Scott Nissen, Managing Russian Knapweed and Annual Weeds During Native Grass Establishment.

Russian knapweed (Rhaponticum repens synonym Acroptilon repens) is an invasive, perennial weed native to Eurasia. In the Arkansas Valley, on the West Slope and even along the Front Range new infestations seem to be appearing everywhere you look. Russian knapweed is well suited to the dry areas of the Western US. Its drought tolerance combined with the fact that the Western US has been in a drought for the past ten years could explain why Russian knapweed infestations appear to be on the increase. It can reproduce by seed, but unlike spotted and diffuse knapweed, Russian knapweed is not highly dependent on seed production to persist. It has extension horizon roots that produced multiple crowns. These crowns develop numerous shoot buds in the fall and it is these shoot buds that produce shoots the next growing season. Once established Russian knapweed spreads very efficiently by sending out horizontal roots in every direction. The ability to spread by vegetative means and its allelopathic root exudates allows Russian knapweed to outcompete most native grasses. Therefore, Russian knapweed control is essential for native grass establishment. Fortunately, there are a number of treatment options for Russian knapweed that can provide up to 2 years of control and these treatment options have significant latitude in terms of application timing. We have been working on Russian knapweed control and native grass establishment for the past four years and have concluded that given sufficient rainfall, significant progress can be made in controlling these infestations. We have had excellent results applying Milestone® (Dow AgroSciences), Tordon® (Dow AgroSciences) or generic equivalents, and Transline® (Dow AgroSciences) or generic equivalents in the spring to rosettes and bolting plants or in the fall or early winter even if there are no living shoots. If there are remnant native grasses and sufficient moisture, the grass will respond. In areas without remnant grasses, we have applied Milestone in spring or fall and seeded native grasses December through April with excellent results. The key to long term Russian knapweed controls appears to be using herbicides to control the roots and shoots and encouraging competition from native grasses. These two components together have provided excellent control for four years. Milestone and Transline are very selective herbicides and do not provide kochia or Russian thistle control; therefore, it is imperative to control these and other annual broadleaf weeds during grass establishment. Vista is very safe on seedling grasses and provides excellent kochia control, but does not control Russian thistle. Combinations of Vista and 2,4-D have provided excellent control when these weeds occur together. Remember, native grasses take time to establish and it may take several years for grasses to provide significant competition. Dr. Scott Nissen, Cameron H. Douglass and Joseph D. Vassios, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523