Spotted Knapweed
Lisa Frost (2007)

Centaurea biebersteinii

Spotted knapweed, Centaurea biebersteinii, of the sunflower family, is not native to Oregon. The earliest recording of its presence in North America was in 1883, arriving through alfafa, clover seed, or discarded soil. It is native to central Europe, and parts of Russia. It got its name because of the spots formed by black margins on the flower bract tips.

Spotted knapweed is a biennial or short-lived perennial. In its first year rosette leaves form at the base, and then in subsequent years it flowers. Rosette leaves are about 8 inches long and 2 wide, deeply lobed once or twice on both sides of the middle vein, lobes being oblong and wider toward the tip. They have taproots, which are deep and thick. Flowering stems can be 8 to 50 inches tall, somewhat hairy, and branched above the middle. Stem leaves are alternate, sometimes slightly lobed. Leaf size gets smaller the higher they are on the stem. The flowers are purple or pink, occasionally white, with 25-35 flowers per head. They bloom June to October, and the flower heads stay on the plant afterwards. They are oblong or oval shaped, single or in clusters of two or three at the branch ends. Leaf like bracts surround the base of the flower head, which are oval and yellow green. The brown oval seeds have pale longitudinal lines and a short fringe on one end.

Spotted knapweed is spreading rapidly throughout the US. It outcompetes native plant species, especially after disturbances. It decreases plant diversity and isn’t utilized by wildlife as food. It may also degrade the soil by causing increased erosion, surface runoff and stream sedimentation. 15 states have designated it as a noxious weed, in 25 it is reported invasive, and is found in all but 5 states. This clearly is significant to the biodiversity and landscape of North America. Methods to eradicate knapweed include using insects to damage different parts of the plant and spraying with chemicals. It is very important to minimize its spread to more areas, which can be done by using weed free hay, not grazing livestock when ripe seeds are present, and cleaning footwear and clothes that have come into contact with it.

Spotted knapweed reproduces by seeds; plants can produce between 500 and 4,000 per square foot per year. Approximately 90% of those seeds are viable at the time of dispersal, and can remain that way for up to 8 years. Dispersal is most commonly just by dropping from the parent plant, but can also be moved by people and other animals, cars, and crop seed or hay. Places like gravel pits, soil stockpiles, and grain elevators effectively distribute the seed to brand new locations. It likes well drained, light textured soils hat get summer rainfall. It easily establishes itself after disturbance but is also able to invade well managed rangelands. The one place it does not do well is in areas of vigorously growing grass in moister areas. Because of its taproot, it is able to get to water far below the surface, which helps it survive better than many other plants.

http://www.nps.gov/plants/alien/fact/cebi1.htm

http://www.invasivespeciesinfo.gov/plants/spotknapweed.shtml