Exotic annual plants are an increasingly important ecological issue in desert ecosystems. In the southwestern U.S.A., the forb Brassica tournefortii is rapidly expanding, and other annuals such as Erodium cicutarium and Schismus spp. are increasing in dominance. Hand weeding B. tournefortii is currently the most common control method employed, but weeding is inadequate and expensive for managing large-scale invasions and few other strategies exist. New methods must be developed to conserve and restore desert ecosystems. Exotic annuals in desert systems have broader germination requirements and more rapid phenologies compared to natives, suggesting that a window for selective control of exotic annuals may occur immediately after exotic seedling emergence. We tested the role of timing in control methods by comparing a cotyledon-stage glyphosate application to a bolting-stage application and to hand weeding B. tournefortii, plus an untreated check. Treatments were tested at two sites dominated by either exotic or native annuals and followed for two years; early application was repeated the second year. Cover and richness were evaluated during seedling and peak flowering stages underneath and between shrubs.

Results/Conclusions

Early glyphosate did not affect native cover, but did reduce exotic cover. Late herbicide negatively impacted both exotics and natives. Natives had little positive response, and then only through hand weeding under shrubs, but the same treatment caused an increase in the exotic E. cicutarium. Results show that the rapid phenology of exotic annuals may be exploited to control exotics while minimizing impacts on natives in desert communities.