Montana Field Guide

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 GENERA
Carduus
* 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.

Pastures, fields, roadsides; valleys (Lesica et al. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

POLLINATORS
The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus borealis, Bombus fervidus, Bombus bimaculatus, Bombus griseocollis, and Bombus impatiens (Colla and Dumesh 2010).

Useful Links:
Montana Invasive Species website
Montana Biological Weed Control Coordination Project
Montana Department of Agriculture - Noxious Weeds
Montana Weed Control Association
Montana Weed Control Association Contacts Webpage.
Montana Fish, Wildlife, and Parks - Noxious Weeds
Montana State University Integrated Pest Management Extension
Weed Publications at Montana State University Extension - MontGuides