Monocarpic biennials or short-lived perennials. Stems erect, branched, 50–200 cm with broad, spiny wings . Herbage glabrate to tomentose, spiny on stems and leaf margins. Leaves clasping, lanceolate, 10–60 cm long, deeply dentate to shallowly pinnately divided into deltoid lobes. Inflorescence 1 to 3 heads at branch tips. Heads discoid; involucres ovoid, 20–35 mm high; phyllaries numerous in 8 to 10 series; the outer spine-tipped, glabrate to tomentose; receptacle flat, fleshy, pitted, naked. Disk flowers perfect, usually purple; corollas slender, 22–25 mm long; style branches fused. Pappus of pinkish, capillary bristles. Achenes obovoid, ribbed, 4–5 mm long, scabrous (
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.
Montana has 15 species of thistles, and only 5 are described below.Bull Thistle –
Cirsium vulgare, exotic and undesirable
* Flower heads are mostly single at stem tips and arranged in an open inflorescence.
* Flower heads have involucres more than 2 cm tall [examine larger heads].
* On the flower head the outer bracts tend to point outwards and upwards, are needle-like and long.
* Leaves are deeply lobed, green beneath with cobwebby hairs and obvious white veins.
* Leaves have many sharp, short spines. Entire plant has spines, some very long, making it difficult to touch without injury.
* Plants are taprooted.
Canada Thistle -
Cirsium arvense, exotic and Noxious
* Flower heads have involucres less than 2 cm tall [examine larger heads].
* Each flower head consists of either male florets or female florets.
* Leaves are arachnoid-villous, but the green leaf remains visible.
* Stems lack an obvious winged stem.
* Plants are strongly rhizomatous.
Wavyleaf Thistle -
Cirsium undulatum, native and desirable
* Upper leaf surface lacks spines AND white-tomentose hairs making it appear gray.
* Involucral bracts tend to point upwards with inner bracts acuminate.
* Flower heads have involucres more than 2 cm tall [examine larger heads].
* Most flower heads not clustered and peduncles more than 2 cm long.
Flodman’s Thistle -
Cirsium flodmanii, native and desirable
* Upper leaf surface lacks spines AND has
sparse white-tomentose hairs making it appear green.
* Involucral bracts tend to point upwards with inner bracts acuminate.
* Flower heads have involucres more than 2 cm tall [examine larger heads].
* Most flower heads are not clustered and some peduncles are more than 2 cm long.
Long-styled Thistle -
Cirsium longistylum, native, Montana endemic, and SOC
* Upper leaf surface lacks spines.
* Inner & outer bracts are wide, scarious, and with erose tips AND outer bracts have a raised, darkened, and resinous keel.
* Flower heads have involucres more than 2 cm tall [examine larger heads].
Musk Thistle -
Carduus nutans, exotic and undesirable
* Flower heads have involucral bracts that are broadly triangular, have smooth margins, and a short spine-tip.
* Heads nod as flowers mature.