Lepidium draba is an invasive perennial herb of the mustard family (Brassicaceae), native to Western Asia and Eastern Europe. It is classified as a noxious weed in 14 states in the US, including Utah. Utah lists L. draba as a class B noxious weed, as it has a moderately large, but controllable population in the state, and is thus considered a priority target for control. Commonly known as Whitetop or Hoary Cress, L. draba was introduced to the US in the 1900’s as a seed contaminant and has since colonized nearly every state outside the southeast, and has made its way into most of southern Canada (see map A). It is now believed to occupy over a quarter of a million acres of private and public land in the United States, and is found in elevations of up to 8000ft in Utah. Most of the western counties of Utah and those surrounding the Wasatch Front are reported be facing Whitetop invasions which can spread fairly rapidly through high seed proliferation and rhizomatous cloning (see map B). L. draba tends to form monocultures in disturbed soils, often from a single individual. These dense strands can crowd out native species, compete with crops for resources, and produce chemicals that are toxic to some animals.

A typical Lepidium draba infestation. Whitetop is known to crowd out virtually any other species. Photo source: http://zipcodezoo.com/Photographers/739/hp350.asp

A typical Lepidium draba infestation. Whitetop is known to crowd out virtually any other species. Photo source: http://zipcodezoo.com/Photographers/739/hp350.asp

Most people in Utah have probably witnessed Lepidium draba invasions without realizing it. From April to July the plant is a common sight along roadsides, in gardens and grazing areas, and on feed lots and water courses--irrigation ditches and natural riparian habitats alike. It can be identified by the somewhat dome-shaped clusters of small white flowers atop their 2-3ft leafy stems. Like nearly all mustards, Whitetop flowers can be further distinguished by their four petals, four sepals and 6 stamen.

Control or management of L. draba can be effectively accomplished through chemical or manual means if applied in a timely fashion.  Whitetop forms deep, dense root systems, so pulling should be done early, before flowering, and tilling should follow to fully remove these systems. Early attempts at removal involved treating infected soils with salt, but were ultimately futile against this proven survivor. Modern chemical management involves repeated application of foliar herbicides, chosen according to plant maturity. Introduction is favored by disturbed soils, providing a clear, if inconvenient route to prevention. More detailed management information is available for download at http://na.fs.fed.us/fhp/invasive_plants/weeds/whitetop.pdf.

Map A. Distribution in the United States and Canada.

Map B. Distribution in Utah.