Low weed and cool-season grass abundances likely necessary for warm-season grass, forb, and shrub establishment

Authors: Rinella, Matthew - Matt; Bellows, Susan - Bartlett

Submitted to: Restoration Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2024
Publication Date: 5/29/2024
Citation: Rinella, M.J., Bellows, S.E. 2024. Low weed and cool-season grass abundances likely necessary for warm-season grass, forb, and shrub establishment. Restoration Ecology. 32(5). Article e14177. https://doi.org/10.1111/rec.14177.
DOI: https://doi.org/10.1111/rec.14177

Interpretive Summary: In the Northern Great Plains, perennial cool-season grasses are easiest to establish, and they sometimes competitively suppress warm-season grasses, shrubs, and forbs. Seeding cool-season grasses at low rates sometimes benefits other seeded plants but risks greater weed abundances. We tried to identify cool-season grass seed rates low enough to allow other seeded plants to establish but high enough to constrain weeds. We varied native cool-season grass seed rates while holding other seed rates fixed. We hypothesized that as cool-season grass seed rates and cover increased, cover of weeds and seeded warm-season grasses, shrubs, and forbs would decrease. However, we also hypothesized weed cover would gradually become independent of grass seed rate due to seeded plants increasing in plots seeded at low rates. Neither hypothesis was supported. Because weed abundances were high, warm-season grasses, shrubs, and forbs apparently experienced similarly intense competition regardless of grass rate, so low rates did not increase seeded plant establishment. Regardless of seed rate, cool-season grass cover did not increase between the second and final (i.e., fourth) growing season, perhaps because of low precipitation. Increasing warm-season grass, shrub, and forb abundances will require controlling weeds in addition to lowering cool-season grass seed rates. Even these steps will not always increase native plant establishment, because native plants sometimes died before herbicide applications became feasible and grass competition became important. Lowering grass rates without implementing weed control risks sites becoming weedy for prolonged periods.

Technical Abstract: Plants seeded to degraded grasslands often fail to establish. In the Northern Great Plains, perennial cool-season grasses are easiest to establish, and they sometimes competitively suppress warm-season grasses, shrubs, and forbs. Seeding cool-season grasses at low rates sometimes benefits other seeded plants but risks greater weed abundances. We tried to identify cool-season grass seed rates low enough to allow other seeded plants to establish but high enough to constrain weeds. We varied native cool-season grass seed rates while holding other seed rates fixed. We hypothesized that as cool-season grass seed rates and cover increased, cover of weeds and seeded warm-season grasses, shrubs, and forbs would decrease. However, we also hypothesized weed cover would gradually become independent of grass seed rate due to seeded plants increasing in plots seeded at low rates. Neither hypothesis was supported. Because weed abundances were high, warm-season grasses, shrubs, and forbs apparently experienced similarly intense competition regardless of grass rate, so low rates did not increase seeded plant establishment. Regardless of seed rate, cool-season grass cover did not increase between the second and final (i.e., fourth) growing season, perhaps because of low precipitation. Increasing warm-season grass, shrub, and forb abundances will require controlling weeds in addition to lowering cool-season grass seed rates. Even these steps will not always increase native plant establishment, because native plants sometimes died before herbicide applications became feasible and grass competition became important. Lowering grass rates without implementing weed control risks sites becoming weedy for prolonged periods.