Japanese stiltgrass is one of the most aggressive, rapidly spreading invasive plants in the eastern United States. Management guidelines state that mowing can help manage Japanese stiltgrass but that mowing is most effective when done late in the season after the plants begin to flower and before they set seed. In this study, I tested the effectiveness of mowing at three different times between mid-June and early September in 2009 and 2010, as well as mowing twice in 1 yr and for two consecutive years. The effectiveness of mowing Japanese stiltgrass was determined by measuring percentage of cover, biomass, seed production, and the number of stems in the summer following mowing. All mowing treatments significantly reduced percentage of cover, biomass, seed production, and the number of Japanese stiltgrass stems the following year. In 2009, all of the mowing treatments significantly reduced biomass, percentage of cover, and seed production. The latest mow, at the end of August, resulted in a slightly greater reduction of cleistogamous seeds. In 2010, the earliest mowing treatment, in mid-June, did not reduce cover and biomass as much as the other mowing treatments. Overall, these results suggest that mowing can be an effective control method for Japanese stiltgrass and that mowing any time after June should be effectively equivalent, although later mowing may provide some marginal advantage.

Nomenclature: Japanese stiltgrass, Microstegium vimineum (Trin.) A. Camus var. imberbe (Nees) Honda.

Management Implications: Mowing is used to control many invasive plant species. For some species, such as Japanese stiltgrass, the timing of mowing within the season is considered important for effective control. The accepted mowing method for Japanese stiltgrass is to mow late in the summer after the plants begin to flower. Mowing earlier in the summer is believed to allow plants to recover and set seed, and is generally considered an ineffective control strategy. However, the evidence to support the importance of late-season mowing is limited and has not been rigorously tested. I tested three timings of mowing (from June to September), and the effect of mowing twice in a year, on Japanese stiltgrass invasions. The earliest mowing treatment (June 18) was less effective than the other timings, but all other times after July 1 significantly reduced cover, biomass, seed production, and the next year's recruitment of Japanese stiltgrass. Some seed production occurred in all mowing treatments, even plots that were mowed after plants began to flower in September. Much of this seed production was from obligately selfing (cleistogamous) flowers that are produced in the axillary nodes of the grass. These flowers are not apparent without close observation and can be produced even under poor growing conditions. The results of this study suggest that Japanese stiltgrass can be controlled reasonably well by mowing in July or later, and that mowing does not have to be restricted to late summer once plants have begun to flower. Some seed production is still likely, however, and mowing needs to be repeated over several years to reduce the seed bank.