Improving Seeding Success in Annual Grass-Invaded Areas Using Pre-emergent Herbicide and Deep Furrowing Techniques

Exotic annual weeds have invaded and transformed western North American ecosystems. Restoration of these invaded sites has been met with low levels of success. Pre-emergent herbicides can effectively control these annual weeds, but this treatment does not allow for the concurrent seeding of desired species. Seeding within a deep, U-shaped furrow following herbicide application may be a method to reduce pre-emergent herbicide effects by transferring the herbicide away from the seed at the time of planting. We investigated this method by spraying plots with or without the pre-emergent herbicide imazapic, then planting with or without a deep furrow. Treatments (i.e., spraying and furrowing) were applied using mechanical equipment within a single pass at six sites. In plots without imazapic, deep furrows generally had higher density and above-ground biomass of seeded species than those without furrows. Similarly, in plots with imazapic, deep furrows generally improved measured plant metrics for the seeded species. For example, plant density in deep furrows ranged from 62% – 97% higher than non-furrow treatments in plots with imazapic and 41% – 89% higher in plots without imazapic. Deep furrows also decreased exotic annual weeds in the first year after planting, but weed reduction was generally more effective when this treatment was applied with imazapic. Overall, this research provides evidence that deep furrows alone can improve seeding success in most instances. Nevertheless, combining herbicide application with deep furrows in a one-pass system should be considered in areas with high weed cover. Due to the substantial soil disturbance caused by deep furrows, this method should be selectively applied, such as constraining the treatment to substantially degraded areas.