Why Is It a Noxious Weed?
This species is very difficult to control once established and is a threat to our natural areas, including subalpine meadows.
How would I identify it?
It is the tallest knapweed in Washington, growing 19 inches to 5.6 feet in height. It is a tap-rooted perennial that typically grows a rosette (cluster of radiating leaves at ground level) the first year and flowering stems the following years.
Solitary flower heads are globe-shaped and range 1 to 3 inches in diameter. Bracts at the base of the flower head are light green to tan and have thin, papery, fringed margins. Lower bracts may have weak spine tips. Flowers are yellow.
Leaves are alternate, may have hairs, gland-dotted and with lower leaves having a stalk and upper stalkless. Leaves become smaller moving up stem. Shape varies, narrow and wider above the middle to narrowly egg-shaped. Edges range from toothed to smooth.
Stems are upright and either unbranched or sparingly branched, terminating in a single flower head. Stems may have some soft hairs.
Fruit Seed Description
Brown seeds are 0.27 to 0.31 inches (7 to 8 mm) long with flattened bristles on one end.
Where does it grow?
It grows in open grassy areas such as fields and pastures. Please click here to see a county level distribution map of bighead knapweed in Washington.
How Does it Reproduce?
It reproduces by seed and reproduction is also possible from root divisions. The rosette typically does not produce a flowering stem the first year. Seeds are dislodged from mature seed head by direct contact or wind, though not wind dispersed.
How Do I Control It?
Pulling bighead knapweed is impractical for large infestations, but small plants may be dug out. When the flowering stem is broken off without removing the tap-root, a new stem will grow from the woody crown, producing another flower head later in the season. Repeated mowing will reduce seed production, and will eventually diminish root reserves.
No biological controls are known for bighead knapweed.
For More Information
See our Written Findings for more information about bighead knapweed (Centaurea macrocephala).