Taprooted biennial. Stems erect, mostly unbranched, 15–80 cm. Herbage sparsely arachnoid-villous to arachnoid-tomentose, often purplish. Leaves petiolate below; blades linear to linear-oblanceolate, 10–40 cm long, shallowly to deeply pinnately lobed. Inflorescence capitate with ca. 5 to 20 subsessile heads subtended by leaf-like bracts mostly longer than the flowers; peduncles 0–1 cm long. Involucres ovoid to hemispheric, 15–30 mm high; phyllaries imbricate in 5 to 10 series, green, not keeled; outer lanceolate, glabrate or arachnoid on the margins with a spine 1–8 mm long; inner linear-lanceolate with a hispid-ciliate to scarious-erose tip. Disk corollas white to purple, 20–26 mm long. Achenes 4–7 mm long (
Lesica et al. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).
On first-glance thistles can look similar, but upon a closer inspection differences become apparent. Thistles belong to the genera of
Cirsium,
Carduus, and
Onopordum, which all have spiny-margined leaves and often have flower heads with spiny bracts. Ecologically, native and non-native thistles are very different.
NATIVE versus NON-NATIVE THISTLES [
Parkinson and Mangold 2015]
Native Thistles* Plants grow relatively sparsely and possess few or gentler spines, intermix with many plant species, and are slow to colonize disturbed ground.
* Flowers provide nectar and pollen for numerous native birds and insects, and forage for some wildlife. For example, elk eat the flowers of Elk Thistle.
* Involucral bracts tend to adhere to the flower head for most of their length (except for the spine).
* Plants are
not rhizomatous except for Flodman’s Thistle which can produce horizontal runner roots.
Non-native Thistles* Plants colonize disturbed ground quickly, often form dense patches, and produce nastier spines - limiting recreational activities, injuring people/animals, and reducing native plant species diversity.
* Flowers provide nectar and pollen for some birds and insects, but not forage for wildlife or livestock.
* Some species are aggressively rhizomatous and outcompete native plants that provide nutritional forage.
* Require management to control, reduce, or remove. Refer to the MANAGEMENT subsection.
DIFFERENTIATING THISTLE GENERACarduus* Stems: Winged.
* Pappus: Barbellate - minutely barbed, narrow bristles. Bristles usually fall separately.
* Flower Head - Receptacle: Not obviously fleshy or honeycombed. Densely bristly. In the flower head, look for bristles between the florets.
Cirsium* Stems: Winged or not winged.
* Pappus: Feathery (plumose) - fine, long hairs on each side of the central axis (rib).
* Flower Head - Receptacle: Densely bristly. In the flower head, look for bristles between the florets.
Onopordum* Stems: Spiny and winged along their entire length.
* Pappus: Barbellate - minutely barbed, narrow bristles. Bristles connected at base.
* Flower Head - Receptacle: Definitively fleshy and honeycombed. No or very sparse and short bristles. In the flower head, look between the florets to find nothing.