Montana Field Guide

PLANTS: Stout, tall, annual forb with erect spiny-winged stems, 3-120 cm tall. Wing spines to 10 mm long with teeth to 5 mm long. Sources: Gaskin et al. 2019; Oregon Department of Agriculture (ODA) 2020

LEAVES: Basal and cauline (stem). Basal leaves taper to winged petioles; blades to 10 cm long, margins pinnately 2-5 lobed, and tomentose on both surfaces. Stem leaves alternately arranged, shorter than basal leaves, and reduced upwards in size. Source: Gaskin et al. 2019

INFLORESCENCE: An array of racemes with erect, purple singular or clustered flower heads occurring at the ends or in axils of branches. Flower heads of involucral bracts, bristles, and disc florets (discoid) on the receptacle, 2 cm or less wide. Involucres cylindrical to ellipsoid, 12-20 mm long by 7-20 mm wide. Involucral Bracts are linear-lanceolate in shape; appressed with loosely tomentose bases, 2-3 mm wide; ascending, into linear appendages 0.5-1.0 mm wide; scarious-margined; distally scabrous on faces and margins; spine tips 1-2 mm long, inner, straight, erect, with armed or minutely armed tips. Disc Floret with stamens/pistil (perfect), 12-15 mm long; lobes about 2.5 times longer than throat. Petals purple. Pappus of bristles, 10-20 mm long. Source: Gaskin et al. 2019

TAXONOMY & NOMENCLATURE
Turkish Thistle was first discovered in North America in 2007 and initially identified as Italian Thistle Carduus pycnocephalus; however, additional locations revealed morphological differences (Gaskin et al. 2019). A collaborative study of entomologists and botanists led by John Gaskin examined the genetics and morphology of numerous thistle species. The study concluded that the plant represented Carduus cinereus, an introduced species, new to North America.

The taxonomy of Carduus cinereus is summarized by Gaskin et al. (2019). Carduus cinereus has been considered part of a species complex involving the closely related C. arabicus, C. cinereus, and C. pycnocephalus, and many intermediate forms. Gaskin et al. (2019) accepted the treatment by Tamamschian (1963), in which the three main taxa are presented as species: C. arabicus, C. cinereus, and C. pycnocephalus (Italian Thistle).

Carduus is an old name for ‘thistle’.

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.

IDENTIFYING CARDUUS SPECIES
The publication by Gaskin et al. (2019) provides a dichotomous key to differentiate Turkish Thistle (Carduus cinereus) from five other Carduus species that occur in the Pacific Northwest.

Turkish Thistle - Carduus cinereus, Not documented in MT, non-native, and undesirable
* Plants: Annual. Plants in Oregon are relatively short, blend in with the landscape, and decay quickly.
* Flower Heads: Involucres cylindric or narrowly ellipsoid, 2 cm or less wide. Usually, pedunculate and loosely clustered at ends of branches. Peduncles are short, if present and usually winged.
* Involucral Bracts: Bracts 0.5-2.0 mm wide, usually narrower than the appressed bases. More or less persistently tomentose, and scarious-margined, especially on upper margins.
* Disk flowers: purple

Spiny Plumeless Thistle - Carduus acanthoides, present, non-native, and undesirable
* Plants: Annual or biennial.
* Flower Heads: Involucres spheric or hemispheric, of less than 2 cm tall and less than 4 cm wide. Several per stem and sessile or short-pedunculate. Generally, erect with maturity.
* Involucral Bracts: Narrowly triangular - less than 2mm wide- with smooth margins and widest at base. Outer (lower) bracts spine-tipped and a short spine-tip.
* Disk flowers: white or purple.

Musk Thistle - Carduus nutans, present, non-native, and undesirable
* Plants: Usually biennial; occasionally annual.
* Flower Heads: Mostly solitary. Involucres of 2-4 cm tall and 4-8 cm wide. Nodding with maturity.
* Involucral Bracts: Broadly triangular – at least 2 mm wide - with smooth margins and wide at mid-length. Outer (lower) bracts spine-tipped.
* Disk flowers: purple.

Turkish Thistle has not yet been documented in Montana.

In Oregon, Turkish Thistle has colonized rugged canyon grasslands scattered with shrubby and forest areas located in a temperate continental climate (ODA 2020). Plants occur in plant communities dominated by native bunchgrasses, including Bluebunch Wheat grass (Elymus spicatus) and Idaho Fescue (Festuca idahoensis). Within these areas, plants grew in dry rock talus on southern aspects and also in deeper soils in more mesic areas.

PLANT-ANIMAL INTERACTSIONS
Turkish Thistle apparently is not grazed by native or domesticated ungulates (UDA 2020).

Plants reproduce by seed. The primary vectors of spread at the infestations in northeastern Oregon are wind, water, and transport by animals (ODA 2020). In Oregon, plants can flower and mature at very short heights, as short as 3 cm! Plants also exhibited short life cycles, and as they senesced the flower heads fell off.

FRUIT
Fruit is a cypsela or an achene. It is a dry, single-seeded fruit that formed from a double ovary. The cypsela are golden-brown to brown, 4-6 mm long, finely nerved (Gaskin et al. 2019).

The infestations in northeastern Oregon are being actively managed. The US Forest Service's noxious weed managers have conducted surveys, released biocontrol agents, and are treating outlying sites. Ultimately, eradication will require financial and staff resources with long-term monitoring. Readers wanting more information should reach out to ODA and the USFS in addition to ODA’s Pest Risk Assessment (2020).

Turkish Thistle apparently spreads easily and is very competitive in native rangeland (ODA 2020). The primary vectors of spread at the infestations in northeastern Oregon are wind, water, and transport by animals (ODA 2020). It is unknown how fire might interact with the species.

Turkish Thistle is likely to reduce forage productivity in rangelands if not controlled or eradicated. Turkish Thistle apparently is not grazed by native or domesticated ungulates (UDA 2020).