After reading last week's article on the noxious weed yellow starthistle, a reader named Phyllis asked me to write about another noxious weed: tall whitetop. She has observed it on properties along Highway 395 in Carson Valley and at the Washoe Lake Wildlife Reserve.
Unfortunately, tall whitetop is very prevalent across Nevada, covering tens of thousands of acres. The state of Nevada, local weed management areas, University of Nevada Cooperative Extension, land management agencies and private landowners have been actively battling this weed since the 1990s.
Whitetope showed up in the U.S. about 1900 as a contaminant in sugar beet seed. In 1983 graduate student Gerry Miller, now with Natural Resources Conservation Service, wrote one of the first articles about how the weed was invading the Truckee Meadows. He had found about three acres. By 1999, it was estimated that the infestation along the Truckee River had expanded to 20,000 acres. It is this rapid expansion and tendency to out-compete native vegetation that makes a weed noxious.
Tall whitetop is a tall dark-green plant with white flowers that look like baby's breath. It is a member of the mustard family. It forms a plant colony that dominates fields and wetlands, growing in dry or wet sites. Its roots range in size from the diameter of a pencil to the size of a person's wrist. They may extend 3-10 feet. These rapidly spreading roots propagate more plants. In addition, a stand of tall whitetop can produce more than 6 billion seeds per acre.
Whitetop can be spread in fill dirt, topsoil moved during construction, erosion, equipment, hay and straw. Most animals will not eat it, so it can destroy a pasture or field for forage. It is quite difficult to control. In general, hand pulling produces more plants, so it is only a viable management technique when there are just a few plants. Generally, whitetop has to be controlled with herbicides applied at bud to early bloom stage. This will prevent seed set.
A second application to any regrowth in the fall appears to give the best control. In order to get good foliar coverage, it is important to remove the previous year's growth by mowing. Only certain chemicals work on whitetop, so read the herbicide label to see if tall whitetop is listed. Always read the entire label and follow it carefully.
Information at 887-2252 or www.unce.unr.edu/publications/files/nr/other/FS9995.pdf
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at firstname.lastname@example.org