large root balls also should be staked. Staking has been found to cause more problems than it has solved. With minimal maintenance budgets, municipal agencies do not have the staff to remove stakes before they begin to harm the tree. Rubber hose collars that tie stakes to the tree often cut into the trunk, causing serious damage. At the same time, there is growing evidence that staking can actually prevent a tree from developing a strong root system to support itself. The best time to plant trees in the city is when conditions best favor growth and when weather conditions cause the least stress. In the Northeast, the most favorable time for planting is early spring after the ground has thawed and the soil is workable. Fall planting can also be successful, but a number of tree species cannot be successfully transplanted then, either because they cannot survive being removed from nursery fields at this time of the year, or because their roots need the longer growing season afforded by spring planting. Drainage Staking Planting Times Urban Tree Maintenance Watering Adequate water is the most important factor for survival and future growth of newly- planted trees. Young trees need at least ten gallons of water a week, including rainfall, during their active growing season. While some cities now own watering trucks to main- tain new plantings, city residents can easily assist by watering young trees with an ordinary garden hose set at low pressure. Watering once a week for about one half hour — more during periods of drought — can be very important for the future growth of new trees. Early morning or evening watering is preferred. Mature trees in good health that are putting out vigorous new growth do not need to be fertilized on a regular basis. Young trees planted in poor soil respond well to an application of a slow release, high nitrogen fertilizer every three or four years. The best time to apply fertilizer is in mid-to-late autumn or early spring just after the ground has thawed. Fertilizer should not be applied in midsummer because it can encourage new soft leaf growth which is highly vulnerable to early frosts. Integrated Years ago, public trees were sprayed with a Pest range of pesticides and insecticides to protect lanci&tnent tms Public resource. But recent research findings and subsequent regulations have severely restricted the number of chemicals that can be applied to the landscape. The principle of Integrated Pest Management (IPM), which originated in the field of agriculture, now guides the arborist profession Urban Tree Maintenance 19 in insect and disease control. IPM is a diagnostic strategy involving a number of techniques, including — but not limited to — the use of chemicals. Urban tree managers must know enough about specific species to ascertain at what point an insect infestation or a disease needs some form of intervention. Biological controls are favored. We have learned in recent years that trees that thrive in urban settings can generally withstand significant amounts of temporary injury from insects and disease before irreversible damage occurs. Certified arborists are qualified to diagnose and manage insect and disease problems. Pruning major limbs of mature trees for either safety or aesthetics is a job for a trained arborist. Minor pruning of young trees can be done by city residents with some professional instruction. Pruning is essential in the follow- ing situations: • Newly-planted street trees should be pruned up to a height of at least six feet. As young trees mature, low growing branches that in- terfere with pedestrian and street traffic should be removed. • Limbs and branches that have been broken through vandalism, carelessness or storm damage should be removed as soon as possible. Studies have shown that trees evidencing neglect are more vulnerable to future vandalism. • Young street trees should be grown with a single stem or leader. Competing stems should be removed. • Sucker growth at the base of trees should be removed as it is unsightly and absorbs much needed water and nutrients. Pruning Incorrect Pruning Correct Pruning Recommended Trees ? IT66SI The following deciduous trees, those that lose their leaves during the winter, are recommended for street and park plan- ting. Except where noted, these trees have the potential to achieve heights of 50' or more, but, due to adverse conditions in urban areas as described above, many will not reach such a large size. Botanical name: Common name: Description: • Special problems: Botanical name: Common name: Description: Special problems: Botanical name: Common name: Description: Special problems: Varieties: Acer campestre Hedge Maple A tough, vigorously growing small maple which achieves