Photographer: Steve Dewey Affiliation: Utah State University Source: Bugwood.org Copyright: (CC BY 3.0)
Whitetop is a deep rooted creeping perennial mustard plant that grows up to 2 feet tall. In general, plants have a gray-green, soft hairy appearance, hence the name hoary. Leaves are blue-green in color with the lower leaves being stalked and the upper leaves having two lobes clasping the stem. Plants have many white flowers with four petals, giving the plant a white, flat-topped appearance. Flowers tend to get a cream colored cast as they mature. Seed capsules are flat and heart shaped with two reddish, brown seeds encased in pods. Their seeds have been used to make pepper, and honey bees make high quality honey from their flowers.
Lepidium draba is a threat to economical an ecological systems in several aspects. It reduces biodiversity by displacing native plants from ecosystems and eventually the animals that are dependent upon those plants for food and habitat. Also, it reduces, crop, pasture, rangeland quality and in turn that reduces foraging quality and quantity.
Plants emerge in very early spring and have bloomed and set seed by early to mid-summer. Whitetop reproduces by seed and by root, and each plant can produce around 3,000 seeds annually. The seeds spread by wind, animals, vehicles, and water. They are often a contaminant in crop seed. Whitetop seeds can germinate within three weeks of being dispersed from the plant. The plant continues to grow both above and below ground until the first frost. The lateral roots grow widely; up to 12 feet the first season and two to five feet each season thereafter. The lateral roots may turn down and grow several feet deep. They may produce rhizomes (horizontal underground stems) that sprout additional plants. A single whitetop plant can produce 450 new shoots in a single growing season.
Whitetop was introduced into the United States in the late 1800’s, most likely via contaminated alfalfa seed.
Native Origin: Eurasia
U.S. Habitat: Whitetop generally grows better in moist sites or areas of moderate rainfall and disturbed sites, including excessively grazed areas, waste areas, roadsides, and open grasslands. Also, it is the plant is common on alkaline soils.
U.S. Present: AK, AZ, CA, CO, CT, DE, IA, ID, IL, IN, KS, KY, MA, MD, ME, MI, MN, MO, MT, ND, NE, NJ, NM, NV, NY, OH, OK, OR, PA, RI, SD, TX, UT, VA, VT, WA, WV, WY
Looks like its congener and fellow invasive plant; the Hairy Whitetop (Lepidium appelianum = Candaria pubescens). However, the whitetop has a heart-shaped, broad, flat seedpod; while the Lepidium appelianum has a small, purplish, globe-shaped pod.
Lepidium draba can be effectively controlled with properly applied herbicides. However, one application of any herbicide will not completely eliminate whitetop. Roundup (glyphosate) applied as a 2% solution at the flower stage has resulted in fairly good control. However, Chemical control is difficult when whitetop has established itself amongst crops. Legumes are sensitive to herbicides and will be lost if sprayed. Crops such as alfalfa, peas, onions, and sugar beets are damaged or killed by pre-emergent and post-emergent herbicides that effectively kill whitetop. There are no registered herbicides for these crops that will kill whitetop and not kill the crop. Fortunately there are also biological options, such as various kinds of livestock. Sheep eat it when it is the most abundant food source, they tend to like the buds. Also, goats eat it and other weeds, just for something to eat.
The spread of Lepidium draba can be prevented by washing vehicles (especially the undercarriage) after driving out of an infested area, this will wash off most of the seeds and plant fragments. Before shutting your vehicle's door, check and make sure that no seed heads are going to be shut in with you, or the next time you open your door, some might fall out and infect yet another area.
References
Stephen F. Enloe (ed). 2007. Weed Management Handbook. Extension Services of Montana, Utah and Wyoming.
Sheley, R.L. and J.K. Petroff. 1999. Biology and Management of Noxious Rangeland Weeds. Oregon State University Press. Corvallis, OR.
Whitson, T.D.(ed.), L.C. Burrill, S.A. Dewey, D.W. Cudney, B.E. Nelson, R.D. Lee, R. Parker. 1996. Hoary cress. Weeds of the West. Western Society of Weed Science, in cooperation with the Western United States Land Grant Universities Cooperative Extension Services, Newark, CA.
Wyoming Weed and Pest Council. Weed Handbook, Series 1-30.
Zouhar, K. 2004. Cardaria spp. In: Fire Effects Information System. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.
Internet Sources
http://www.fs.fed.us/database/feis/plants/forb/carspp3/all.html
http://luirig.altervista.org/flora/taxa/index2.php?scientific-name=lepidium+draba
http://smslrwma.org/invasives/Cardaria/index.html
http://www.mtweed.org/whitetop/
http://www.newyork.plantatlas.usf.edu/Plant.aspx?id=628
http://www.unce.unr.edu/publications/files/nr/2002/FS0256.pdf
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