Knapweeds
Knapweeds are herbaceous biennials or perennials. Seedlings emerge throughout the growing season and rosettes persist year-round. Knapweeds bloom in summer with white to purple flowers. As part of the aster family, knapweeds have composite flowers with each flowerhead made up of many tiny florets. Knapweeds also have bracts at the base of their flowerheads. Characteristics of the bracts are key for distinguishing knapweed species. Seeds are dry, brown, and elliptic in shape with tufts of hair to aid wind dispersal.
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Look Alikes
Canada thistle’s (Cirsium arvense) flower quantity, size, and shape can be similar to knapweeds. Canada thistle leaves are noticeably spiny.
Resource Table
| Classification | Common Name | Scientific Name | Other Scientific Names | Resources | Videos | Weed ID slides ppt (AGR 375) | National Distribution Map |
|---|---|---|---|---|---|---|---|
| Prohibited | Diffuse knapweed | Centaurea diffusa | Link 1 Link 2 | Centaurea spp. | National Map | ||
| Restricted | Brown knapweed | Centaurea jacea | Link | Centaurea spp. | National Map | ||
| Restricted | Black knapweed | Centaurea nigra | Link 1 Link 2 | Centaurea spp. | National Map | ||
| Restricted | Tyrol knapweed | Centaurea nigrescens | Link | Centaurea spp. | National Map | ||
| Prohibited | Russian knapweed | Leuzea repens | Rhaponticum repens, Acroptilon repens, Centaurea repens | Link 1 Link 2 | Centaurea spp. | National Map | |
| Prohibited | Yellow star thistle | Centaurea solstitialis | Link | Centaurea spp. | National Map | ||
| Restricted | Spotted knapweed | Centaurea stoebe | Centaurea biebersteinii, Centaurea maculosa | UWEX Factsheet Link | Video | Centaurea spp. | National Map |
Species Summary Table
| Scientific Name | Common Name | Leaves | Flowers | Bracts | Seeds |
|---|---|---|---|---|---|
| Centaurea diffusa | Diffuse knapweed | Alternate, finely divided, tiny hairs give grey appearance | Usually white, sometimes pinkish to purple; July – Sept. | Multiple lateral spines + one prominent spine on tip (1/3” long) | Urn-shaped seed heads; seeds with no tufts |
| Centaurea jacea | Brown knapweed | Lance-shaped, undivided; leaves smaller at top | Rose to lavender to purple, 1 -1 ½”; June – Oct. | Rounded, wide, + brown at tip; often appear raggedly torn | Tan-colored seeds with fine hairs, no tufts |
| Centaurea nigra | Black knapweed | Lance-shaped, narrow end at base, widest at middle; leaves smaller at top; edges smooth to lightly serrated | White to purple; June – Oct. | Black | Tan, hairy seed ~1/10” long; may have small dark bristle at one end |
| Centaurea nigrescens | Tyrol knapweed | Basal leaves lance-shaped, narrow end at base, widest at middle; upper leaves lance- shaped, smaller at top | Lavender to purple, 1 – 1 ½”; July- Aug. | Green to pinkish with parallel veins; triangular brown to blackish tips with fringes on each side | Dry brown seed, finely hairy, may have tufts |
| Centaurea stoebe | Spotted knapweed | Grey-green to blue-green, somewhat hairy, deeply lobed into narrow segments | Pink to purple, 1”; June – Oct. | Green to purplish-brown with parallel veins; brown to black tip with fringes | Brown, finely hairy, tuft or short light brown to white hairs |
| Centaurea solstitialis | Yellow star thistle | Grey-green, covered in woolly hairs, often lobed on lower half of leaf | Yellow, ½-3/4”; July – Oct. | Light green, covered in cobwebby hairs, multiple spreading spines at tip with one pointed yellowish spine ~1” long | Dry seed; center seeds with tufts of white hairs, perimeter seeds without tufts |
| Leuzea repens | Russian knapweed | Grayish-white, deeply lobed near base; entire or serrate leaves in upper plant | Pink to purple, ½”; July – Sept. | Green at base, slightly hairy at tip, no fringes or spines | Ivory-colored seeds with tufts |