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Management Of Hoary Cress
Hoary Cress, similar to perennial pepperweed, is a creeping perennial mustard that is well-adapted to moist habitats, especially sub-irrigated pastures, rangeland, roadsides, and ditch banks. Like many noxious weeds, these species dramatically reduce biodiversity and forage production, and displace native plant communities and wildlife habitat (Sheley and Stivers 1999). Typical of many mustards, hoary cress can form dense monocultures, similar to other noxious weeds, such as leafy spurge. In addition, hoary cress contains glucosinolates that are toxic to cattle. This serves as a significant threat to Larimer County's cattle industry. In Larimer County, there are an estimated 3,100 and 1,200 acres of hoary cress and perennial pepperweed, respectively (May and Beck, unpublished data). Management of this weed will protect Larimer County's agricultural community as well as recreational resources such as native flora and fauna.
Mechanical Control
Due to the rhizomes of this perennial weed, mechanical control provides minimal control. Diligent digging can provide control of very small infestations. Hand-pulling of aboveground plant parts is ineffective. Successful digging requires complete plant removal within 10 days after weed emergence throughout the growing season for two to four years. Cultivation 6 inches deep must be repeated within 10 days of weed emergence throughout the growing season for two to four years.
Chemical Control
In general, effective chemical control requires multiple applications. Timing and application rate are crucial for successful control. Again, application rate is critical; MORE DOES NOT NECESSARILY MEAN BETTER CONTROL. Always read the label when applying a herbicide. The label will be able to tell you how much to apply per acre. If you have any confusion as to how much to chemical to apply to your tank, please call our office and we will assist you as soon as possible.
Campaign (glyphosate + 2,4-D) may be applied at 54 oz/A to control hoary cress. However, the glyphosate in Campaign will injure and kill perennial grasses and use of Campaign in successive years should be avoided. Escort (metsulfuron) applied at 0.75 to 1.0 oz/A or Telar (chlorsulfuron) applied at 1 oz/A are effective if combined with an agricultural surfactant at 0.25 % v/v (equivalent to 1 qt of surfactant per 100 gallons of spray solution). The optimum time to apply Escort and Telar for hoary cress control is in the spring when it is flowering or in the fall. These herbicides can be purchased at an agricultural products supplier (please check the yellow pages for specific stores).
Biological Control
No effective agents for Colorado at this time, however, research in this area persists.
Cultural Control
Hoary cress might be managed temporarily with herbicides or, in the future, with insects, but long-term reductions must include planting competitive plant species to occupy bare ground once infested by the weed. Many improved grass species can be seeded in late fall or winter when seedbeds are properly prepared. An initial herbicide treatment is important to suppress problem perennial weeds before seeding.
Important grass characteristics to be considered for long-term control of problem weeds such as hoary cress should include 1) adaptation to the soil and climate; 2) ease of establishment; and 3) competitiveness with weeds.
Resources
Sheley, R.L. and J. Stivers. 1999. Whitetop. In: Biology and Management of Noxious Rangeland Weeds. Oregon State University Press, Corvallis. pp. 401-407.