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Growth Suppression of ‘Kenblue’ Kentucky Bluegrass Using Plant Growth Regulators and Degree Day Application Timing

Dep. Crop and Soil Sci., Michigan State Univ., East Lansing, MI 48824
Dep. of Agronomy, Ohio State Univ., Columbus, OH 43210

^* Corresponding author.

Plant growth regulator (PGR) applications for seedhead and vegetative growth suppression have given inconsistent results from year to year when application timing is based on calendar days. Application timing based on growing degree days (GDD) should provide uniform and consistent results over time. These field studies were conducted to determine the window of application, based on GDD, for seedhead and vegetative growth suppression of Kentucky bluegrass. Mefluidide (N-|2,4-dimethyl-5-[[(trifluoromethyl)-sulfonyl] amino] phenyl] acetamide) at 0.28 kg ha–¹ and amidochlor (N-[(acetylamino) methyl]-2-chloro-N-(2,6-diethylphenyl) acetamide) at 2.8 kg ha–¹ were applied to a uniform stand of Kentucky bluegrass [Poa pratensis (L.) ‘Kenblue’] at 25, 50, 75, 100, 125, and 150 GDD. Trials were conducted at East Lansing, MI in 1985 and 1986, and Columbus, OH in 1986. Soil type at East Lansing was an Owosso-Marlette sandy loam complex (fine-loamy, mixed, mesic Typic Glossoboric Hapludalf). Soil at Columbus was a Brookston silty-clay loam (fine-loamy, mixed, mesic Typic Agriaquoll). Application of PGR between 25 and 125 GDD provided excellent (>86%) seedhead control. Mefluidide and amidochlor applications at 150 GDD gave 31 and 24% seedhead control, respectively, indicating that seedhead control was lost with this application timing. Clipping weights at East Lansing varied between years with only the 150 GDD application in 1985 and the 75 GDD application in 1986 differing significantly from the other application timings. However, the 75 GDD treatment in 1986 had reduced PGR activity because of rainfall 5 h after application. Application timing of PGR appeared to have no significant effect on the degree of vegetative growth suppression. However, GDD timed PGR applications are a valid technique for seedhead control in Kentucky bluegrass.

Received for publication April 30, 1987.