Once established, Knapweed is very difficult to eradicate.


Habitat: It requires loamy, well drained soils


with neutral pH and full sun. It is drought tolerant and prefers warm temperate or continental climates with dry summers.


Identification: Stems: Are erect, unbranched or spar-


ingly branched near the tips, growing 50-170 cm tall. They are covered with long, soft hairs, which are slightly cobwebby. Leaves: Are lance shaped, 10-30


cm long, and slightly undulating with pointed tips. Basal and lower stem leaves have short petioles (leaf stem). Leaf margins are smooth or shallowly dentate. Upper stem leaves are five to 10 cm long, sessile (no petioles), and slightly undulate with smooth margins. Leaves are covered with short hairs and are dotted with resin glands. Flowers: Are disc-shaped, borne


singly and composed of many yellow florets, with clusters of reduced leaves directly below the flower heads. Involu- cres (flower base) are 25-35 mm diam- eter and covered with one to two mm wide brown bracts having fringed edges


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and occasionally weak spines. Seeds are indehiscent, seven to eight mm long and with flattened bristles five to eight mm long.


Prevention: Many infestations result from garden


escapes. Do not grow as an ornamental. Using bighead knapweed in dried flower arrangements facilitates seed dispersal.


Control: Grazing: Cattle will consume bighead


knapweed but this is not considered a control method. Invasive plants should never be considered as forage. Cultivation: Cultivation can be effec-


tive as long as it is deep enough to cut the roots off below the root crown and must be repeated for at least two seasons. Care must be taken not to


Information courtesy of


transport root pieces that could start new infestations elsewhere. Mechanical: Repeated mowing will


suppress seed production but plants will regrow. Repeated cultivation, where suitable, is an effective control method. Small patches can be hand pulled or dug out but the entire taproot must be removed to prevent resprouting. Chemical: Currently no selective


herbicides are registered for use on bighead knapweed. Always check prod- uct


labels to ensure the herbicide is


registered for use on the target plant in Canada by the Pest Management Regulatory Agency. Always read and follow label directions. Consult your local Agricultural Fieldman or Certified Pesticide Dispenser for more informa- tion. Biological: None researched to date.


abinvasives.ca | info@abinvasives.ca Early Spring 2015 • 7


Photo by Dwight Sipler.


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