Species specifics: Whitetop

Photo Credit : Brother Alfred Brousseau, hosted by the USDA-NRCS PLANTS Database

Fast facts

Nonnative invasive perennial mustard that is classified as noxious in many U.S. states.
Toxic to cattle.
Also known as hoary cress, ball cress or hairy whitetop, thanks to its snow-like flower cover at the top of the lobes that first blooms in April.

What to look for

Whitetop (Cardaria draba), also known as hoary cress, ball cress or hairy whitetop, is a creeping perennial mustard that is highly invasive, toxic to cattle and classified as noxious in many parts of the United States.

The invasive weed is part of the mustard family and grows up to 2 to 3 feet tall, with lower leaves that are blue-green and lance-shaped, and upper leaves that feature two lobes clasping the stem. In April, the tops of the lobes give way to somewhat flat-topped clusters of white flowers, giving the plant a white, flat-topped appearance and its common name, whitetop.

With a deep vertical taproot that branches laterally and extensively – one plant can produce up to 450 shoots without competition – whitetop spreads by both shoots and seeds. A single plant can produce 1,200 to 4,800 seeds each year that drop in the fall and quickly establishs seedlings that overwinter as small rosettes. Plants die back to their roots in late summer before emerging again shortly after the newly established rosettes emerge in early spring of the following year.

Because of its extensive root systems and tendency to develop dense monocultures, whitetop outcompetes favorable forages by displacement and outcompeting for soil moisture. The weed contains glucosinolates that are toxic to cattle, which typically avoid the plant unless no other forages are available.

Where it is found

Whitetop prefers open, unshaded areas where it can receive at least 12 to 16 inches of moisture annually; however, it can withstand extensive drought. Recently disturbed areas are especially susceptible to whitetop spread.

A common nonnative invasive species across the West, whitetop is found in much of Arizona, California, Colorado, Montana, New Mexico, Nevada, Oregon, Utah, Washington and Wyoming. County-level outbreaks and invasive watches have also been reported in Kansas, Nebraska, North Dakota and South Dakota. Numerous states classify this weed as noxious including Arizona, California, Utah and Washington.

How to treat it

Apply 3.3 ounces of Chaparral™ herbicide per acre early in the spring to actively growing rosettes or to regrowth before budding. The addition of 2,4-D amine at 2 pints per acre is recommended.

Treatment after bloom is generally less effective; however, applications can be made to fall regrowth before the first killing frost.

Chaparral™ has no grazing or haying restrictions for any class of livestock, including lactating dairy cows, horses (including lactating mares) and meat animals prior to slaughter. Label precautions apply to forage treated with Chaparral and to manure and urine from animals that have consumed treated. Consult the label for full details. Chaparral is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state.