Invasive Plants of California's Wildland

Three species of hoary cress are found in California: heart-podded (Cardaria draba), lens-podded (C. chalepen-sis), and globe-podded (C. pubescens). Globe-podded hoary cress is not listed as noxious in California. These Cardaria species are single-stemmed, upright perennial herbs, less than knee high, with upper lobed leaves clasping the stem. The inflorescence is typically flat-topped and generally dense with white flowers. Lower leaves form a basal rosette and are somewhat hairy and lance-shaped; higher on the plant the leaves clasp the stem with two ear-like lobes and have fewer hairs. Flowers have four sepals with white margins and four petals narrowing to a claw at the base. Plants tend to form dense monospecific mats with shoots connected by white underground rhizomes (Hick-man 1993, Robbins et al. 1974).

These three Cardaria species are differentiated mainly by the shape of their fruit pods. The pods of heart-podded hoary cress are heart-shaped at the base. The pods of the lens-podded hoary cress are circular. The pods of both heart-podded and lens-podded are flattened in cross-section and have no hairs. The pods of globe-podded hoary cress are globular or spherical and are covered with fine hairs (Hickman 1993, Robbins et al. 1974).

Cardaria draba and C. chalepensis grow in many habitats and areas of the state, except in the Mojave and Colorado deserts (Barbe 1990). C. draba occurs in wet and dry grasslands, scrubs, and arid areas with alkali soils. It is most often found in open, unshaded areas on disturbed, generally saline soils, but can grow on almost any soil. It generally is found at elevations of less than 4,000 feet, but it is known from elevations of over 8,000 feet in Utah and over 6,000 feet in Montana. It survives heavy frosts and snowfall, but may favor wetter sites in harsher climates (Cook 1987, Robbins et al. 1974).

Cardaria chalepensis concentrations exist in grainfields and hayfields and along roadsides in Siskiyou County, the Sacramento-San Joaquin Delta, and in Kings and Tulare counties (Barbe 1990). It is particularly common in the north on red-brown soils and in disturbed, generally saline soils and fields (Mulligan and Findley 1974).

Hoary cress is native to central Europe and western Asia, probably centering on Turkey, Georgia, Syria, Iraq, Iran, and Armenia. It has now spread to all continents and is particularly common in many parts of North America.

Cardaria species occur as crop weeds throughout the Middle East, Europe, Australia, and New Zealand. Cook (1987) reports that C. draba has been considered England’s most serious weed pest since 1949, and the most serious weed pest for the Wimera region of Australia.

Hoary cress appeared in New York in 1898, apparently introduced from ship ballast. In 1910 it was introduced into alfalfa fields in the southwestern United States from imported seed from Turkestan (South Australia Dept. of Agriculture 1973; Chipping 1992). It was first reported in California near Yreka (Siskiyou County) in 1876 (Robbins 1940).

Seed is commonly spread in hay and forage such as cut alfalfa, in soil attached to livestock and farm equipment, and by flowing water. Seed may be spread by wind along highways (South Australia Dept. of Agriculture 1973; Chipping 1992). Despite prolific seed production, spread by seed is likely not the most important means of spread. Many infestations remain virtually the same size year after year in spite of annual seed production (Parsons 1992).

Plants also spread by means of extremely persistent root systems, which consist of extensive rhizomes from which shoots emerge (Mulligan and Findley 1974). Another method of dispersal is through movement of root fragments in mud carried by livestock and vehicles, spread by highway maintenance, carried in streams, and spread by tillage (Cook 1987, Robbins et al. 1974). Even very small pieces of root are capable of growth. Infestations in areas with frequent disturbance, such as cultivation, regularly increase in size and density (Parsons 1992).

Cardaria draba establishes monospecific mats that exclude most or all other herbaceous vegetation. C. chalepensis forms dense infestations that crowd out forage plants in meadows and fields. By displacing native vegetation used by wildlife, both species negatively affect native fauna as well. These Cardaria species are strong competitors for nutrients and moisture. In Australia C. chalepensis slowed water drainage, increasing flooding. Gophers increased and, as a result, the amount of soil disturbance also increased, increasing colonization rates of Cardaria species. Australians consider C. chalepensis toxic to grazing stock (Chipping 1972, South Australia Dept. of Agriculture 1973).

Growth and reproduction are better understood for Cardaria draba than for C. chalepensis, although the two may be quite similar. C. draba reproduces by seed and expands by creeping roots. Flowering is generally from March to June, but may occur as early as December in mild coastal climates. In large stands that are close to the water table a few flowering plants may be encountered year round. In early spring, infestations may resemble carpets of snow (Cook 1987, Robbins et al. 1974, Mulligan and Findlay 1974). Under stressful conditions flowers may develop on stems just four to six inches (10-15 cm) high with just one branch, but in well watered conditions flowering may start when the plant is as small, but continue until many flowering branches have developed and plant height approaches twenty inches (50 cm). The plant is self-incompatible, is pollinated by insects, and can produce 1,000 to 5,000 seeds per stem, with seed viability of about 80 percent. Seeds are small, with 550,000 seeds per kilogram. Plants appear to produce few seeds in dry years and are prolific seeders in wet years. Seedbanks are generally depleted in three years under both irrigated and non-irrigated conditions. Seed survives in uncomposted cattle dung.

