University of Washington \ Volume 2, Number 1 January 1983 GROWTH REGULATORS FOR TURF by R. P. Freeborg. Condensed from Plants and the Landscape, Summer, 1983, Purdue University. At Purdue University, we began examining growth regulator compounds in the late 1 960 1 s to early 70 ' s . At first, the initial objective was to find a growth regulator that would either eli- minate or reduce the frequency of the mowing required, thus reducing fuel and labor costs and equipment depreciation. So far, we have not found a compound that can sati factori ly eliminate mowing entirely. Our efforts have more recently been directed at finding a growth regulator that will reduce the mowing frequency requirement to perhaps every third or fourth week within a three month period. ^'•compound that will be available in limited Quantity this year is EL500. It has proved to be a very good growth inhibitor. It en- hances the color of the plant and promotes an improved root system. Our test plots have gone ninety days without mowing and without thinning or discoloration of the turf. This product will be marketed under an experimental use permit as "Cutless" from Elanco. As we examine growth regulators, we must also be concerned about what is happening to the plant under the surface of the soil. To ac- complish this, we established a greenhouse test wherein sprigs of bluegrass are planted and then treated with a growth regulator. Thirty days after treatment we harvest them, measure top growth, count rhizomes and tillers, and evaluate root development. EL500 performed very well in this test. The plants treated with this product had dark green color, adequate inhibition, and an ex- ceptionally healthy root system.. Failure to inhibit seedhead development seems to be the only major drawback to EL500. |A growth regulator that has interested us for some years is Ethephone, sold as Ethrel by Union Carbide. One of its unique character- istics is that it tends to dwarf the plant moderately. Compared to other growth reg- ulators, it does not have the potential for as prolonged a period of inhibition, but it does keep all parts of the plant growing aoout equally. A major difficulty is the tendency toward species response, so that if you have a bl uegrass-rye-fescue mix, you will find that each is inhibited at a dif- ferent rate. This results in surface irreg- ulari ties . A more recent development in growth regula- tors has come from Monsanto. It is identi- fied as M0N4621 (wettable powder) or M0N4623 (granule). It is a good growth inhibitor; it enhances turf color and it provides good seedhead inhibition. This compound will soon be available to the turf industry on a limited basis under an experimental use permit. A problem that is associated with the use of growth regulators is in fact the result of their success as inhibitors. In a normal healthy turf, new leaf growth continually masks or hides the older lower leaves as they die. In an inhibited turf, natural senescence continues at a normal rate, and, if the plant is under stress, the rate will accelerate. The inhibited leaf growth cannot hide the dead foliage. This results in a thin, dis- colored turf. The ability of most growth regulators to sup- press seedhead development has aroused in- terest in these compounds as a means of con- trolling a plant species. By reducing seed development over a period of time, weeds like Poa annua can eventually be reduced until their level becomes low enough to control with a preemergent herbicide. With proper timing and use, one can effect a potential reduction of new plants in the future. Two products currently available have the potential for seedhead suppression or selec- tive suppression of annual grass growth. One of these is Embark, a compound that provides good prolonged growth inhibition. It also gives excellent seedhead suppression of Poa annua without severe inhibition of grass species in a stand of turf. The other product is marketed as Rubigan ( EL 222 ) . It is a fungicide used for control of various turf diseases. In our early work with it, we began to see that it inhibited Poa annua more than it inhibited the bluegrass. Further testing revealed that it will selec- tively suppress Poa annua and, over a period of time, with frequent use it will tend to eliminate Poa annua in a stand of cool season grass. Rubigan, although it is not a seed- head inhibitor, has this special ability to influence Poa annua . To some degree, these two products represent where we stand today. We are not only con- sidering growth regulators as a means of possibly reducing mowing frequency require- ments and labor costs, but we are also seeing them as selective herbicides. 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