a mature height of only 25'-30'. Native to Europe and Western Asia, the Hedge Maple has proven to be an excellent street tree for small spaces, particularly where utility wires are a problem. Very tolerant of dry locations, it is easily transplanted and pest free. It grows naturally as a low branching tree. Only single stemmed tree form specimens should be used for street planting. Acer ginnala Amur Maple The Amur Maple is a very drought-tolerant and cold-hardy tree native to China. It has small, finely textured foliage that turns brilliant red in the autumn. Like the Hedge Maple the Amur Maple is a small tree growing to only 20'-25'. It is a little used tree which is excellent for screening or grove planting. It is also a low branching, shrubby plant. Only pruned tree forms should be used for street planting. Acer platanoides Norway Maple One of the most widely planted shade trees in the eastern and mid-western U.S. Native to Europe, it was introduced to the U.S. during the Colonial period. Very tolerant of urban conditions, it is fast growing and relatively pest free. Bright yellow-green flowers appear in early April before the leaves unfold. It has outstanding yellow autumn color. The Norway Maple's large, leathery leaf creates dense shade, making it difficult to grow other plants beneath it. Additionally, it is surface-rooted, and can cause sidewalk problems. Cleveland — an upright growing type Columnar — a very narrow, upright form for narrow sidewalks and parking areas Crimson King — summer foliage is red, an excellent specimen tree Emerald Queen — rapid growing, with glossy foliage Summershade — bred for heat resistance Recommended Trees 21 Botanical name: Common name: Description: Special problems: Varieties: Botanical name: Common name: Description: • Special problems: Botanical name: Common name: Description: Botanical name: Common name: Description: Special problems: Acer rubrum Red Maple The small red flowers of Red Maples growing in wet low- lying woodlands are one of the first signs of spring in New England. A recent additon to street planting lists, the Red Maple has shown itself adaptable to a variety of urban conditions. It is pyramidal when young, forming a round headed crown when mature. Known for its brilliant orange to red autumn color. This tree is very effective for grove planting. While the Red Maple is a rapid grower, it has a tendency to brittleness and is occasionally damaged in heavy, wet snowstorms. Columnar — narrow, upright form October Glory — brilliant crimson autumn color, holds leaves longer in the autumn Red Sunset — early orange/red autumn color Aesculus hippocastanum Baumanni Baumann Horse-chestnut Native to Europe where it has been used as an avenue tree for centuries, the Horse-chestnut is known for its very showy white flower panicles in mid-May. The Baumann variety is recommended for city planting as it has sterile double flowers which do not produce fruit. The Horse-chestnut has large rather coarse leaves which produce deep shade. The tree does not have significant autumn color. Horse-chestnuts are susceptible to a number of leaf disfigur- ing diseases. They are best used as a specimen tree for park areas. Ailanthus altissima Tree of Heaven The Ailanthus is the self-propagating inhabitant of empty lots, back alleys and other abandoned sites. Native to China, it was brought to this country in an early experiment with silkworm production. It escaped cultivation and adapted quickly to urban sites. Not a tree to be deliberately planted, but one to respect for its tenacity and adaptability. Carpinus betulus European Hornbeam A densely branched, compact and symmetrical tree widely used for screening in Europe. While it naturally holds its bottom branches it can be pruned up for a very successful street tree. It has dark green birch-like foliage and upright, twiggy branches. The European Hornbeam should be more widely planted. The European Hornbeam is not as drought-tolerant as some other species and should be planted in areas with a good water source. Mature specimens do not transplant well. Botanical name: Common name: Description: Special Problems: Botanical name: Common name: Description: I Special problems: Varieties: Botanical name: Common name: Description: Special problems: Varieties: Botanical name: Common name: Description: Cercidiphyllum japonicum Katsura This very beautiful tree , native to Japan, has small pink buds opening to light green heart-shaped leaves. Autumn color is a brilliant yellow-gold. Pest-free, the Katsura is a compact upward branching tree when young, but matures into a more broad headed specimen. In nature, Katsuras are often multi- stemmed. Be sure to specify a single-stemmed specimen for street planting. Very