Montana Field Guide

Taprooted perennial with 1-few erect or ascending stems that are arachnoid-tomentose or sometimes glabrate. Flowers are pink to purple, 18-25 mm long, tubes 7-9 mm, throats 5.5-7.5 mm, lobes 4-8 mm; style tips 3-5.5 mm. The 1 to several heads are borne singly or in 2-3-headed clusters at the tips of the main stem and branches in an open, congested, or flat-topped fashion, heads sometimes also in present in distal axils. Peduncles 0-15 cm. Leaf blades are linear to oblong, oblanceolate, or elliptic, 5-25 × 0.6-7 cm, unlobed and merely spinulose or spiny-dentate to regularly pinnatifid, typically with 5-8 pairs of lobes, well separated, and usually with broad, U-shaped sinuses to crowded, linear to triangular-ovate, ascending-spreading to retrorse, merely spinulose to coarsely dentate or few lobed, primary spines are slender, 2-7 mm long. Lower leaf surfaces are usually densely arachnoid-tomentose, while upper surfaces are typically green, glabrous or less commonly thinly to densely gray-tomentose; basal leaves often present at flowering, spiny winged-petiolate; principal cauline well distributed along stem, gradually reduced upwards, lower leaves usually winged-petiolate, while mid and upper leaves typically are sessile, leaves decurrent along stem for 1.5-3.5 cm.
Involucres are ovoid to campanulate, 1.8-2.7 × 1-2 cm, thinly arachnoid-tomentose or glabrate. Bracts are in 6-7 series, ± imbricate, green or with a dark subapical patch or appendage, linear to linear-lanceolate, margins entire, abaxial faces with narrow glutinous ridge; outer and middle bases appressed, apical appendages spreading to stiffly ascending, linear-lanceolate to acicular, entire, spines spreading or ascending, stout, 2-7 mm, often flattened; apices of inner stiffly erect or sometimes flexuous, narrow, flat. Pappus is 14-16 mm long. (adapted from FNA treatment vol 19).

Flowering in June-July.

Similar species include Cirsium canescens and C. flodmanii both of which occur in eastern Montana. The former can usually be distinguished by having the upper leaf surfaces being slightly tomentose and by the leaf lobes being over 3.5 times as long as wide. The latter species usually has stem leaves that only clasp the stem or are short-decurrent compared to the typically, long-decurrent leaves (middle leaves with wings usually >15 mm long) of C. pulcherrimum. A technical manual should be consulted for positive identification.

Sparsely-vegetated soils of washes and steep, eroded gullies in dissected or badlands topography. Associated species include Juniperus scopulorum, Chrysothamnus nauseosus, Artemisia ludoviciana, Atriplex confertifolia and Elymus trachycaulis. In other areas, this species is listed as typically growing in rocky or stony soils. However, in our area the species has also been noted from fine-textured soils.

The effects, if any of the introduced Rhinocyllus weavil on this species are unknown. Additional information on this species' ecology and management in Montana are lacking at this time.

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 appositus, Bombus auricomus, Bombus bifarius, Bombus borealis, Bombus centralis, Bombus fervidus, Bombus flavifrons, Bombus frigidus, Bombus huntii, Bombus mixtus, Bombus nevadensis, Bombus rufocinctus, Bombus sylvicola, Bombus ternarius, Bombus terricola, Bombus sitkensis, Bombus occidentalis, Bombus pensylvanicus, Bombus bimaculatus, Bombus griseocollis, Bombus impatiens, Bombus insularis, Bombus suckleyi, Bombus bohemicus, and Bombus flavidus (Thorp et al. 1983, Mayer et al. 2000, Wilson et al. 2010, Colla and Dumesh 2010, Colla et al. 2011, Koch et al. 2012, Williams et al. 2014, Tripoldi and Szalanski 2015).

STATE THREAT SCORE REASON
Reported threats to Montana's populations of Wyoming Thistle indicate impacts at one population due to chemical herbicide application (MTNHP Threat Assessment 2021). Wyoming Thistle has a stature similar to non-native thistle species, and it is occasionally targeted by chemical weed control applicators in error.