Native Forb Establishment following Application of Aminopyralid or Clopyralid

Interest exists in planting mixed forb–grass prairies in the midwestern United States. Aminopyralid or clopyralid can be used to suppress competition from invasive plants prior to seeding prairies. As these active ingredients are known to persist, concern exists that reductions in forb establishment could occur. We tested whether common midwestern forb species could tolerate an application of aminopyralid or clopyralid alone or in combination the summer prior to seeding, and whether fall dormant or spring seeding date influenced establishment. This experiment was performed in Beresford, SD, and Arlington, WI, where aminopyralid (54 or 123 g ae ha−1), clopyralid (237 and 420 g ae ha−1), or aminopyralid+clopyralid (54+237 g ae ha−1) were applied to a prepared seedbed in July of 2009. Ten forbs were seeded in November 2009 as a dormant seeding and in April 2010 as a spring seeding at both locations, and establishment was assessed 12 and 24 mo after treatment (MAT). Results were site and species specific. Time of seeding was an important driver of plant counts at both locations 12 and 24 MAT. In Wisconsin at 12 MAT, 60% of species studied exhibited higher counts in the spring seeding. This trend persisted in some, but was not consistent across all 10 species. In South Dakota, 80% of species studied had higher counts at 12 and 24 MAT, but differences were species specific and often differed from those studied in Wisconsin. Those species that had higher counts in spring seeding at 12 MAT, maintained higher counts at 24 MAT. Forbs planted in plots treated with herbicides did not differ from plots left untreated at either location. Results suggest native forbs typically seeded in the upper Midwest can tolerate these herbicides when applied at least 4 mo prior to seeding.