African rue is a poisonous, perennial forb that readily invades salt-desert shrub and sagebrush-steppe rangelands. Information detailing options for integrated management of African rue is lacking. To date, a few studies have researched the efficacy of different herbicides for controlling African rue, but none have investigated integrated approaches to its management. Broadcast applications of imazapyr at three rates (0.275, 0.55, and 0.85 kg ae ha−1) were made, with and without a prior mowing treatment, to African rue when it was in full bloom. Imazapyr resulted in significant reductions in both the cover and density of African rue, regardless of application rate or mowing treatment (P < 0.05). Mowing had no effect on African rue cover or density (P > 0.05). Higher rates of imazapyr resulted in significant reductions in the cover of native perennial bunchgrasses (P < 0.05), whereas the low rate did not affect perennial grass cover, regardless of mowing treatment (P > 0.05). Integrating a mowing treatment with imazapyr applications was less effective for controlling African rue than applying herbicide alone. Mowing before imazapyr application did not increase survival of perennial grasses. Our results suggest that the recommended rate of imazapyr for controlling African rue (0.85 kg ae ha−1) could be reduced by as much as one-third on dry floodplain ecological sites within the northern Great Basin without comprising its effectiveness for controlling African rue. This lower rate would reduce nontarget damage to native perennial grasses, which are the dominant functional group in the herbaceous understory. Less damage to native perennial grasses would probably accelerate understory recovery and help prevent invasion by other invasive species.
Nomenclature: Imazapyr, African rue, Peganum harmala L.
Management Implications: African rue (Peganum harmala L.) is a poisonous, exotic, perennial desert forb that has the potential to cause extensive ecological and economic losses in the western United States. Broadcast applications of imazapyr at three rates (0.275, 0.55, and 0.85 kg ae ha−1) were made, with and without a prior mowing treatment, to African rue during full bloom. The current rate recommended for control of African rue using broadcast applications of imazapyr is 0.85 kg ae ha−1. Results of this study suggest that this rate can be reduced by as much as one-third on sagebrush–steppe rangeland without comprising the short-term effectiveness of imazapyr for controlling African rue; however, follow-up treatments or restorative actions may be required to sustain long-term control of African rue and to prevent invasion by exotic annual species. In addition, our results indicate that perennial bunchgrasses readily recover from imazapyr application rates of 0.55 kg ae ha−1 or less, whereas rates of 0.85 kg ae ha−1 resulted in suppression of desirable perennial grasses for 2 yr or more. Limiting damage to native perennial grasses is important because this functional group dominates the understory in noninvaded plant communities and, therefore, is critically important for preventing invasions by exotic plants in sagebrush–steppe rangelands.