effective when planted in groves. The Katsura — not as heat or drought resistant as some other species — should be grown in a situation where there is adequate moisture. Fraxinus pennsylvanica lanceolata Green Ash A vigorous growing, drought-resistant, deep-rooted tree that is excellent for difficult urban sites. The Green Ash has dark green compound foliage that turns yellow in the autumn. It transplants easily. A rather coarse tree that should be reserv- ed for planting where other more distinguished species won't survive. The Green Ash drops a profusion of seeds which germinate easily, therefore it is preferable to plant seedless varieties when possible. Marshall's Seedless — a very popular seedless variety for street planting Summit — an upright growing clone of the species that is very hardy, but does produce seeds Ginkgo biloba Ginkgo or Maidenhair Tree The Ginkgo is one of the most interesting and picturesque trees for city planting. Called a living fossil because geological evidence dates the tree back 150 million years, the ginkgo has become one of the most successful street trees in eastern cities. Its distinctive fan-shaped leaves are bright green in summer and turn a clear yellow-gold in autumn. Easily transplanted, it is very tolerant of pollution and wind. The Gingko needs full sun and well drained soil. It is asymmetrical when young, growing into a broad and open tree when mature. The fruit of the female tree can be a nuisance on city streets, so only the male of the species is recommended. Sentry — upright, columnar form Gleditsia triacanthos inermis Honeylocust The Honeylocust has been one of the most widely planted trees on city streets in the last twenty years. Extremely adaptable to city conditions, it has finely textured compound foliage which creates a very desirable dappled sunlight. It can tolerate limited root space better than many other species. Only the inermis varieties, which are thornless and often podless, should be planted on city streets. The Honeylocust Recommended Trees 23 Special problems: Varieties: Botanical name: Common name: Description: ^W Special problems: Botanical name: Common name: Description: 41 Botanical name: Common name: Description: Special problems: Varieties: Botanical name: Common name: Description: Botanical name: Common name: Description: leafs out late, drops its leaves early and has a light branching structure, making it an excellent tree for passive solar uses. Overplanting of this species in many cities has resulted in monoculture concerns. The use of this species should be evaluated against the number in the total urban population. Halka — rapid growth, horizontal branching Moraine — large with spreading crown Quercus palustris Pin Oak The branching pattern of the Pin Oak is its most distinctive feature. Upper branches are ascending while lower branches descend almost to the ground. It has smaller leaves than the Red Oak which turn a brilliant deep red to bronze in autumn. The leaves then often turn brown and stay on the tree throughout the winter. The graceful branching pattern of the Pin Oak can be a problem if the ascending lower branches are not regularly pruned up if the tree is used for street planting. Sophora japonica Japanese Pagoda Tree The Japanese Pagoda Tree has large white pyramidal flower clusters in August well after other trees have bloomed. The flowers and fern-like compound foliage make this a very valuable tree for street or park planting. It transplants easily and tolerates poor soil and dry conditions. The long seed pods often remain on the tree all winter. This is a large, spreading tree which needs plenty of room to grow. Tilia cordata Littleleaf Linden Known in Europe as the "Lime Tree," this magnificant tree is unequaled for formal avenue planting. It has dark green heart-shaped leaves which turn a clear yellow in autumn, and small white flowers in late June. Like all good avenue trees, it is tolerant of poor soil and road salt. It has distinctive dark grey bark and a graceful branching pattern. The Littleleaf Linden is susceptible to Japanese beetles and aphids which leave a sticky residue on leaves. It is also prone to basal sucker growth which can be unsightly if not removed. Greenspire — oval form with distinctive narrow top cut Tilia tomentosa Silver Linden The Silver Linden is a symmetrical upward branching tree with dark green foliage whose undersides are silver grey. A beautiful street or specimen tree, the Silver Linden is difficult to propagate so is not always readily available in the nursery trade. Zelkova serrata Japanese Zelkova The Zelkova, a member of the elm family, has been planted Special problems: Varieties: Botanical name: Common name: Description: Special