Taprooted biennial. Stems erect, often branched, 20–100 cm. Herbage arachnoid-tomentose throughout; stems and leaf undersides densely white-tomentose. Leaves petiolate; blade lanceolate to obovate, 5–20 cm long, undulate, deeply pinnately lobed to subentire. Inflorescence: heads mostly solitary at stem tips, forming open corymbiform arrays; peduncles 1–10 cm long. Involucres ovoid to hemispheric, 25–35 mm high; phyllaries imbricate in 8 to 12 series, green; outer lanceolate, sparsely arachnoid mainly on the margins with a darkened, resinous keel and spreading spine-tip 3–5 mm long; inner linear-lanceolate, acuminate. Disk corollas pink to purple, 25–40 mm long. Achenes 6–7 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 closer inspection differences become apparent.
Thistles belong to the genera of
Cirsium,
Carduus, and
Onopordum. They are separated by:
Cirsium* Feathery (plumose) pappus, which have fine, long hairs on each side of the main bristle.
* Receptacle of flower head has bristles. Look between florets within a flower head to find them.
Carduus* Capillary pappus, which are minutely barbed, narrow bristles.
* Receptacle of flower head has bristles. Look between florets within a flower head to find them.
Onopordum* Receptacle of flower head has no bristles. Look between florets within a flower head to find nothing.
* Entire lengths of stems have spiny wings.
* Foliage is silvery gray.
Native versus Exotic (Source: Parkinson and Mangold 2015)
* Native thistles tend to have involucral bracts adhere to the flower head for most of their length (except for the spine).
* Native thistles tend to grow scattered across a habitat, spreading slowly with disturbance, and contribute to plant diversity.
* Exotic thistles grow quickly with disturbance, form dense patches that interfere with access, and through competition often reduces plant diversity.
Montana has 12 species of Cirsium, and only 5 are described below.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.
* Most resembles Flodman's Thistle.
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].
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.
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.
Scotch Thistle –
Onopordum acanthium, exotic and undesirable
* Receptacle of flower head has no bristles.
* Entire lengths of stems have spiny wings, becoming broad and spiny.
* Foliage is silvery gray and can grow taller than 6 feet.
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.
