Herbicide residues and perennial grass on establishment perennial pepperweed sites

Abstract

Perennial pepperweed (Lepidium latifolium L.) is a creeping rooted exotic weed that has infested native hay meadows, riparian areas and agronomic fields throughout the western United States. This highly invasive species causes major losses in forage quality and creates numerous management problems. On many sites infested with perennial pepperweed, a near mono-culture exist. Sustainable suppression programs require the establishment of a competitive perennial species. You can not establish seedlings of a perennial competitive species without some initial substantial reduction in perennial pepperweed stands through weed control. Because tillage is not feasible with this creeping rooted species, herbicidal weed control is the primary option. Experience has shown, that the massive and extensive root system of perennial pepperweed can not be completely eliminated with one application of a herbicide. This means that repeated applications of a selective herbicide are required after the perennial seedlings of a competitive species are established. Perennial pepperweed is a broadleaf species that is some what susceptible to applications of 2,4-D. Therefore, the choice revegetation species is limited to a perennial grass that is resistant to 2,4-D applications at low rates as a seedlings and moderate rates once established. The saline/alkaline nature of the soils where perennial pepperweed is often found limit the adapted perennial grasses to tall wheatgrass (Elytriga elongata [Host] Nevski). The herbicide chlorsulfuron has been shown to be more effective in initially controlling perennial pepperweed than 2,4-D. We determined that applications of chlorsulfuron at rates sufficient to control perennial pepperweed resulted in herbicidal residues that severely reduced or eliminated the establishment of tall wheatgrass seedlings. Application of 2,4-D at flower budding for perennial pepperweed (June), followed by seeding tall wheatgrass in the fall (October), and application of low rates of 2,4-D over the wheatgrass seedlings the next spring (May), gave the best grass seedling establishment and suppression of the perennial weed.