Annual grass invasion is transforming the western US and driving a need for restoration techniques that can both reduce exotic annual grass abundance and allow revegetation of native species. Pre-emergent herbicides can provide control of annual grasses, but when applied concurrently with direct seeding efforts, the herbicide can also impact seeded species. Indaziflam is a relatively new herbicide that may provide extended control of exotic annual grasses, but little is known about its effects when applied at the time of seeding. 

In this study, we compared indaziflam to imazapic, a popular herbicide used in restoration efforts, to understand how indaziflam affects plant establishment of a native species, bluebunch wheatgrass Pseudoroegneria spicata (Pursh) Á. Löve. We created furrows on half our treatments to limit herbicide concentrations and potentially create a safe-site for seeding bluebunch wheatgrass. 

During the two-year study, indaziflam provided consistent control of the annual weed, downy brome Bromus tectorum L., whereas imazapic control decreased sharply with time. Indaziflam and imazapic decreased bluebunch wheatgrass seedling emergence by 96 and 46%, and two-year plant density by 91 and 65%, respectively, compared to non-herbicide treatments. Both herbicides reduced aboveground biomass of bluebunch wheatgrass by over 85% two years after seeding/herbicide application. 

Furrow treatments mitigated imazapic effect on bluebunch wheatgrass, but did not limit the impacts by indaziflam. 

Herbicide can be used in conjunction with direct seeding efforts, but mitigation of the effects to native seeds will depend on herbicide specifics such as mode of action and soil mobility. 

Plant Measurements

Bluebunch wheatgrass seedling emergence was characterized at the end of April 2018 by individually counting all live seedlings in each row of the entire plot. Bluebunch wheatgrass aboveground biomass was sampled and dried in late August 2019, two years after the initial planting. Biomass samples were collected by clipping all aboveground biomass at ground level. Downy brome cover was measured visually during the last week of May 2018 and May 2019. Bluebunch wheatgrass density was counted simultaneously as biomass was destructively sampled. Both bluebunch wheatgrass plant density and aboveground biomass were sampled once, 2 years after planting. All bluebunch wheatgrass sampling was done on the center row of three side-by-side replicate rows to limit edge effect.

Downy brome cover estimates were made visually to the nearest 1% using a circular metal hoop (Bonham et al., 2004). The hoop used was 1 m in diameter and placed over three side-by-side rows of the same treatment. Percent of total ground area occupied by downy brome within the hoop was estimated visually using the total hoop area to the nearest percent. These hoops were placed in the same position during both years of the study. Downy brome cover values were estimated for each treatment (4 per sub-block) averaged to the sub-block level, such that each herbicide treatment had one statistical repetition at the block level.