Photo Credit
Photo courtesy of Paul Alaback, University of Montana.

Spotted knapweed

Scientific name
Centaurea stoebe
Plant Family
Plant Group
The only knapweed in the U.S. with black tipped involucral bracts, the leaf-like structures that surround the cup upon which the flowers are attached.
Identification Hints

There are many species of knapweed that have been introduced to North America, four of which are among the worst noxious weeds in the western US. Spotted knapweed is the only one with black tipped involucral bracts (the leaf-like structures that surround the cup upon which the flowers are attached), creating a spotted appearance.The flowers are generally purple in color. The garden plant Bachelor's buttons (Centaurea cyanea) is similar and also can occur in disturbed habitats. However, it has smooth involucral bracts and entire leaves, but is not a major weed.

Did You Know?

Common camas is a stout, robust, 12-28 inches (30-70 cm) tall plant with a dense inflorescence. Camases are liliaceous, perennial herbs that grow from an edible bulb. The leaves are long and narrow, grass-like, and emerge from the base. The flowers are light to deep blue; more than 3 flowers in an inflorescence may be open at one time. Flowers have 6 tepals, 6 stamens, and 3 stigmas. The inflorescence is a spike-like cluster borne on a leafless stem that is held above the leaves. Common camas is distinguished from great camas (Camassia quamash ssp.quamash) by the following: the flowers are slightly irregular, with the lowest tepal curving outward away from the stem; the anthers are bright yellow; the plant is relatively short and stout, with shorter flower stalks and smaller bulbs; and there is no waxy powder on the leaves. Common camas blooms from April through June. The fruits are barrel-shaped to three-angled capsules,splitting into three parts to release many black, angled seeds.

Leaves
The leaves are once or twice divided into lobes on each side of the center vein. They are alternate and generally pale green to blue-gray in color and are about 1 to 3 in (2.5 to 7.6 cm) long. The basal leaves (basal rosette) have pinnate lobes. The upper leaves are more narrow (linear) in shape.Seedlings initially have entire grayish green leaves.
Flowers
: Flowers are pink to light purple and are borne on the tips of the branching stem. Flowers are disc flowers only (no ray flowers), though outer florets look similar to ray flowers since they are enlarged. Florets are arranged in clusters above an urn-shaped involucre (ring of bracts (modified leaves)) that has rows of bracts with hard, black, fringed tips, giving it a spotted appearance.
Fruits
The seeds are brownish, less than a 0.25 in (0.64 cm) long, notched on one side of the base, with a short tuft of bristles at the tip. Each plant can produce 400 or more seeds per flower stalk which can result in thousands of seeds per plant.
Habitat
Spotted knapweed was introduced from Eurasia and is now distributed throughout most of North America except parts of the Southeast. It is found on open disturbed sites, industrial sites, agricultural fields, pastures, roadsides,grasslands, open woods, mountains, and stream banks. It will readily invade lawns if they are not well watered or aerated.
Bloom Time
Numerous flowers are produced from early June to November. In northern areas it flowers until snowfall or the soil freezes. The seeds may germinate from spring through early fall.
States
AL, AR, AZ, CA, CO, CT, DE, FL, GA, HI, IA, ID, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MT, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OR, PA, RI, SC, SD, TN, UT, VA, VT, WA, WI, WV, WY