Invasive Plants of California's Wildland

A coarse biennial, bull thistle (Cirsium vulgare) is distinguished from other thistles by the following combination of characteristics. Leaf blades, especially those that are larger and deeply lobed, are rough to the touch like medium sandpaper and dark green. Stems of bolted plants appear winged because leaf blades continue along the petioles and several inches down the stems. Flowerheads are one to two inches wide and one and a half to two and a half inches high with deep purple flowers. The bristles on the pappus (thistledown) are feathery. This characteristic distinguishes thistles in the genus Cirsium from those in the genus Carduus, which have unbranched, thread-like bristles.

Bull thistle is widespread in California and is most common in coastal grasslands, along edges of fresh and brackish marshes, and in meadows and mesic forest openings in the mountains below 7,000 feet (2,120 m). It is most troublesome in recently or repeatedly disturbed areas such as pastures, overgrazed rangelands, recently burned forests and forest clearcuts, and along roads, ditches, and fences. Even small-scale disturbances such as gopher mounds promote bull thistle establishment and survival (Klinkhamer and de Jong 1988, Randall 1991). It can also colonize areas in relatively undisturbed grasslands, meadows, and forest openings.

Native to Europe, western Asia, and North Africa, bull thistle is now naturalized and widespread throughout the United States (including Hawaii and southeast Alaska) and southern Canada and on every other continent except Antarctica (Grime et al. 1988). It probably was introduced to eastern North America during colonial times as a contaminant in seed and/or ballast (Moore and Frankton 1974) and to scattered locations in western North America in the late 1800s or early 1900s (Jepson 1911). By 1925 it had been reported in California from the San Francisco Bay Area, Central Valley, Klamath region, North Coast, and the northern Sierra Nevada (Robbins 1940).

Bull thistle reproduces only by seed, and individual plants set seed only once before dying. Large individuals may produce tens of thousands of wind-dispersed seeds.

Bull thistle invades a variety of wildland habitats, where it competes with and displaces native species, including forage species favored by native ungulates such as deer and elk. In addition to outcompeting native plant species for water, nutrients, and space, the presence of bull thistle in hay decreases feeding value and lowers market price (McDonald 1994). In pastures and irrigated rangeland it may interfere sufficiently with livestock grazing so that live-weight gain is significantly reduced (Hartley 1983).

McDonald and Tappeiner (1986) noted that bull thistle often dominates recently clearcut forest areas in the Sierra Nevada of California, and experimental work in a replanted clearcut indicated that infestations limit growth of ponderosa pine saplings (Randall and Rejmánek 1993). Bull thistle also colonizes and maintains high population densities for up to six years in clearcuts in redwood and mixed evergreen forests in northwestern California (Glusenkamp, pers. comm.).

Bull thistle is a rosette-forming biennial. Seedlings grow slowly until soil temperature rises in the spring. Absolute growth rates for thistle rosettes are slow for two months after sowing, even under ideal conditions. Only after two months do seedlings expand their leaves and accumulate dry weight rapidly. Seedlings quickly grow into rosettes, nearly stemless plants with leaves clustered radially at the soil surface or pressing against adjacent vegetation. They typically remain in the rosette phase through the first growing season and the following winter. Some studies indicate that rosettes must be exposed to cold temperatures before they will bolt and flower and may also require a minimum daily period of light following exposure to cold (Klinkhamer et al. 1987a, 1987b).

Most plants remain in the rosette stage for one year, then bolt, flower, and set seed in the second growing season, although some individuals may bolt and flower in the first year and others may not bolt until their third, fourth, or fifth year. Seeds are released in late summer or fall and may germinate after rains in fall or the following spring. Seed viability is high, often above 90 percent, and germination is stimulated by soil moisture.

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Flowering may occur from early June until the first snowfall or hard frost; in California there is a pronounced peak in July and early August. Seeds ripen and are released from early July through October, occasionally later along the coast. Studies indicate that most seeds fall within three feet of the parent plant, but up to 10 percent may travel distances of more than ninety feet, even on days with little wind (Klinkhamer et al. 1988, Randall unpubl. data). A single flowerhead can produce from forty to over 250 seeds, and individual plants may have anywhere from one to 475 flowerheads or more. Bull thistle reproduces only by seed, and individual plants die after setting seed.