problems: Botanical name: Common name: Description: Special problems: Varieties: Botanical name: Common name: Description: ♦ Special problems: Varieties: in many areas as a substitute for the American Elm. Smaller and far less majestic than the American Elm, it is nonetheless a valuable city tree. It has a dense upward branching pattern and oval leaves with serrated edges, and an attractive deep red to orange autumn color. The Zelkova is generally pest free and resistant to Dutch Elm disease. Because of its dense branching structure, the Zelkova often needs corrective pruning as it matures. Village Green — a fast growing, extremely hardy form Shademaster — straight trunked with ascending branches Skyline — pyramidal form, rapid growth Liquidambar styraciflua Sweetgum The Sweetgum is grown for its star shaped leaves and its ability to tolerate poorly drained soils. It is pyramidal when young growing into a round headed tree when mature. Brilliant orange to purple autumn color. Native to low-lying areas of the eastern U.S. Transplant when young. The very attractive spiky fruit can be a problem near children's play areas. Platanus x acerifolia London Plane The London Plane is one of the largest and most widely- planted street trees in the world. It is a cross between the P. occidentalis, the American Sycamore and P. orientalis, the Oriental plane. The London Plane has proven to be extremely adaptable to city streets. Identifiable by its light exfoliating bark, it has large, rather coarse leaves and an open branching pattern. It has a conspicuous fruit ball which is visible on the tree throughout the winter. The London Plane tree is susceptible to anthracnose which does not kill the plant but seriously disfigures the foliage and leads to a general weak condition. Bloodgood — a clonal variety that is anthracnose resistant and the only variety now recommended for city planting Pyrus calleryana Callery Pear This is a small (mature height 30') glossy leaved tree that has become enormously popular for street planting in the past 20 years. It has a profusion of small white flowers in late April before the leaves appear. It prefers full sun and adequate moisture but will survive in drier conditions. The Callery Pear is an attractive, formal tree that is a good choice for areas with overhead wires and other restrictions. It can never be a substitute for our more majestic shade trees. Mature plantings of Bradford have been very susceptible to breakage and splitting because of poor branching structure. Aristocrat — stronger branching, disease resistant Chanticleer — strong upright form, excellent autumn color Redspire — attractive oval crown, large flowers Recommended Trees 25 Botanical name: Common name: Description: ^^r Quercus rubra (Q. borealis) Red Oak This is an excellent oak for street, boulevard or parkway planting if sufficient space is available both for root and crown growth. It is deep rooted and transplants easily. Slow to establish itself, it eventually becomes a vigorously growing and long-lived tree. It needs well drained acid soil, and has deep red autumn color. I Irees: Evergreen trees are a very important part of urban planting as they provide year round interest and serve as excellent screening devices. Evergreen plants are not as tolerant of air pollution, drought and poor soil as many deciduous species. The following trees have been successful in many different types of urban sites. All should be grown in full sun; and may reach a mature height of 50 ' or over. Botanical name: Common name: Description: Botanical name: Common name: Description: Botanical name: Common name: Description: Botanical name: Common name: Description: Abies concolor White Fir The White Fir is a stiffly pyramidal tree with bluish green needles. Native to the western U.S., it withstands heat and drought. Its horizontal branching pattern and unique color make it an excellent park specimen tree. When planted in groups, it makes an excellent wind break for large sites. Pinus nigra Austrian Pine The Austrian Pine has long, stiff dark green needles. Very tolerant of salt spray and poor soils, it is an open, wide spreading tree that is most effective planted in groups. The Austrian Pine adjusts well to windy, exposed sites but suffers in areas of extensive paving because of reflected heat. Pinus thunbergii Japanese Black Pine An irregularly shaped, picturesque tree, the Japanese Black Pine is native to coastal Japan. It has long, dark green needles and does very well in windy, dry coastal sites. When planted in groves, the Japanese pine can be an effective windbreak. It gains its asymmetrical shape as it gets older. Tsuga canadensis Canadian Hemlock The Canadian Hemlock is a soft, graceful tree with