Common Name: bull thistle
Scientific Name: Cirsium vulgare
Code: CIVU
Group: Dicot
Family: Asteraceae (Sunflower)
Growth Habit: Forb/ Herb
Duration: Biennial

What does it look like?

Bull thistle is a striking plant that stands 1 to 6 feet tall. It generally has one stem that spreads widely above the middle of the stem and has extremely prickly hairy leaves. The lower leaves grow from 4 to 16 inches long and the upper leaves become smaller with spiny wings. The terminal flowers found at the end of the stems can be solitary or clustered with 1.5- to 2-inch wide, dark purple blooms. The seeds are straw colored and about 1/8-inch long, striped lengthwise with brown or black, with a protrusion at one end, and tipped with plume-like hairs. Bull thistle blooms between July and September (BLM 1995).

Bull thistles are distinguished from other thistles by their larger leaf blades with deeper lobes that feel like sandpaper to the touch, along with larger flower heads which are a deeper color of purple (Bossard et al. 2000).

Where does it grow?

This species is native to Europe but can be found anywhere where there is evidence of ecological disruption. Bull thistle prefers disturbed soils and grows in pastures, roadsides, disturbed areas, edges of dry meadows, and log landings. In areas that have been heavily logged and overgrazed, bull thistle can form dense patches and make native re-vegetation difficult due Bull Thistle's rapid spreading, its large size, and prominent spines.

Is it in our watershed?

Bull thistle is common throughout the King Range and can be found along roadsides, streams and pastures throughout the Mattole watershed.

Why is it a problem?

Bull thistle is a highly aggressive species that grows in a variety of habitats. It outcompetes native plant species for water, nutrients, and space, often eliminating forage species for native wildlife and crowding out desirable grazing species on pasturelands. Bull thistle has also taken over recently-clearcut forest areas inhibiting re-growth of native tree species (Bossard et al. 2000).

How do you get rid of it?

Manual Control: It is best to cut the plants at ground level BEFORE the flowers turn purple in order to stop seed production. If the heads have already turned purple, they can be cut but must be placed in bags and burned to prevent spreading of seeds. Dead plants without flower heads can be left on site.

Prevention is the most cost-effective technique. Whenever an obvious ground disturbance occurs, seed the area with native plant seeds to encourage regeneration by desired species rather than exotics (NPS, 2001).

Mechanical Control: Cutting or mowing bull thistles shortly before plants flower can be a successful control method, but uneven flowering times may make more than one treatment necessary (Bossard, 2000). Mow as close to the surface as possible. If plants are cut above the terminal bud before the stems elongate, they likely will regrow. It is important to mow before the flowers start showing color because plants mowed after that will likely produce some viable seed. Mowing for several years will reduce the root vitality of the perennial species and will prevent seed production, reducing the seed reserve.
Any type of crop rotation will eliminate Bull Thistles since they cannot tolerate tillage or crop competition. However, this practice may not be suitable for rolling, sloping, and erodible fields.

Habitat Manipulation: In the Mattole, bull thistle seems to heavily sprout following the use of heavy machinery. Following any large-scale soil disturbances, re-plant the area with rapid growing, native vegetation to restore the balance of the natural plant community and prevent the growth of invasive species that thrive in disturbed habitats.

Biological Control: Two insects, Urophora stylata and Rhinocyllus conicus, have USDA approval for bull thistle control in California. U. stylata had limited success on coastal sites after their release in 1997 and 1998 (Villegas and Coombs 1999 in Bossard, 2000). Neither species has had substantial success in controlling bull thistle populations as yet.

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photographs:
Unity Peterson

References:
Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/

NPS 2001. Exotic Plant Species List. Scientific and Common Names of known Exotic Plant Species in the Redwood National and State Parks. National Park Service. http://www.nps.gov/redw/epsplist.htm