Mechanical methods: Bull thistle can be controlled by mowing or hand cutting shortly before plants flower (Harris and Wilkinson 1984, Randall 1991). If cut too early in the season, plants will resprout and flower before the first frost. The uneven flowering times may make more than one treatment necessary. Gill (1938) reported that bull thistles that were cut while flowering were not capable of producing and releasing viable seeds. Plants from Yosemite Valley that were cut at the root crown a few days after their first flowers appeared and then laid on the ground produced abundant viable seed (Randall pers. observation). Thus it may be important to remove cut stems from the area.

Less than 5 percent of adult thistles cut at the soil surface resprouted, while over 80 percent of adult thistles in control plots survived and flowered (Randall 1991). Mean height and number of inflorescences were lower for plants that resprouted (twenty-five inches or 63 cm and 3.7 flowerheads) than for adults in control plots (thirty-three inches or 85 cm and 15.8 flowerheads). Subsequent work (Randall unpubl. data) indicated that about 4 percent of the thistles cut two to four inches (5-10 cm) above the soil surface a month before flowering will resprout.

Variations in seed production have more influence than variations in adult mortality on population fluctuations of biennials (de Jong and Klinkhamer 1988b). De Jong and Klinkhamer (1988a) found seed input limited establishment of bull thistle seedlings on coastal dunes in the Netherlands. Even if some plants resprout, manual control may reduce bull thistle populations by limiting seed production. The distance seeds are dispersed is positively correlated with the height at which they are released (Sheldon and Burrows 1973), so cutting may reduce the spread of seed even when plants resprout and produce seed.

Two USDA approved insects, Urophora stylata and Rhinocyllus conicus, have been released for bull thistle control in California. Neither has been successful in controlling bull thistle populations in California to date, but U. stylata shows some promise in coastal sites where it was released in 1997 and 1998 (Villegas and Coombs 1999). U. stylata is a gall-forming fruit fly (Tephritidae) with a narrow host range that has been released at sites in Canada and the United States (Harris and Wilkinson 1984, Julien 1987). Harris and Wilkinson (1984) found that over 90 percent of the flowerheads examined in some British Columbia populations had been attacked by the flies, and the number of larvae per flowerhead increased with the percentage of flowerheads attacked. They calculated that an infestation of this intensity reduced the population’s seed output by over 65 percent.

Rhinocyllus conicus is a weevil (Coleoptera, Curculionidae) that attacks species of Cirsium, Carduus, and Silybum. It has been used as a biocontrol agent for several species in these genera. Studies indicate that there are local populations or strains with strong preferences for particular host species (Zwolfer and Harris 1984). One strain collected from bull thistle in France was released in 1984 in British Columbia, where it bred well through 1986 (Julien 1987). A strain of R. conicus introduced to southern California to control Carduus pycnocephalus L. and Silybum marianum (L.) Gaertn. was rarely found on bull thistle in the release area, although it destroyed an estimated 55 percent of its intended hosts’ seeds (Goeden and Ricker 1986a). Unfortunately, the weevils were occasionally found feeding on two native Cirsium species (C. californicum Gray and C. proteanum T. Howell) (Goeden and Ricker 1986b). R. conicus also attacks several other native Cirsium species in the Rockies and the Great Plains and has been found to limit populations of at least one of these native thistles (Louda et al. 1997, Louda 1998).

Bull thistle is relatively easily controlled with herbicides. Several Agricultural Extension bulletins recommend 2,4-D at 0.5 kg/ha; dicamba at 0.15 kg/ha; picloram (not registered in California) at 1 kg/ha, and various tank mixes of these chemicals for control of bull thistle (Fawcett and Nelson 1981, Harris and Wilkinson 1984). Timing of herbicide application is important, with the exact date dependent upon life cycle stage. Autumn or spring application is recommended to control rosettes (Fawcett and Nelson 1981).