Herbicide Treatment and Timing for Controlling Kentucky Bluegrass (Poa pratensis) and Tall Fescue (Festuca arundinacea) in Cool Season Grasslands of Central Kentucky, USA

The goal of this study was to evaluate the efficacy of four herbicides for controlling invasive Kentucky bluegrass (Poa pratensis L.) and tall fescue (Festuca arundinacea Schreb.) in idle grasslands by monitoring post-treatment plant communities. Experiments were conducted in two remnant savannas in central Kentucky. With the exception of domestic grazing, these sites were chosen because they were undisturbed by agriculture and development. Single treatments of 0.2 kg ai/ha imazapic, 0.2 kg ai/ha clethodim, 0.2 kg ai/ha imazapic + 1.1 kg al/ha glyphosate, 0.03 kg ai/ha sulfosulfuron, and an untreated control were implemented in fall 2004 and spring and summer 2005 at each site in a randomized complete block design. Plant communities were monitored 3, 10, 12, 14, and 15 months after the final treatments were implemented. At 15 months post-treatment, tall fescue cover was reduced in all treatment plots relative to the untreated control, with the greatest reduction in tall fescue cover occurring in the summer treatment plots. Spring applications of each herbicide were more effective for Kentucky bluegrass control than summer or fall treatments. All herbicide treatments reduced Kentucky bluegrass compared to untreated plots. The most effective herbicide treatment was imazapic + glyphosate, which reduced Kentucky bluegrass cover to less than 7%. However, the resulting bare ground facilitated invasions by other aggressive exotic species, notably musk thistle (Carduus nutans L.). Spring applications of clethodim reduced Kentucky bluegrass cover relative to the untreated plots, and resulted in the highest number of native plant species compared to all other plots. This conservation of forb cover provided by clethodim may be of importance in managing areas impaired by this aggressive, invasive grass.