Long-styled Thistle has simple or branched stems that are 50-60 cm tall and up to 15 mm thick at the base; plants are perennial, producing daughter rosettes that live for two more years. The basal leaves are shallowly lobed, moderately spiny, green and glabrous above, and densely white-hairy below; the lower leaves are narrowly lance-shaped, up to 15 cm long, and lobed to 1/3 or less the leaf width. The smaller upper leaves are more ovate in outline with mostly entire margins and numerous marginal spines; the herbage is covered with long, tangled, white hairs. The inflorescence usually consists of a terminal cluster of 2 to several flower heads and a number of side branches which reach up to 15 cm long and have fewer heads; each flower head is ca. 30 mm high, 25 mm wide, and subtended by a few reduced leaves. The involucral bracts occur in two main series: the outer are narrowly lance-shaped with a yellowish, dilated, and fringed apex tipped by a slender spine; the inner bracts are more lance-shaped and longer. The white disk flowers are 20-22 mm long; ray flowers are absent. There are numerous tawny bristles which form a pappus at the tip of the achene.
Flowering in late June-August.
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, forming dense patches that interfere with access, and through competition often reduces plant diversity.
Montana has 12 species of Cirsium, and only 6 are described below.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].
* Resembles Hooker Thistle the most.
Hooker Thistle -
Cirsium hookerianum, native and desirable
* Heads mostly clustered.
* Flower head stems (peduncles) are more than 2cm long.
* Flower heads have involucres of 2-3 cm tall [examine larger heads].
* Involucral bracts not erose, dilated, and scarious, but do have cobwebby hairs.
* Upper leaves mostly do NOT enclose the flower heads.
* Leaves are densely tomentose beneath and succulent.
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.
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.
