JoAnne Skelly: Perennial pepperweed, aka ‘tall whitetop’

Visiting the garden today at the Greenhouse Project, I was discussing perennial pepperweed, Lepidium latifolium, management with Cory King, the Project’s manager, farmer and educator. Controlling this noxious invasive weed organically is difficult. The most effective method of management unfortunately is chemical. However, because the amount of the infestation is small, repeated deep digging for two or three years may work.

Perennial pepperweed, often called tall whitetop, is one of the most troublesome noxious weeds in Nevada. It grows in all 17 counties, and despite many efforts to control it, populations continue to expand. A member of the mustard family (Brassicaceae), perennial pepperweed commonly grows 2 to 4 feet tall but can reach 8 feet in wet or shady areas. Young seedlings of perennial pepperweed can be difficult to recognize, as they look like seedlings of other mustards.

Perennial pepperweed is difficult to control due to its competitive nature and extensive, rapidly spreading root system. Creeping roots are the most important method of reproduction for this weed. Efforts often focus on excluding it from uninfested areas. Digging can be used in situations where only a few plants are present. It is most effective in moist loose soils. New plants readily sprout from root fragments, so hand-pulled or dug areas must be monitored frequently and new growth pulled or dug as soon as it appears. Perennial pepperweed is very difficult to control, and sites must be monitored for up to five or more years and retreated when needed. Absence of plants one year after treatment should not be interpreted as success. New shoots from deep roots that survive an initial growing season after treatment may emerge during a subsequent growing season.

I mention that perennial pepperweed is noxious as well as invasive. Noxious is a legal term. The Nevada Revised Statutes defines a noxious weed as “any species of plant which is, or likely to be, detrimental or destructive and difficult to control or eradicate.” All landowners are responsible for controlling noxious weeds on their property. NRS 555.130 states, “Every person owning, controlling or occupying lands in this state shall cut, destroy or eradicate all weeds declared and designated as noxious before such weeds propagate and spread and whenever required by the State Quarantine Officer.”

If your property has perennial pepperweed on it, control it early and often so it doesn’t take over the entire site. For more detailed information, including chemical options, go to www.unce.unr.edu/publications/files/nr/2010/sp1007.pdf.

JoAnne Skelly is associate professor and Extension educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.