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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Cirsium vulgare (Savi.) Tenore (Cirsium lanceolatum Scop.)
Bull thistle, spear thistle (Asteraceae)


Current level of impact
Known locations in RMNP: Moraine Park, Club Lake Trail. Common in open meadows, and ponderosa pine savannas on the east side of park. Found in some late succession communities.
Assessment: An intermediate number of patchy distributed populations. When added together, all populations would cover an estimated area less than 5 hectares.

Distribution
Origin: Introduced from Europe but native to Asia.
Geographic distribution: Widely established in North America. Bull thistle has been introduced many times as a seed contaminant. Scattered through out Colorado.
Ecological distribution: Meadows, fields, roadsides, and other disturbed sites. A weed of pastures, montane and found at elevations up to 9000'.
Soils: On rich, rather moist soils. Common on calcareous soils, or those rich in bases.

Reproduction
Biennial forb, reproduces only by seeds and plants die after they set seed. A true biennial in that it produces a rosette in the first year, and in the second year it flowers and dies. Flowers July to October.
Seed production: A single inflorescence may produce up to 250 seeds, the number of inflorescence per plant vary, but individuals with more than 60 are not uncommon. Average fruit production is nearly 4000.
Seed germination: Studies in coastal dune populations and in British populations have produced little evidence of seeds persisting in soil from year to year.
Seed dispersal: Seeds possess a hairy pappus, and are well suited for wind dispersal.
Germination: Germination commonly takes place in autumn. In one study, bull thistle germination was not inhibited by dense cover, however, subsequent seedling survival was reduced.

Competition
Highly competitive weed. Studies have found that the spread of bull thistle is favored by trampling and soil disturbance. In Yosemite valley, the heaviest infestations were in areas that were heavily used by visitors. Digging by pocket gophers may also be important disturbance that favors the spread of bull thistle.

Control
Bull thistle will not withstand cultivation. Studies in Yosemite show that mechanically cutting the thistles at the soil surface is an effective method of control. Infested fields should be mowed before seeds have a chance to ripen. A program that involves cutting should be maintained for at least 4 years.
Chemical: 2,4-D can be used to control bull thistle. 2,4-D should be sprayed while plants are still in rosette stage because plants become resistant as flower stalk is produced. If plants are too large before 2,4-D is applied, mow areas to prevent seed production and spray 2,4-D to inhibit regrowth (Lorenzi and Jeffrey 1987).

References

Forcella, F. and H. Wood. 1986. Demography and control of Cirsium vulgare (Savi) 
    Ten. in relation to grazing. Weed Research 26:199-206.

Jong, T.J. de and P.G.L. Klinkhamer. 1988. Population ecology of the biennials 
    Cirsium vulgare and Cynoglossum offcinale in a coastal sand-dune area. Journal 
    of Ecology 76:366-382.

Klinkhamer, P.G.L. and T.J. de Jong. The importance of small-scale disturbance for 
    seedling establishment in Cirsium vulgare and Cynoglossum officinale. Journal 
    of Ecology 76:403-413.

Lorenzi, H.J. and L.S. Jeffrey. 1987. Weeds of the United States and Their Control. 
    Van Nostrand Reinhold Company, New York. 355 pp.

McCarty, R.J. and J.L. Hattling. 1975. Effects of herbicides or mowing on musk 
    thistle seed production. Weed Research 15 :363-367.

Randall, J.M. 1988. Population dynamics of bull thistle, Cirsium vulgare, in 
    Yoesmite Valley. In Proceedings of the Symposium on Exotic Pest Plants. U.S. 
    Dept. of Interior, Washington D.C. 261 -277.

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