DECEMBER 2007

Washington State University
WHITMAN COUNTY EXTENSION

STEVE'S

Hoary Cress
Also Known As: Whitetop, heart-podded hoary cress, pepperwort, white weed.

Hoary cress is a Class C Noxious Weed: Non-native plants that are already widespread in Washington State. Counties can choose to enforce control, or they can educate residents about controlling these noxious weeds.

Hoary cress ( Cardaria draba ( L. ) Desv. ) Hoary cress, a member of the mustard family, is native to central Europe and western Asia. It is a single-stemmed, deep-rooted perennial forb that grows about 10-24 inches high. Stems are generally erect, although they can bend or lodge with age. Stems and leaves are blue-green to gray-green in color. Lower leaves form a basal rosette and are somewhat hairy. Mid to upper stem leaves are alternate, oblong to lance-shaped, with margins entire and sometimes toothed, ranging from 3/4 inch to 3 inches long and slightly pubescent to glabrous (lacking hairs). Upper leaves clasp the stem with two ear-like lobes. The inflorescence is typically flat-topped, consisting of a compact cluster of small, white, four-petaled flowers. Fruits of the plant are pods shaped somewhat like an inverted heart. The plant’s heart-shaped pod distinguishes this species of Cardaria from the lens-podded and globe-podded species. The pod of this plant usually contains two seeds. Its seeds are rounded at one end and narrowed to a blunt point at the other, and are about 1/12 inch long and dark reddish-brown in color.

                                                    Rosette stage
Hoary cress reproduces by seed. Upon germinating, plant seedlings develop an extensive root system. The root system consists of vertical and lateral roots, both of which produce adventitious buds that develop into rhizomes and new shoots. Within a three-month period, roots can extend a foot from the stem and add nearly fifty new shoots and over eighty buds. Rosettes emerge in early spring and typically flower from May to June, although plants usually do not flower the first year. Hoary cress plants set seed by midsummer. A single mature plant produces between 1,200 to 4,800 seeds each year. Although seeds can remain viable in the soil for three years, the germination rate drops significantly after the first year. While its seed can be dispersed by human activity, livestock, vehicles/equipment and flowing water, hoary cress depends primarily on its extremely persistent root system to spread. Even very small root fragments are capable of growth. That explains why disturbances, such as cultivation and irrigation, promote the plant’s spread and density.

Hoary cress is an aggressive plant and strong competitor for nutrients and moisture and can form dense monocultures on disturbed habitats. Once established, hoary cress can reduce crop yields and displace native plants. By displacing native vegetation, hoary cress reduces biodiversity and forage production. Wildlife habitat and its fauna are also adversely affected by the plant.

Hoary cress is most often found in open, unshaded, disturbed ground and prefers alkaline soils, but can grow in a variety of soil conditions.  It can be found in fields, croplands, pastures, waste areas, and along roadsides and ditch banks.

Hoary cress growing along highway

Control

As is the case with most perennial invasive weeds, the best option for long-term control of the plant is an integrated management system that combines different control methods.

Prevention

The first objective for hoary cress control should be to contain and control known infestations and prevent the weed from spreading to new areas. Because hoary cress’s primary means of spread is its extensive root system, care must be taken to minimize the spread of root fragments. Machinery and vehicles should be cleaned of seeds and plant fragments before leaving infested areas. Because livestock can spread the plant, grazing should not occur during flowering and seed set. Also, beware of contaminated hay and crop seed.

Mechanical

Mechanical control (hand pulling, digging, cultivation) is effective only if the plant’s entire root system is removed. Realistically then, hand pulling and digging work on only small infestations. Because cultivation is actually a common cause of the plant’s spread, its effectiveness depends on timing and persistence. Cultivation should start when the plant is at the bud or flowering stage and be repeated frequently (every ten days or so--often enough so that no green leaves form) throughout the growing season for several years to destroy the root system and deplete the seedbank. Again, to prevent plant spread, tillage equipment should be cleaned of plant fragments before leaving an infested area.

Mowing offers limited results. If done before the plant seeds, mowing can reduce plant biomass and seed production and slow the plant’s spread, but it does not provide long-term control and should be combined with other control methods (herbicides) to increase efficacy.

Fire

Prescribed burning is not recommended. Fire may actually enhance hoary cress populations since the Cardaria species is more adept at regenerating than competitive vegetation.

Chemical

Hoary cress can be effectively controlled with properly applied herbicides, keeping in mind the importance of timing of application and product selection. Most studies recommend that herbicides be applied at bud or bloom stage, and repeat treatments are usually necessary. Noncrop herbicides such as Escort and Telar (metsulfuron-methyl and chlorsulfuron) can be quite effective, but must be accompanied by a non-ionic surfactant. Plateau (imazapic) provides effective control, but must be applied at bloom stage. Roundup (glyphosate), applied at the budding or flowering stage, will provide fairly good control, but is non-selective. Although 2,4-D is somewhat effective, Tordon 22K, a broad spectrum herbicide which provides excellent control of many rangeland weeds, is ineffective on whitetop species.

Refer to the label for all herbicides
Always follow label directions prior to use

Biological

No biological controls consisting of natural enemies are currently available in the United States, although host specificity tests are being conducted to determine the suitability of biocontrol agents for release. The stem gall mining weevil, Ceutorhynchus cardaria has excellent host specificity and is currently being screened in Switzerland.                                                 
         
 
                                                           
  Stem gall mining weevil

                                                                                                                                                               Weevil larvae
         Grazing

Sheep and goats readily graze young plants, but hoary cress becomes bitter as it matures and loses nutritive value compared to other forages. Sheep have been shown to reduce hoary cress populations. Since the plant is considered at least mildly toxic to some livestock, managers should use caution when allowing animals to graze infested lands. Grazing alone may not control hoary cress since the plant is able to resprout using energy stored in its rhizomatous rootstock.

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Questions: contact Steve Van Vleet or by phone, (509) 397-6290