Germination in these Cardaria species typically occurs in autumn, with the plant overwintering as a rosette and flowering the following spring. Plants can also germinate in spring or early summer and over-summer as a rosette, but flowering still is delayed until the following spring. Seeds can germinate thirty-five to forty-two days after they are released from the fruit, and in California generally do so after the first rains. C. draba sometimes appears after a grass fire; the heat presumably breaks the dormancy of seeds lying in or on the soil (Parsons 1992).

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Basal rosettes of Cardaria draba are formed three to four weeks after sprouting. The rosettes may be formed from seed or, more commonly within infested areas, from sprouts that arise from rootstocks. Plants usually do not flower the first year. After twenty-five days a plant may be rooted to a depth of 10 inches (25 cm) and may have up to six lateral roots with buds. Shallowly buried buds may form rosettes, while deeper buds form new rhizomes. Hoary cress has a deep, penetrating root system, numerous underground buds, and large food reserves. The extensive root system spreads horizontally and vertically with frequent shoots arising from the rootstock. Within three months roots can extend a foot from the stem, with nearly fifty new shoots and over eighty buds. In the absence of competition, one plant can produce 455 shoots the first year (Cook 1987). Energy is stored in the rootstock during the growing season, and new plants are produced from joints in the roots. The roots may survive complete removal of shoots for a period of one season without noticeable loss in vigor; plants suffer visibly the second consecutive year. Plants must be cut off at depths greater than 20 inches (50 cm) below the soil surface to prevent regeneration from underground parts. Even root fragments will readily regenerate, allowing C. draba to be spread by any vector that can carry a root fragment.

Manual/mechanical methods: Tillage may control infestations if started at flower bud time and continued every ten days throughout the growing season. Slightly longer intervals may be possible at different times of year, but it is essential that no green leaves be allowed to form. This deprives rootstock fragments of energy, but the process may have to be continued for at least three growing seasons to deplete the seedbank. Care should be taken not to spread fragments of the plant out of the infested area on tillage equipment.

Prescribed burning: Cardaria species apparently are favored by fire through removal of competition.

Flooding: For control of Cardaria draba, flooding to a depth of six to ten inches (15-25 cm) for about three months can produce 90 percent control of the plant (Cook 1987, Fryor and Makepeace 1978, Pryor 1959, Robbins et al. 1974). However, short-term submergence lasting a week has no effect on the plant (Chipping, pers. observation).

Insects and fungi: No USDA recommended biological control agents exist, and potential introductions from the native range are complicated by the large numbers of cruciferous crops. Although the mite Acerea draba is effective in sterilizing plants of Cardaria draba, it is also found on commercial crops, as is the aphid, Aphis armoracea (Cook 1987).

Grazing: Grazing is not effective on Cardaria draba, as it survives and resprouts using energy stored in its rhizome-like rootstock. In C. chalepensis young plants may be grazed to the ground by cattle and sheep, which also ingest seed heads.

Although C. chalepensis contains glucosinolates and can be mildly toxic, nutritional levels are adequate to meet the requirements of most livestock, especially in early growth stages. Problems arise as the foliage becomes coarse and bitter as it matures, when plants have low nutritive value compared to other forages (Cook 1987, Robbins et al. 1974).

These plants actually are spread by grazing, as cattle ingest seed heads and may become vectors for plant fragments.

Most research on chemical control of Cardaria species has focused on cropland, usually alfalfa, clover, or wheat fields. Experiments commonly include combinations of herbicides and other non-chemical methods in association with the herbicide treatment. Check with your county agricultural agent to determine which of the possible chemical means of control are currently registered for use in wildlands in California.

Different forms of 2,4-D have been tried with limited success in northern California, although Canadian trials have had success with applications at 1 to 2 lbs/acre, repeated for three years to remove the seedbank.

Mixes of 2,4-D ester and dicamba have been applied by aircraft, and mixes of 0.50 2,4-D and 0.25 each dicamba and R-11 surfactant have worked in roadside applications of one gallon of the mix in 100 gallons of water. Airplane application inevitably affects non-target plants and carries with it the danger of drift of the herbicide to non-target areas and surface water.

Chlorsulfuron, which is selective for broadleaf plants, has been used on California rangeland at 0.33-1 oz/acre with limited success, but has a half-life of four to six weeks and affects non-target species. Chlorsulfuron at 0.50-2 oz/acre has been successful in roadside applications in central coastal California. Glyphosate at 1 pt/acre produces 80 percent control at the budding or flowering stage, but is also non-selective (Cook 1987, Fryor and Makepeace 1978, Pryor 1959, Robbins et al. 1974).