The Thistles
List of Noxious Thistles and their Key Identifying Features
You can also check out the Thistles of Colorado Guide
| Latin Name | Common Name | Origin | Flower | Leaves | Stems | Height | Growth Form |
|---|---|---|---|---|---|---|---|
| Carduus nutans | Musk thistle | Nonnative, noxious | Purple; single very large nodding flower at end of stem; purple bracts | Deeply lobed, dark green with light green midrib; spiny margins | Spiny winged | 1-6 feet | Biennial |
| Onopordum acanthium | Scotch thistle | Nonnative, noxious | Purple; large upright flowerheads; bracts are spine-tipped | Toothed with slight lobe; attach directly to stem; hairy, giving grayish appearance to plant | Spiny winged, hairy | To 10 feet | Biennial |
| Cirsium arvense | Canada thistle | Nonnative, noxious | Light purple to white; multiple small flower heads in clusters on each stem | Whitish underside; spiny-tipped irregular lobes; attach directly to stem | Smooth with no spines; ridged | 1.5-4 feet | Perennial (from crown and rhizomes} |
| Cirsium vulgare | Bull thistle | Nonnative, noxious | Pink to purple; occur in clusters at ends of branches | Tops of leaves are hairy and prickly (unidirectional}; underside is cottony with prominently raised main veins | Spiny winged | 1-6 feet | Biennial |
| Carduus acanthoides | Plumeless thistle | Nonnative, noxious | Pink, red, or purple; single or in clusters of 2-5 at end of branch | Basal leaves have spiny lobes; stem leaves blend into stem; leaves with prominent midrib (like musk thistle) | Spiny winged; extend to flower head | To 6 ft | Biennal or annual |
