Japanese stiltgrass seedhead suppression with plant growth regulators and glyphosate

Background/Question/Methods

Japanese stiltgrass (Microstegium vimineum) is an invasive annual grass which decreases species diversity in woodland habitats throughout the southeastern United States by displacing desirable species.  As an annual, Japanese stiltgrass (JSG) relies on seed production for local recruitment and landscape dispersal, which can remain viable in the soil for 3 to 7 years.  Recent studies suggest that most seed germinate the first year after they are shed; thus, seedhead suppression should greatly reduce population growth and make long-term management more feasible.  Postemergence herbicides have been examined for JSG control previously; however, no have examined effects of plant growth regulators (PGRs) as seedhead suppressors for JSG.  Our objective was to determine the effectiveness of PGRs and low rates of common herbicides for JSG seedhead suppression while minimizing injury to desirable species. 

Studies were conducted in Floyd and Blacksburg VA in 2004, and Martinsville and Blacksburg VA in 2006. Treatments were in a factorial arrangement with three application timings prior to seedhead production: mid-August, early-September, and mid-September for: glyphosate (0.54kg/ha), ethephon (PGR @ 10.9kg/ha), and mefluidide (PGR @ 0.38kg/ha). Other treatments applied at only one timing (early September) included ethephon (17.8kg/ha), fluazifop (0.27kg/ha), glyphosate (1.36kg/ha), imazapic (0.49kg/ha), and mefluidide (0.76kg/ha). 

Results/Conclusions

Ethephon and mefluidide applied mid-August reduced JSG seedhead production by at least 80%, and only 1% of ethephon-treated plants produced seedheads compared to 95% by control plants rated in late-September. Glyphosate completely eliminated seedhead production with mid-August and Early-September applications.

Non-target native plant injury of roundleaf greenbriar (Smilax rotundifolia), blue violet (Viola sororia), tufted knotweed (Polygonum caespitosum) and yellow crownbeard (Verbesina occidentalis) was up to 45% by mefluidide and glyphosate and 20% by ethephon. Grass species: knotroot foxtail (Setaria geniculata), tall fescue (Festuca arundinacea), and hard fescue (Festuca brevipila) were injured 40% by mefluidide and 70% by glyphosate.  Ethephon did not injure grasses.  Whitegrass (Leersia virginica), present at one site, and was injured <30% by all treatments. Individuals with <30% injury typically recover, while individuals with injury >30% can recover but with a competitive cost. Injury above 60% is generally lethal

Both PGRs suppress JSG seedheads with minimal effect on surrounding vegetation and warrant further investigation as novel treatments for invasive species management. Glyphosate completely suppressed seedheads at 0.54kg/ha but caused significant injury to surrounding vegetation.  Lower rates may prove safer to surrounding vegetation while providing a cost effective way to manage this invasive grass.