Hand Pulling Following Mowing and Herbicide Treatments Increases Control of Spotted Knapweed (Centaurea stoebe)

Abstract

Extensive areas in the upper Midwest have been invaded by spotted knapweed, and effective management strategies are required to reestablish native plant communities. We examined effects of mowing, mowing plus clopyralid, or mowing plus glyphosate in factorial combination with hand pulling and burning on knapweed abundances on a knapweed-infested site in western Michigan. We applied mowing and herbicide treatments in summer 2008, and seeded all plots with native grasses and forbs in spring 2009. We conducted the knapweed pulling treatment from 2009 to 2012 in July. The prescribed burn was conducted in April 2012. By 2012, hand pulling reduced adult knapweed densities to 0.57 ± 0.12 m−2 (0.053 ± 0.011 ft−2) (mean ± SE), which was 5.8% of nonpulled treatments, juvenile densities to 0.29 ± 0.07 m−2 (2.1% of nonpulled treatments), and seedling densities to 0.07 ± 0.06 m−2 (2.6% of nonpulled treatments). After 3 yr, hand pulling reduced seed bank densities to 68 ± 26 m−2 as compared to 524 ± 254 m−2 in nonpulled treatments and 369 ± 66 m−2 in adjacent untreated areas of the study site. Without hand pulling, effects of mowing or mowing plus glyphosate were short-lived and allowed knapweed to rapidly resurge. In comparison, although a single mowing plus clopyralid treatment maintained significantly reduced densities of knapweed for 4 yr, by 2012 knapweed biomass in the nonpulled clopyralid treatment was approximately 60% of that in the other nonpulled treatments. Burning had minimal impacts on knapweed densities regardless of treatment combination, probably as a result of low fire intensity. Results demonstrated that persistent hand pulling used as a follow-up to single mowing or mowing plus herbicide treatments can be an effective practice for treating isolated spotted knapweed infestations or for removing small numbers of knapweed that survive herbicide applications.

Native Plant Establishment Success Influenced by Spotted Knapweed (Centaurea stoebe) Control Method

“…In addition, based on soil testing performed at the onset of the study (MacDonald et al 2013), we found that while soil properties did not differ significantly among the different treatment combinations, there was a trend toward lower gravel on plots assigned to the pulling treatments (PERMANOVA; F = 3.19, p = 0.09). Since variation in gravel content could affect soil moisture holding capacity and thus plant response, when a significant pulling effect was found, the data were re-analyzed using gravel as a covariate to ensure the apparent treatment effect was not related to underlying variation in soil properties.…”

“…Knapweed was the dominant invasive plant at our study site prior to the initiation of our study with 60% to 70% cover based upon the total pre-treatment knapweed density (236 ± 16 m-2 SE, , MacDonald et al 2013). Previous studies at this site demonstrated that native grass establishment and fire reintroduction could successfully control knapweed (MacDonald et al 2003(MacDonald et al , 2007, but these approaches resulted in a community with very low native diversity.…”

“…Of the species we seeded, only blackeyed Susan exhibited reduced establishment on clopyralid treatments (Table 4), consistent with this known sensitivity. The relatively minor impact of clopyralid on native forbs that we observed may have been related to the sand to loamy sand soil of our study site (MacDonald et al 2013), as clopyralid is lost rapidly from sandy soils by leaching (Dow AgroSciences 2010, 2011.…”

“…Prior to establishment of the Bass River Recreation Area, the site was highly disturbed by extensive gravel mining in the mid-1900s (MacDonald et al 2003), leaving a persistent ruderal plant community infested by spotted knapweed (MacDonald et al 2003(MacDonald et al , 2007(MacDonald et al , 2013. Knapweed was the dominant invasive plant at our study site prior to the initiation of our study with 60% to 70% cover based upon the total pre-treatment knapweed density (236 ± 16 m-2 SE, , MacDonald et al 2013).…”

“…Controlled burning of the designated plots (n = 24) took place on April 2, 2012 (MacDonald et al 2013). Unseasonably warm weather during March of 2012 resulted in advanced plant phenology causing us to burn earlier than is optimal for knapweed control (Emery andGross 2005, MacDonald et al 2007).…”