short, dark green needles. It needs moist, well-drained soil and good air circulation. It holds its bottom branches and withstands regular shearing, making it an excellent screening tree. The hemlocks can tolerate more shade than other needle evergreens, and are a good choice when an evergreen tree is desired for a semi-shaded location. Ornamental Trees: The following deciduous trees are recom- mended for their outstanding flowers and fruits. They can all be planted under the right conditions in city parks and gardens, but they are not recommended for street planting. They seldom exceed 30' in height at maturity. Botanical name: Cornus florida Common name: Flowering Dogwood Description: Despite serious problems with anthracnose this beautiful native tree can still be planted in urban areas if sited correctly and given responsible care. The Flowering Dogwood offers year-round interest: white flower bracts appear in May before the leaves, summer foliage is attractive, bright red fruit appear in the autumn; and the light grey bark and open branching add to the winter landscape. Special problems: The Flowering Dogwood needs very specfic growing condi- tions to keep it healthy. Very sensitive to heat and drought, it needs lightly shaded conditions, particularly for its root area, a good source of moisture, and well-drained soil. Botanical name: Common name: Description: ft Cornus kousa Japanese or Kousa Dogwood The Kousa Dogwood is a recent introduction to city plantings and an often spectacular specimen tree. It blooms in mid- June and often holds its creamy white flower bracts well into summer. It has bright red fruit and excellent autumn color. The Kousa Dogwood is naturally multi-stemmed but can be pruned into a single trunked tree form. Extremely effective when planted in groups, the Kousa Dogwood has proven very tolerant of urban conditions. Grow in full sun. Botanical name: Common name: Description: Cornus mas Cornelian Cherry Despite its common name, this plant is not a member of the cherry family but a dogwood native to southern Europe. It is grown primarily for its great profusion of small yellow flowers which bloom in late March, well before most other flowering trees. It is shade tolerant and its small leaves turn a soft red- orange in autumn. The Cornelian Cherry is a shrubby plant but it, too, can be pruned into tree form if necessary. Botanical name: Common name: Description: Crataegus lavallei Lavalle Hawthorn The Lavalle Hawthorn has a great display of white flowers in late May and bright red fruit that remains on the tree most of the winter. It can be used as a specimen tree in a park setting or as screening tree for large areas. The Lavalle Hawthorn has is essentially thornless and very resistant to air pollution. Recommended Trees 27 Botanical name: Common name: Description: Special problems: Botanical name: Common name: Description: Botanical name: Common name: Description: Botanical name: Common name: Description: 'WQ&~ Varieties: Botanical name: Common name: Description: Crataegus phaenopyrum Washington Hawthorn More upright and columnar than the Lavalle, the Washington Hawthorn is highly ornamental, with a profusion of flowers and fruit, and exceptional orange to red autumn color. It is highly resistant to rust fungi, and does well in coastal condi- tions. The Washington Hawthorn is often multi-stemmed, but can be obtained with a single stem as well. Because it has many sharp thorns this tree should not be placed near children's play areas or along a walkway. Magnolia x soulangiana Saucer Magnolia The Saucer Magnolia is one of our most popular flowering trees, known for its large, waxy pinkish white flowers which appear in late April. It is a low branching, shrubby tree with a distinctive light grey smooth bark. The furry flower buds swell in late winter and are a first sign of spring in the city. A protected site is helpful to shield flowers from late frosts. Magnolia stellata Star Magnolia The Star Magnolia has fragrant, white star-shaped double flowers. It blooms about a week earlier than the Saucer Magnolia. It is also somewhat smaller and more upright in habit. The Star Magnolia has smaller leaves and consequently is a finer-textured tree. Malus Crab Apple The flowering Crab Apple is a reliable, showy park tree, more drought-resistant than other ornamentals. It is most successfully planted in groves or allees. There are many varieties ranging from small, shrub-like plants to taller forms. Flower colors range from white to deep pink. Fruits vary in size and color and persist well into the winter. Crab Apples need full sun and well drained soil. The Siberian Crab (M. baccata) and the smaller Sargent Crab (M. sargen'tii) are very hardy and resistant to rust