Did you know?
A single whitetop plant can send out 400 shoots in a year!
How did whitetop get here?
Whitetop, or hoary cress, was introduced from southwestern Asia. Introductions likely occurred multiple times in shipments of contaminated alfalfa seed from Turkistan into North America over a period of 40-50 years. Whitetop was first found in California in 1876.
What are its characteristics?
This is an aggressive perennial forb that is somewhat tolerant of salty soil, and grows up to 2 feet tall. Many small white flowers are produced, giving a white-topped appearance. Plants most aggressively establish where extra water is available, like swales, irrigated fields without frequent cultivation, and in riparian areas. Seeds establish new stands when transported by water, vehicles, farm machinery, or contaminated hay and crop seeds. Palatability of whitetop decreases as plants mature. The foliage becomes coarse and bitter, and nutritive value decreases.
The stubborn persistence of this weed is due to an extensive root system. New shoots arise as roots creep laterally beneath the surface of the soil. A single plant can send out 400 shoots in a year. Vertical roots may penetrate the soil to a depth of 15 feet and allow the plant colony to withstand extensive drought. Only about one quarter of the plant is seen aboveground while the rest is belowground, hoarding water and nutrients. New populations should be aggressively controlled to prevent seed spread to new areas. It can look similar to perennial pepperweed, except it is shorter with more compact flowerheads, and its seed pods do not open at maturity.
Why is whitetop a problem in the Great Basin?
Infestations rapidly establish dense stands and decrease rangeland health. Invasion potential is greater under heavy grazing or other disturbances. It damages wildlife habitat by crowding out beneficial plant species. Whitetop is considered by many to be at least mildly toxic to livestock. Frequent burning under cheatgrass dominance may enable whitetop to gain greater dominance and become a bigger problem.
How can we fight this weed?
Cattle and sheep will graze whitetop, but don’t offer much control. It takes plowing once a month for 2-4 years to remove colonies once established. Flooding an area for 2 months can also eliminate infestations. Mowing followed by an herbicide application on regrowth is likely the most feasible method of control. The Weed Management Handbook on the University of Wyoming Extension website offers current herbicide information. Fire will not kill perennial plants, and seedlings can grow quickly following burns. Where possible, shrub establishment may provide long-term suppression of whitetop colonies. Regardless of the control method, an intensive management process is required in heavily infested areas.
-Drawing: USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. Illustrated flora of the northern states and Canada. Vol. 2: 165.
-Flower: Oregon State University Jed Colquhoun photo Collection.
-Plant: Oregon State University Jed Colquhoun photo Collection.
-Encycloweedia. California Department of Food and Agriculture. Available: (http://www.cdfa.ca.gov/phpps/ipc/weedinfo/cardaria.htm, 27 September 2006).
-USDA, NRCS. 2006. The PLANTS Database (http://plants.usda.gov, 26 September 2006). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
-Whitson, T. D., et.al. 2000. Weeds of the West., 9th ed. Western Society of
Weed Science, Newark, CA.
-Zouhar, Kris 2004. Cardaria spp. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: (http://www.fs.fed.us/database/feis/, 26 September 2006).