Does Applying Indaziflam and Imazapic Together Improve Restoration of Annual Grass-invaded Rangelands?

Invasive annual grasses are degrading millions of hectares of Intermountain West rangelands. Restoration of perennial vegetation in annual grass-invaded rangelands is a high priority. Of particular interest is controlling annual grasses in rangelands that still have co-occurring perennial vegetation. The goal of restoration actions in these communities is to control the annual grasses and promote co-occurring perennial vegetation. Indaziflam and imazapic, applied as pre-emergent herbicides, have both been used for this purpose. Indaziflam often has less than desired control in the first year but can control annual grasses for multiple years. In contrast, imazapic has effective control in the first year, but control is short-lived. Land managers have recently started tank-mixing these two herbicides to potentially alleviate their individual shortcomings and theoretically achieve more effective, long-term annual grass control. However, little is known about the effectiveness of aerially applying these herbicides together, particularly compared with just applying indaziflam, and the effects on co-occurring perennial vegetation. We investigated the effects of applying indaziflam individually and in combination with imazapic at three sites (two in Oregon and one in Washington). Applying indaziflam and imazapic in tandem provided better control of annual vegetation and promoted perennial vegetation, although site differences influenced treatment effects. Applying indaziflam individually controlled annual vegetation but did not generally generate a response from perennial vegetation. Tank-mixing indaziflam and imazapic improved first-year control compared with only applying indaziflam, but control was still better in the second year after treatment, suggesting that a greater rate of imazapic than used in this study may be needed to achieve better first-year control. The results of this study suggest that applying indaziflam and imazapic in tandem may be an effective strategy for controlling invasive annual grasses and promoting co-occurring perennial vegetation.