disease. Syringa reticulata (Amurensis japonica) Japanese Tree- Lilac This small tree is not well known but it is excellent for city planting. It blooms in mid-June and has has creamy white flower heads and shiny red-brown bark. The Tree Lilac can be grown with many stems or with a single trunk. It does not, however, have the distinctive fragrance of the shrub lilacs. Resources • Libraries and bookstores carry a wide selection of books on trees. The following titles are suggested for more detailed information on tree species and tree care. They are all available at the Massachusetts Horticultural Society Library. Dirr, Michael. Manual of Woody Landscape Plants, Champagne, Illlinois, Stipes, 1983. Harris, Richard W. Arboriculture: Care of Trees, Shrubs and Vines in the Landscape. Englewood Cliff, N.J., Prentice-Hall, 1983. Hudak, Joseph. Trees for Every Purpose. New York, McGraw-Hill, 1980. Moll, Gary and Sara Ebenreck editors. Shading our Cities: A Resource Guide for Urban and Community Forests. Washington, D.C. Island Press, 1989. Wyman, Donald. Trees for American Gardens. New York, Macmillan, 1965. • The following are very useful for current technical information on urban tree management. Journal of Arboriculture, published monthly by the International Society of Arboriculture National Urban Forestry Conference Proceedings, published every 3-4 years, cosponsored by the American Forestry Association, and the National Urban Forest Council. Metropolitan Tree Improvement Alliance (METRIA), Proceedings, published irregularly Urban Forest Forum, published bimonthly by the American Forestry Association, P.O.Box 2000, Washington, D.C, 20013. This free urban forestry newsletter is written for citizens and professionals. • The following academic and governmental research centers are doing important field research in urban horticulture and are sponsoring hybridization programs aimed at expanding the number of varieties suitable for urban planting. Urban Horticulture Institute Cornell University Ithaca, New York Urban Vegetation Laboratory The Morton Arboretum Lisle, Illinois Center for Urban Horticulture University of Washington Seattle, Washington National Arboretum Washington, D.C. • A number of cities have developed model tree planting and tree maintenance programs. Especially notable are those of: D Dayton, Ohio D Milwaukee, Wisconsin D Minneapolis, Minnesota D Oakland, California Resources 29 • Nonprofit groups which have successfully supplemented the work of municipal agencies include: D The New York City D San Francisco Friends Street Tree Consortium of the Urban Forest □ Trees Atlanta D Trees for Dallas Certified arborists can be located through the National Arborists Association 174 Route 101 Bedford, N.H. 03102 603-472-2255 Massachusetts Arborists Association 1357 Washington St., Suite 5 West Newton, MA 02165 617-332-8683 • The Stockbridge School of the University of Massachusetts has the oldest program in the United States for training arborists. For more information contact: Stockbridge School University of Massachusetts Stockbridge Hall Amherst, MA. 01003 413-545-2222 • The Massachusetts Horticultural Society Library was founded in 1829 and is considered to be the finest and largest horticultural library in the world. The adjoining bookstore has an excellent selection of current titles. The Massachusetts Horticultural Society Library (and Bookstore) Horticultural Hall 300 Massachusetts Avenue Boston, MA 02115 617-536-9280 • Landscape professionals and municipal tree wardens can contact the following agencies for further information on trees: Shade Tree Laboratory University of Massachusetts Amherst, MA 01003 413-545-2402 Shade Tree Management and Pest Control Bureau The Mass. Department of Environmental Management 100 Cambridge St. 19th Floor Boston, MA 02202 617-727-3159 Citizens with questions about trees can contact their local tree warden. Photo Credits *~* Nancy Anne Dawe Title Page: Alex S. Maclean, Landslides Page 4: Phyllis Andersen Page 7: Boston Parks and Recreation Dept Page 8: Sasaki Associates Page 11: o Margaret S. Connors Page 12: Mary Jane Hayes Cover illustration: Robin Jareaux TREES for Urban Landscapes Trees are an essential part of an ideal city, giving character and order to streets. They link neighborhoods and give them a sense of continuity and history. We assume that tree-lined streets are part of our urban heritage. Yet cities have never provided benevolent conditions for tree growth. This booklet will help provide an understanding of urban trees — their requirements for survival and their contribution to the city. — Phyllis Andersen The Massachusetts Horticultural Society Horticultural Hall 300 Massachusetts Avenue Boston, Massachusetts 02115