Cogongrass [Imperata cylindrica (L.) Beauv.] response to herbicides and disking on a cutover site and in a mid-rotation pine plantation in southern USA

Cogongrass [Imperata cylindrica (L.) Beauv.], an alien grass listed as a federal noxious weed, is invading the forestlands of the southeastern USA threatening their ecological and economic integrity. Two field studies were conducted to quantify the effects of herbicides and disking on cogongrass that had infested a cutover site and an adjacent 17-year-old loblolly pine stand. Foliar cover, foliar biomass and rhizome biomass were quantified at 6-month intervals for 2 years. Imazapyr and glyphosate were applied at rates up to 1.12 and 8.96 kg ai ha−1, respectively, as single or split applications. Disking was post, herbicide application on the cutover site only. Evidence for immature cogongrass swards was present in both studies, which led to significant temporal, or community-level growth effects for all three measured responses. With respect to main effects, the herbicide treatments temporarily controlled the cover and aboveground biomass, with an almost negligible impact on rhizome biomass, in the cutover study. Disking had no significant effect on foliar re-growth, but did decrease rhizome re-growth, across all levels of time and herbicide treatments. The split imazapyr treatment had the slowest recovery rate (31% of the control treatment), regardless of disking effects. All the herbicide treatments resulted in reduced cover, foliar biomass and rhizome biomass in the mid-rotation study, 12 months after treatment (12 MAT). However, recovery was evident at the end of 24 months. Comparison of herbicide treatments between the cutover and mid-rotation studies revealed that competition from the 17-year-old pines did not significantly enhance herbicide control of cogongrass, although there was a trend of lower biomass productivity associated with the mid-rotation study. For example, rhizome biomass was 26% lower for the control in the mid-rotation study when compared to the non-disked control in the cutover study, in terms of relative growth 2 years after treatment. The overall effect of disking, herbicides (single dose or split applications), and pine tree competition was to provide a “window for desirable plant re-establishment” that may last up to 2–3 years. Long-term control may be possible with repeated herbicide applications and the use of fast growing, evergreen trees that can provide continuous heavy shade.