Cogongrass, an invasive grass native to Asia, has infested thousands of hectares in the southeastern United States. Although numerous studies have examined cogongrass control, no published studies, to our knowledge, have tested strategies for cogongrass eradication. Cogongrass has a persistent, thick rhizome mat but an ephemeral seedbank; therefore, successful eradication methods must largely focus on the rhizomes. A field study to evaluate specific herbicide treatments and application timings for cogongrass patch eradication was conducted at two locations in southwestern Alabama. Herbicide treatments included glyphosate at 4.48 kg ai ha−1, imazapyr at 0.84 kg ai ha−1, and a tank mix of glyphosate and imazapyr at the same rates. Treatments were applied in May, August, or October for 3 consecutive yr, and the May glyphosate treatment included a second annual application each October. Cogongrass visual control, shoot biomass, rhizome biomass, rhizome depth, and total nonstructural carbohydrate (TNC) content were sampled during the course of the study. Cogongrass response to treatments varied by location but by 36 mo after initial treatment (MAIT), complete elimination of cogongrass shoot and rhizome biomass and 100% visual control was achieved in several herbicide treatment–timing combinations at both locations. These included glyphosate plus imazapyr at any application timing, imazapyr in August or October, and glyphosate applied in May and October each year. TNC levels of surviving healthy rhizomes were not affected by herbicide treatments, but a seasonal pattern was observed. The maximum live-rhizome depth was not influenced by any treatment, indicating that herbicides were not preferentially leaving deeper, surviving rhizomes. These results demonstrate, for the first time, that the entire rhizome layer of cogongrass can be eliminated within 3 yr with multiple treatment options and that cogongrass patch eradication is possible for many land managers.
Nomenclature: Glyphosate, glyphosate plus imazapyr, imazapyr, cogongrass, Imperata cylindrica (L.) Beauv. IMPCY.
Management Implications: Cogongrass is one of the most difficult weeds to manage because of its aggressive growth and persistent rhizomes, which often survive initial herbicide treatments. Historically, research efforts have been focused on cogongrass control, but no published studies have tested strategies for eradication. The present study is the first documented research to demonstrate complete elimination of cogongrass in 18 to 36 mo using repeated, annual herbicide applications. Treatments included glyphosate, imazapyr, and a tank-mix of both applied in the spring, summer, or fall for 3 consecutive yr. Verification of eradication was based on a highly rigid criterion involving measurements of cogongrass visual control, shoot biomass, rhizome biomass, rhizome depth, and total nonstructural carbohydrate (TNC) content over 3 yr. Cogongrass response to treatments varied by location. By 36 mo after initial treatment, the glyphosate plus imazapyr treatment applied at any timing, the imazapyr treatment applied in August or October, and the glyphosate treatment applied in May and October each year resulted in complete elimination of cogongrass shoot and rhizome biomass. The maximum live-rhizome depth (16 cm ± 2 SE) was not influenced by any treatment. During the 3-yr period, herbicides did not affect TNC levels of surviving rhizomes, indicating that repeated treatments directly killed rhizomes, rather than slowly exhausting energy reserves. We are not suggesting that cogongrass can be eradicated from the southeastern United States; however, with repeated glyphosate or imazapyr herbicide treatments, land managers do have a feasible means of eradicating cogongrass patches.
