Keys to Identification
- The flowers are bright yellow with distinctive spines at the base
- Forms pea-green rosettes that quickly turn silvery-green as they become full grown plants
- The stems are winged and can be up to two ft long
- Each plant produces thousands of seed
- Yellow starthistle covers millions of acres in the west but it has only been found in a handful of locations within Colorado.
- It is critical that this plant be eliminated wherever it is found.
This information courtesy of the Colorado Natural Areas Program
St. Barnaby’s thistle
Colorado Noxious Weed List A
New in Colorado – call your county weed supervisor if you find this plant!
Flower heads are yellow, located singly at the ends of branches. Flower heads are distinguished by sharp, straw-colored thorns, which are up to 0.75 inches long.
Yellow starthistle has two types of seeds, plumed and plumeless.
Basal leaves are deeply lobed while the upper leaves are entire and sharply pointed.
Mature plants are 2-3 feet tall and have rigid, branching, winged stems that are covered with cottony hairs (Whitson et al. 1996).
Seedlings have oblong, tongue-shaped cotyledons (Herzog and Randall 1998).
Yellow starthistle causes a neurological disorder called chewing disease (equine nigropallidal encephalomalacia) in horses that eat it (Maddox et al. 1985).
Yellow starthistle is a pioneering plant that becomes established on disturbed land. It forms dense infestations, reduces the available edible forage, and exhibits a suspected allelopathic effect on some associated species (Maddox et al. 1985).
Habitat and Distribution
Yellow starthistle invades rangelands, pastures, roadsides, cropland, and wastelands. It is intolerant of shade and requires light on the soil surface for winter rosette and taproot development (FEIS 1998). Yellow starthistle is capable of establishing on deep, well-drained soils as well as on shallow, rocky soils that receive from 10-40 in of annual precipitation. In the Pacific Northwest, yellow starthistle favors sites that were formerly dominated by big sage-brush, bluebunch wheatgrass, Idaho fescue and Sandberg bluegrass (Sheley et al. 1999).
Well established in the Pacific coast states, and spreading west. So far, fewer than a dozen infestations of yellow starthistle have been reported in Colorado.
Introduced from Europe, where it is native to the Mediterranean region.
Seedlings usually emerge in the fall, form rosettes, and begin growing a taproot. Root growth continues throughout the winter. Yellow starthistle bolts in late spring and flowers June through August.
Mode of reproduction
Plants usually produce 700 – 1,000 seeds perplant, but vigorous plants may produce up to 170,000 seed (Herzog and Randall 1998, FEIS 1998).
Seeds may remain viable for several years (Herzog and Randall 1998).
Plumed and plumeless seeds are dispersed at different times. Plumed seeds are dispersed by wind shortly after maturity. Plumeless seeds remain in the seedhead until it disintegrates in the fall or winter.
FEIS – Fire Effects Information System [Online] (1996, September). Prescribed Fire and Fire Effects Research Work Unit, Rocky Mountain Research Station (producer), US Forest Service. Available: www.fs.fed.us/database/feis/ [1998,March 12]
Hastings, M.S. and J.M. DiTomasso. 1996. Fire controls yellow starthistle in California grasslands. Restoration and Management Notes 14:124-128.
Herzog, P. and J. Randall. 1998. Element stewardship abstract for Centaurea solstitialis, yellow starthistle. The Nature Conservancy. Available 7/30/98. Internet: http://tncweeds.ucdavis.edu/esadoc/documents/centsol.html
Maddox, D.M., A. Mayfield, and N.H. Poritz. 1985. Distribution of yellow starthistle (Centaurea solstitialis) and Russian knapweed (Centaurea repens). Weed Science 33:315-327.
Sheley, R.L., L.C. Larson and J.S. Jacobs. 1999. Yellow starthistle, In: R.L. Sheley and J.K. Petroff (eds.) Biology and management of noxious rangeland weeds. Oregon State University Press, Corvallis. pg. 408-416.