Timing of seeding after herbicide application influences rates of germination and seedling biomass of native plants used for grassland restoration

Natural resource managers commonly use herbicides to control invasive weeds. This practice, however, can lead to secondary invasion by other invasive plants, unless measures such as seeding are taken to promote natives. Although there is evidence that seeding immediately after herbicide treatment adversely affects germination, there is little information on optimal timing between spraying and seeding or variation among species in herbicide sensitivity. We investigated effects of picloram and aminopyralid on seed performance of 10 native plants. We separated the herbicide application and seeding by 0, 3, 6, 9, and 11 months in a greenhouse, and recorded the rates of germination and germinant biomass after 6 weeks. In addition, we installed 72 field plots to test the effects of fall- and spring-treated plots on seed performance. In the greenhouse, herbicides negatively impacted germination of four species over the entire 11-month trail whereas six showed less sensitivity over time. Results from spring-treated field plots were largely consistent with those from the greenhouse: rates of germination and biomass were lower in herbicide-treated plots than those in control plots for 75% of seeded species. In fall-sprayed plots, however, the adverse effects of herbicides were only apparent for 25% of seeded species. Results from greenhouse and field studies combined indicate that herbicides can have strong adverse effects on germination, but that actual effects in field settings will be based on complex interactions between species traits, field conditions, and management choices. Thus, site-specific trials will ultimately be the best method for making inference to particular restoration sites.