Centaurea nigra

Name and Family

Black knapweed, Centaurea nigra, is a member of the aster family (Asteraceae).


  • Black knapweed is a perennial plant native to the Mediterranean, 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.
    • It grows best in disturbed, well-drained soils and full sun.

Identification Guide

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 30-150cm in height
  • Stems are covered with fine white hairs, making plant appear woolly
  • Leaves on lower part of the plant are 5-25cm 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, 15-18mm 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 – 3mm, and finely hairy with a pappus of black bristles

What it does in the ecosystem

  • Black knapweed tends to grow in dense clumps that form a monoculture, crowding and outcompeting native plant species. In this way, infestations reduce the species richness and biodiversity of an ecosystem. 
  • The plant changes soil chemistry, making growth difficult for other nearby plants while making the soil more suitable for its own needs. This is called allelopathy. 
  • The plant is able to regenerate from stem and root fragments, but reproduces primarily by seed. Annual seed production can be as high as 18,000 seeds per plant.
  • The plant prefers habitats with full sun, and can tolerate a wide range of soil conditions. 
  • Black knapweed is drought tolerant, allowing it to outcompete other plants in times of stress.
  • Black knapweed is unpalatable to livestock and will reduce the value of pastureland over time. 
  • Black knapweed can increase soil erosion and sediment retention, and alter water movement in an area, affecting crucial components of ecosystem health. 
  • Black knapweed will increase fire risk over time as dead plant material builds up. 
  • For humans, the plant also poses a small risk as a skin irritant. 
  • Has allelopathic qualities that allow it to inhibit the growth and survival of other plant species.



Before selecting a control method, consider the following:

  • The size of the infestation. 
  • Resource availability. 
  • The amount of effort you are willing/able to expend. 
  • Often, multiple control methods are used simultaneously. 
  • Consider the local ecosystem and what other organisms or ecological processes may be affected by management.
  • Ideally, management of black knapweed should be done early in the growing season before flowering occurs. 
  • As knapweed can cause skin irritation, it is important to wear gloves and cover your skin when managing the plant. 
  • Focus on smaller, younger populations first and foremost in your removal efforts.These are known as satellite populations, and are generally responsible for the bulk of an invasive species’ outward expansion.


  • Small infestations can be controlled by hand-pulling and digging to remove the entire taproot. 
  • If the root cannot be removed, it should be cut with a spade about 3 cm below the soil surface to remove at least the root crown. 
  • Look around the base of larger plants for the basal rosettes.
    • These are first-year plants and should be dug out as well. 
    • The rosettes have weak stems and, like larger plants, can resprout from broken taproots.
      •  Because of this, care should be taken when digging or pulling to remove the whole plant without breakage.
  •  Removing the root crown should be enough to kill the plant, as it does not have rhizomes.


  • Larger infestations can be mowed. 
  • This does not kill the plant but can prevent further spread if undertaken before the plant goes to seed.  
  • This process will need to be repeated annually as black knapweed seeds can persist in the soil for up to five years.  


  • The PEIISC does not provide advice for chemical control methods at this time. 
  • If using chemical controls, it is imperative that all local legislation and manufacturer’s instructions be followed during application.


When wrapping up a management project:

  • It is important to properly dispose of all invasive plant material. 
  • Black knapweed, like other invasive species, should never be composted. This is especially true when managed after flowering, as the flowers can still go to seed in a compost heap! 
  • Collect plant parts in clear plastic bags marked with “invasive plant” or “black knapweed”. 
  • As this plant is an irritant, it would be well advised to double bag the plants to prevent any accidental dispersal during transit and help keep any waste handlers safe. 
  • Place the bags in your normal residential waste collection (BLACK BIN).
    • Up to two additional bags can be placed beside the waste cart if it is full.
  • For loads larger than this, reach out to the PEIISC for specific disposal instructions. You may need to obtain a special waste permit.
  • Be sure to thoroughly clean all gear and clothing before moving offsite to prevent further spread of black knapweed. 
  • Revisit the area year after year to continue management, as the plant will resprout until its seed bank is depleted (about 5 years). 
  • If possible, replant the area with an appropriate native species to help regenerate the ecosystem. 


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Black knapweed identification and control: Centaurea nigra – King County

Black knapweed (Centaurea nigra) | Fraser Valley Invasive Species Society (

Black Knapweed | Invasive Species Program | Nebraska (

Washington State Noxious Weed Control Board Black Knapweed Information

Capital Region Invasive Species coulack_knapweed__factsheet_v5f84de052e7e16533860dff00001065ab.pdf (

Biology And Biological Control Knapweed Of [PDF] [5pepttpk4420] (