Montana Field Guide

Taprooted perennial with a brown-wooly branched crown. Stems erect, spine-tipped, 15–40 cm, branched. Herbage glabrate to weakly tomentose with milky sap. Leaves alternate, cauline, linear, entire, 5–30 mm long, inconspicuous. Inflorescence heads solitary on upper spine-like branches. Heads ligulate; involucre cylindric, 7–13 mm high; phyllaries linear-oblanceolate in 2 series, the outer short; receptacle flat, naked. Ray flowers 3 to 5, perfect, pinkish. Pappus 2 series of minutely barbed bristle in 2 series, tan. Achenes tubular, 6–8 mm long, 5-ribbed, glabrous (Lesica et al. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Flowering occurs in July - August.

These plants can be confused by their common name, scientific name, and/or initial appearance because at some point they can exhibit green or brown stems that are leafless or inconspicuously leafy.
Here is how to differentiate them:

Spiny Skeletonweed - Pleiacanthus spinosus, native, SOC:
* In Aster Family; pinkish flowerheads occur solitary on upper branches. About 3 florets per flowerhead.
* Achenes are not beaked, but are tubular.
* Plants have a milky sap.
* Plants have hairs, being glabrate to weakly tomentose.
* Plants are taprooted with a brown-woolly branched crown.
* Stems are spine-tipped and branched.

Rush skeletonweed - Chondrilla juncea, exotic and Noxious:
* In Aster Family; yellow flowerheads occur at the tips of stems or in the axils of leaves. About 7-15 florets per flowerhead.
* Achenes (fruits) have a beak of 5-6mm long.
* Stems, leaves, and inflorescence have milky sap.
* Plants have hairs being hirsute to glabrate. On the lower 10-15 cm (4-6 inches) of the stem the hairs point downward while the upper stem is lacks hairs or has a few rigid hairs.
* Plants are taprooted perennial forbs.

Rush Skeleton-plant - Lygodesmia juncea, native:
* In Aster Family; pinkish flowerheads occur at the tips of stems. About 5 florets per flowerhead.
* Achenes are not beaked, but sometimes are fusiform.
* Plants lack milky sap.
* Plants are without hairs (glabrous).
* Plants are rhizomatous perennial forbs.

Tall Tumble-mustard - Sisymbrium altissimum, exotic:
* In Mustard Family; flowers with 4 (pale) yellow petals and 4 green to yellowish sepals.
* Stems are leafy with large pinnately lobed leaves.
* In fruit or during the winter stems appear leafless. Plants branch, becoming round.
* Fruit is a long and narrow silique that is equal in width to its stem (pedicel).

In Montana, spiny skeletonweed is known from Madison County, where it occupies arid grasslands on stony loam at lower elevations (5000-6400 feet). Agropyron spicatum typically dominates, however Stipa comata, Festuca idahoensis, Bouteloua gracilis, and ground cover of Selaginella densa are locally co-dominant or abundant. Associates include Antennaria rosea, Artemisia frigida, Astragalus adsurgens, Astragalus terminalis, Heterotheca villosa, Gutierrezia sarothrae, Koeleria macrantha, Musineon divaricatum, Oxytropis sericea, Phlox hoodii, Poa secunda, and Senecio canus.

Spiny skeletonweed reproduces strictly by seed. Its paucity of leaves and its occurrence in sparsely vegetated, semi-arid grassland or shrubland suggest that this species is adapted to full sun and may not compete well with other plants for light. The spine-tipped branches undoubtedly deter grazing.

Field observations have yielded no indication as to whether this species is affected by livestock grazing. However, grazing would likely favor this plant because it is probably unpalatable and would benefit from a reduction in competing palatable grasses.

STATE THREAT SCORE REASON
Reported threats to Montana's populations of Spiny Skeletonweed (Pleiacanthus spinosus) are currently assigned as unknown. Potential threats due to residential development, non-native species, and herbicide application need to be substantiated before a threat rank can be assigned (MTNHP Threat Assessment 2021).