Hoary alyssum (Berteroa incana L.) is a non-native invasive plant that is on noxious weed lists in California, Idaho, Michigan, Montana, and Washington. This weed can be toxic to horses, reduce pasture forage value, and decrease plant and pollinator ecosystem services. Managing hoary alyssum is difficult because it has a long flowering period, during which individual plants simultaneously flower and produce seeds. Consequently, improper herbicide application may kill hoary alyssum flowers but not seeds. Hoary alyssum reproduces exclusively by seed, therefore, this study seeks to understand how different herbicides affect hoary alyssum seed biology, specifically seed production and viability. As such, this study will provide hoary alyssum control recommendations and enable land managers to make more judicious herbicide application decisions. We collected hoary alyssum plants from six rangeland sites that had been treated with various herbicides by invasive plant managers across southwestern Montana in summer 2016. Hoary alyssum plants at each site were at different developmental stages when treated. We randomly selected approximately 20-30 hoary alyssum plants from treated and non-treated areas at each site about four weeks post-treatment. Seeds from these plants were counted to determine seed production and analyzed for viability using tetrazolium tests.
Results/Conclusions
At four of the six sites, herbicide treatments significantly reduced hoary alyssum seed production by 49-98% compared to non-treated areas. Notably, herbicide treatments significantly reduced seed production at all sites that were sprayed at early developmental stages, that is before 50% of hoary alyssum flowering stems had seed pods. All herbicide treatments, except for Telar + 2,4-D, significantly decreased hoary alyssum seed viability. Seed viability in non-treated areas ranged from 36-73%, whereas seeds from treated areas, except those treated with Telar + 2,4-D, exhibited 0-21% viability. Escort + Milestone most consistently reduced seed viability to 0.75% and 2.5% at two sites. While invasive plant managers should rely on more than just herbicides to reduce weed populations, the judicious use of herbicides within an integrated pest management program can sustainably maintain pastures and livestock operations. Our research suggests that the application of some herbicides early during hoary alyssum flower and seed pod development can effectively reduce seed production and viability, controlling this invasive plant’s sole method of propagation.