Montana Field Guide

PLANTS: Shrubs that grow to 100 cm tall. Stems are spiny, tomentose, and highly branched. Source: Lesica et al. 2012

LEAVES: Alternately arranged on stems. Primary leaves are converted to spines about 1 cm long. Secondary leaves grow in fascicles [born on a short shoot in the axil of the primary leaf]. Fascicled leaf blades are simple with smooth (entire) margins, linear in shape, lack hairs (glabrous), and 7–15 mm long. Source: Lesica et al. 2012

INFLORESCENCE: Yellow flowers occur in heads which grow from leaf axils. Involucre is cylindric to obconic, 8–12 mm high. Involucral bracts are 4 to 6 in number and tomentose. Heads consist of 4-6 disk florets (discoid) that are prefect (both sexes). Disk florets have a tube longer than the throat, petals of 7–12 mm long, pistil with a flattened, branched style, and a pappus of white capillary bristles. Fruit is an achene. Source: Lesica et al. 2012

Shrubs flower from May to June (Strother 1974).

Montana has 2 Tetradymia species:

Short-spine Horsebrush - Tetradymia spinosa, SOC
*Stems: Spiny.
*Flower Heads: Grow from leaf axils.

Gray Horsebrush - Tetradymia canescens
*Stems: Unarmed.
*Flower Heads: Grow in clusters (corymbiform) at tips of stems.

Possible look-alikes include yellow-flowered shrubs in the Sunflower (Asteraceae) Family:

Tetradymia - Horsebrush
*Flower Heads: Ray florets are absent and pappus is of capillary bristles.
*Twigs: White-tomentose (hairy) with or without spines andUpper Leaves are fascicled.

Artemisia - Sagebrush
*Flower Heads: Arranged in a raceme or panicle, involucral bracts in 4-7 unequal rows, and pappus is absent.
*Leaves: Usually lobed, but some species with simple leaves.

Ericameria - Goldenbrush / Rabbitbrush
*Flower Heads: a) Ray florets are absent and pappus is of capillary bristles, or b) Some ray florets are present.
*Twigs: a) White-tomentose (hairy) without spines and upper Llaves are mostly not fascicled, or b) glabrous and greenish.

Chrysothamnus - Rabbitbrush
*Flower Heads: Ray florets are absent and pappus is of capillary bristles.
*Twigs: Glabrate to puberulent and leaves are resin-dotted.

Gutierrezia sarothrae - Broom Snakeweed
*Flower Heads: Arranged in flat-topped clusters (corymbiform), ray florets present, and pappus is of jagged (erose) scales.
*Plants are short, often less than 30 cm tall.

Lorandersonia linifolia - Spearleaf Rabbitbrush, PSOC
*Flower Heads: Ray florets are absent and pappus is of capillary bristles.
*Twigs: Mostly without hairs (glabrous), has a whitish epidermis, and leaves lack glands.

Sagebrush steppe in the plains and valley zones of Montana (Lesica et al. 2012).
Across its range, shrubs grow in very sand soils between 850 and 2,400 meters in Alkali Sink, Shadscale Shrub, or rarely in Pinyon-Juniper Woodland habitats (Strother 1974).

Horsebrush species are common, but seldom abundant in the arid landscapes of western North America (Strother 1974). Unlike certain Sagebrush (Artemisia species, Horsebrush species don't form dense, impenetrable stands. Often colonies consist of no more than a few hundred individuals, and commonly can be found in loose groups of 25 to 50 shrubs. Further local colonies tend to exhibit uniform morphology, probably because they represent the local breeding population.

PLANT PHYSIOLOGY
All Horsebrush species have primary and secondary leaves. Primary leaves grow on elongated shoots where as, secondary leaves are borne in fascicles that grow from the axils of primary leaves (Strother 1974). In Short-spine Horsebrush the primary leaves are spines, but in other Horsebrushes they can take on other anatomical forms. Primary leaves are relatively long-lived, up to a few months or up to several years for the spined species, like Short-spine Horsebrush. In the armed Horsebrush species, the transformation of primary leaves into spines is caused by growth of fibers that continue into the decurrent bases of the spines, but was not present with axial fiber tissues. Secondary leaves are short-lived, tending to dry-up and whither within a few weeks of development. The longevity of secondary leaves is correlated to the dryness of the habitat and relative specialization of the plants that bear them.

Shrubs reproduce by seed.

FRUITS
Achene: Obconic in shape, 5-ribbed, 6–8 mm long, and densely minutely-hairy (puberulent).

FLOWERING
For Horsebrush species the onset of flowering correlates with the temporal distribution and amount of rainfall (Strother 1974). Within an individual Horsebrush shrub, flowering is highly synchronized (within 2 to 4 days) (Strother 1974). This level of synchronization is characteristic of other xerophytes within the Sunflower (Asteraceae) family. It is a strategy that allows the plant to reproduce quickly when the proper amounts of rainfall arrive. This synchrony is actually exhibited by the entire local colony of a given species.

POLLINATORS
Horsebrush shrubs are visited by a variety of pollinators, including moths, bees, flies, and beetles (Strother 1974). Fertility by pollination is generally high (Strother 1974).

GERMINATION and ESTABLISHMENT
In Strother's studies on Horsebrush shrubs, he found fertility to be high and pollinators to be abundant, yet seedlings were often not observed (Strother 1974). Strother surmised that this resulted in a low turn-over rate within colonies which contributed to the lack of variation found in individuals of the same colony. In extremely dry years, Strother observed that individuals and even whole colonies might not flower at all and others that flower do not produce fruit, and reasoned that the cause was from a lack of water. Further the lack of fruit production may be more limiting for the species than the rate of germination or establishment. In Montana, conditions that limit germination, establishment, and fruiting intrinsically reduce the plant's viability. Consequentially this can lead to small population sizes and low genetic diversity. Colonies may be highly sensitive to local selection pressures.

STATE THREAT SCORE REASON
Reported threats to Montana's populations of Short-spine Horsebrush are currently assigned as unknown. The thick thatch developing from Cheatgrass (Bromus tectorum), Field Fluffweed (Filago arvensis), and Common Mullein (Verbascum thapsus) at one site is speculated as making it difficult for new Short-spine Horsebrush plants to establish (MTNHP Threat Assessment 2021). General droughty conditions are a potential threat to plants at all locations(MTNHP Threat Assessment 2021). Information is needed to quantify the threat type(s), severity, scope and timing.