Black Knapweed (Centaurea nigra)

Black Knapweed (Centaurea nigra)

Black Knapweed is a perennial plant native to the Meditarranean, but is also naturalized throughout Europe.  Black Knapweed was first introduced to North America in the early 1900s as an ornamental species, and is now present is most Canadian provinces.  It tolerates a wide range of conditions and habitats, however it grows best in disturbed, well-drained soils and full sun.  It’s a member of the Aster family.

 

IDENTIFICATION

Here are some key features that may help to positively identify Black Knapweed:

  • 1st year plants form rosettes of leaves close to the ground
  • 2nd year plants produce flowering, ascending stems
  • 2nd year plants can reach 30-150 cm in height
  • Stems are covered with fine white hairs, making plant appear woolly
  • Leaves on lower part of the plant are 5-25 cm long, with leaf stems (petioles) and are lance-shaped
  • Leaves decrease in size moving up the stem and may lose leaf stems
  • Flowers occur singly at the end of stems and branches, from June- October
  • Flowers are composed of 40-100 purple (sometime white) tubular florets
  • The base of the flower is oval to globe-shaped, 15-18 mm in diameter and covered with stiff bracts, which are black/brown in the center and have long black fringes
  • Prolific seed producer – seeds are tan coloured, 2.5 – 3 mm, and finely hairy with a pappus of black bristles
  • Has allelopathic qualities that allow it to inhibit the growth and survival of other plant species

 

Black Knapweed is considered a priority species for the PEI Invasive Species Spotters Network.  If you see this plant, please report your sighting.