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Scotch Thistle - Onopordum acanthium
Other Names:  Scotch Cotton-thistle, Scotch Cottonthistle

Non-native Species

Global Rank: GNR
State Rank: SNA

Agency Status
USFWS:
USFS:
BLM:
MNPS Threat Rank:
C-value: 0

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General Description
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 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Diagnostic Characteristics
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.
* Montana has only 1 species.

Scotch ThistleOnopordum 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.

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 17 species of Cirsium, and only 5 are described below.

Bull ThistleCirsium 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.

Species Range
Unknown/Undetermined
 


Range Comments
Introduced to most of U.S.; native to Eurasia (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Observations in Montana Natural Heritage Program Database
Number of Observations: 256

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density

Recency

 

(Observations spanning multiple months or years are excluded from time charts)



Habitat

Management

References
  • Literature Cited AboveLegend:   View Online Publication
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
    • Parkinson, Hilary and Jane Mangold. 2015. Guide to Exotic Thistles of Montana and How to Differentiate from Native Thistles. EB0221. Montana State University Extension, Bozeman, Montana.
  • Web Search Engines for Articles on "Scotch Thistle"
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Citation for data on this website:
Scotch Thistle — Onopordum acanthium.  Montana Field Guide.  .  Retrieved on , from