Tall whitetop, a member of the mustard or Cruciferae family, grows one to three feet tall, but may reach eight feet in wet areas. It forms colonies that dominate fields and wetlands. The base of the stem is semi-woody, growing from a woody crown at the soil surface. The course roots are pencil-size in diameter or larger. Small flower clusters with six to eight blossoms bloom by mid-June and usually again in fall if conditions are right.

Do not confuse tall whitetop with the noxious weed "hoary cress" or "whitetop" (Carderia draba) that grows usually much shorter and blooms in May. It grows along the Carson River and in waste places in Nevada.

Tall whitetop is a prolific plant, capable of producing over 6 billion seeds per acre. Although most are shed in the fall, some seeds remain on the plant over winter. Seedlings of tall whitetop are difficult to recognize, as they look like seedlings of other mustards.

This weed expands by creeping underground stems (rhizomes) that grow three to ten feet long and send up shoots to form new plants. Plants can grow from parts of rhizomes as small as 1/10th of an inch in thickness.

Tall whitetop commonly travels in rivers and irrigation systems as seeds and rhizomes from eroded banks over long distances to infest new areas. Flood irrigation carries tall whitetop into and throughout native hay meadows, pastures, and other irrigated lands.

No benefit has been found for this plant, but it has many bad points. Tall whitetop is a supremely competitive plant. It crowds out desirable vegetation, destroys wildlife habitat, impairs scenic value, and decreases biodiversity. In riparian zones, it interferes with the regeneration of willows and cottonwoods, reducing the cover and food available for birds, especially nesting waterfowl.

Tall whitetop in pastures and hay fields lowers the quality of the feed. Both protein content and digestibility are reduced, devaluing the hay nutritionally and economically. Infested hay should not be sold or transported off the property to prevent the spread of tall whitetop. While livestock do not like tall whitetop, they will eat it if it is the only food available. The accumulation of dead, woody stems inhibits grazing.

Tall whitetop is hard to control due to its competitive nature and rapidly spreading roots. Because it is found throughout the West, its eradication (complete elimination) would be very expensive and thus, is unlikely. Instead, the goal is to stop its spread using integrated weed management.

Integrated weed management uses many cultural, mechanical, biological, and chemical weed control techniques in a planned approach. Cultural control of whitetop involves maintaining healthy stands of desirable plants with appropriate irrigation, fertilization and species selection. Mechanical control, including digging, mowing or tilling, is ineffective, encourages plants to sprout from their crowns and rhizomes, and is not recommended. Shading or smothering with thick mulches does not work either.

Livestock may play a role in control of the whitetop in saltgrass meadows, where they eat the flower stalks, reducing seed production. In irrigated pastures, however, cattle avoid whitetop even during drought years when preferred forage species are depleted. Cattle eat the leaves in the early spring, if the coarse, previous year’s stalks are removed. There is some concern that tall whitetop is poisonous to livestock. Research is underway at the Poisonous Plant Research Laboratory, USDA, Logan, Utah, to determine if the plant is harmful to livestock.

The search is underway for a biological control for tall whitetop. So far, no insects or diseases are available to kill or debilitate the plant. Extreme care is needed to avoid introduction of an insect or disease that could go to a valuable crops species or infest any of the eleven perennial lepidium species in the mustard family that are native to the western United States, one of which is endangered. The influence of a potential biological control organism on these closely related species must be clearly established before their use will be allowed.

Current research is focusing on the use of herbicides. They are most effective when the flower stalk has grown, but prior to bloom. This prevents 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, burning or brush beating. Follow all label directions with care, do not spray during windy weather, and never apply chemicals to open water — ponds, ditches, streams or wetlands.

Whenever chemicals are used around wet areas, extreme care must be taken to avoid contaminating waterways. Rather than using a spray applicator or risking herbicide drift onto the water, apply the chemicals with a wipe applicator. This device contains a sealed reservoir of herbicide and a porous wicking applicator at one end. The chemical is wiped or painted onto individual plants. Without drift, nearby, desirable vegetation is not affected by the herbicide.

Chlorsulfuron (Telar (R)) and metsulfuron-methyl (Escort (R)), two sulfonylurea compounds, give the best control of the herbicides studied to date. In test plots, Dr. James Young of the Agricultural Research Service has shown that chlorsulfuron provides better control than metsulfuron when applied at 1.5 oz/acre. This chemical, which is a broadleaf weed killer, does not kill grass or grass-like species. It is not registered for use in wet areas, but shows promise for use in range and roadside applications.

Unfortunately, it is expensive and kills most broadleaf plants, including native shrubs and trees. If tall whitetop can be controlled with only one or two applications of chlorsulfuron or metsulfuron-methyl, however, it may be less expensive than other alternatives that require repeated applications over several years.

Another effective herbicide on tall whitetop is imazapyr (Arsenal(R)). It is a non-selective herbicide, which kills all vegetation indiscriminately. Imazapyr is labeled for use on non-croplands such as road and utility rights-of-way. It is most effective when applied to actively growing foliage.

The phenoxy herbicides (variants of 2,4-D) result in death of the shoots of tall whitetop, but the roots and crown buds may rapidly sprout and grow. Repeated applications may be required for up to five years to starve out the root system. The only herbicide labeled for use in water which has been shown to be somewhat effective is Weedar-64 (R), a 2,4-D amine salt. Glyphosate (Roundup (R)) is less effective than 2,4-D in controlling tall whitetop.

Site-specific chemical control recommendations are given in Table 1. [Editor’s note: Table 1. is not reprinted in this issue, but is available for review at the NWPA office, 560 W. Williams Ave.] Research is continuing on the control of this weed. Many questions still remain to be answered about the effectiveness of herbicides and which application methods are best.

The Tall Whitetop Task Force was formed in 1992 to study best management for the control of this weed. Individuals from Conservation Districts, the University of Nevada Cooperative Extension, the Pyramid Lake Paiute Tribe, the Agricultural Research Service and many local, state and national governmental agencies are participating in this effort. Please support the work of the Task Force and participate in activities sponsored by the group.