Chemical Regulation of Grass Growth. II. Greenhouse and Field Studies with Intensively Managedd Turfgrasses

Chemical growth retardant treatments have been used on a limited scale to decrease vegetative growth and reduce mowing maintenance requirements of turfgrasses. A number of experimental chemical retardants have been developed recently. Three greenhouse experiments and five field studies were conducted from 1970 to 1972 at Southern Illinois University to evaluate .experimental and commercial chemical growth retardants on several species and varieties of intensively managed turfgrass. Seven chemical retardants were tested in the greenhouse with potted Kentucky bluegrass (Poa pratensis L.) and in field studies with Kentucky bluegrass and other species. Treated grasses were evaluated weekly as to growth reduction, and on selected dates as to color maintenance, stand losses, and weed infestations. MON-820 (N-phosphonomethylimini-diacetic acid) retarded growth of Kentucky bluegrass more than Slo-Gro (diethanolamine salt of 6-hydroxy-3-(2H)-pyridazinone; maleic hydrazide or MH). However, losses of the green color often were greater with MON-820 treatments than with MH treatments. The responses of MBR-6033 (3-trifluoromethyl-sulfonamido-p-acetotoluidide) and MON-845 [N,N-Bis (phosphonomethyl) glycine] were similar those of MON-820 and MH. MON-139 (phosphonomethylglycine, isopropylammonium salt) and MON-464 (phosphonomethylglycine, calcium salt) were phytotoxic and caused pronounced color losses and grass stand reductions. High rates of C-19490 (chemical configuration not released by CIBA GEIGY Corp.) were required to retard grass growth. Pronounced color losses of the grass, serious stand reductions, and resulting weed infestations occured after use of higher rates of several chemicals. A concurrent herbicide application may be necessary to control weeds in Kentucky bluegrass plots treated with certain growth retardants. Residual chemical-retarding effects were noted in autumn-treated plots when they were evaluated in early spring of the next year. However, few effects were observed in autumn evaluations of spring-treated plots. MON-820 treatments evoked marked growth reduction of a number of Kentucky bluegrass varieties and with varieties of tall rescue (Festuca arundinacea Schreb.), bermudagrass (Cynodon dactylon L.), and zoysiagrass (Zoysia japonica Steud.). Differential varietal and species responses were noted with a given rate of MON-820. These responses complicate the process of recommending a general chemical rate for a wide array of turfgrasses. Most experimental and commercial growth retardants tested showed some degree of phytotoxicity on several intensively managed turfgrasses. Their use would be more useful on rough turfgrass areas such as highway roadbanks than on home lawns and similar areas.