Plant Profiles Common Knapweed

As the world now moves into the beginning of October, one invasive species is coming to the close of its blooming period.  The common knapweed (Centaurea nigra) is a species of plant originating in Southern Europe and the British Isles but that has been introduced to the United States in the past few centuries.  Since its introduction, the common knapweed, also known as the black knapweed or the lesser knapweed, has been classified as a Class B noxious weed by the United States Department of Agriculture.  According to the Federal Noxious Weed Act passed in 1974, Class B noxious weeds are those that are “not native to the State, [are] of limited distribution statewide, and pose a serious threat to the State.”  


The common knapweed grows to be about three feet tall and topped with a purple flower similar to a thistle.  Green leaves grow from the stalk of the plant and grow smaller as they approach the flower itself.  According to the Bricksfield Country Park located in Hampshire, each blade of these flowers is connected to a brown bract at its base and forms a white seed between June and October which help to quickly spread the weed.


This species of weed can tolerate some of the worst habitats, excluding soils with thick clay contents, which is vital for its speedy invasion of new areas.  The USDA reports that cases of common knapweed invasion have been found in at least 28 states in the US as well as in six Canadian provinces.  It is often found in grassy areas and sections of ground along well traveled locations as these are the places that seeds can be most easily transferred.  


According to information collected from Nicholas Culpeper’s field guide to medicinal herbs, Complete Herbal, knapweed has been used to treat wounds and stop bleeding of the mouth and gums in the seventeenth century.  The weed was also used at one point to treat an inflammatory disorder known as catarrh.  

The weed also has many non-medicinal, and more modern, uses.  The attractive purple flower of the knapweed lends itself to usage in flower arrangements and, according to Culpeper, makes it a great “magnet for many species of butterfly.”  The flowers of the plant have also been eaten on salads, although ingesting or using the plants for medical purposes is not recommended by the experts at the Bricksfield Country Park.