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.
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.
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.
FOLLOW-UP AND REMEDIATION
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.