Montana Field Guide

Slender Thelypody is an herbaceous biennial or short-lived perennial with solitary, simple or branched stems that are 3-8 dm high and arising from a taproot. The lower leaves are 6-20 cm long and have petioles and broadly lance-shaped, entire-margined blades. The upper leaves are smaller, narrowly arrow-shaped, and they lack petioles. Foliage is glabrous or nearly so and has a thin, waxy coating. Flowers are densely clustered on ascending stalks in cylindric inflorescences that expand greatly when in fruit. Each flower has 4 separate sepals that are 3-7 mm long, 4 separate, lavendar petals that are 7-14 mm long and 1-3 mm wide, and 4 long and 2 short stamens. The ascending, straight, cylindric siliques are 18-53 mm long and less than 1 mm wide.

Flowering and fruiting from the end of May to mid-July.

Differentiating Montana's three Thelypodium species requires a technical key, such as those by AL-Shehbaz in FNA 2010, Lesica et al (2012), and Giblin et al. [eds.] 2018. Other genera can also appear similar. Thelypodium species have cylindrical siliques while species of Arabis have flattened siliques.

Slender Thelypody - Thelypodium sagittatum subspecies sagittatum, SOC
* Stem Leaf: Auriculate-clasping, mostly sessile.
* Inflorescence: Terminal racemes do not considerably elongate in fruit (opened flowers crowded at tip).
* Flowers: Petals light to deep violet-purple, 1-3 mm wide. Anthers 2-3 mm long.
* Fruit: Siliques often straight, 0.5-1.2 mm wide.
* Seeds: Flat.
* Habitat: Moist, often alkaline meadows that dry by summer, desert plains to lower mountains. In the montane zone of Montana.

Northwestern Thelypody - Thelypodium paniculatum, SOC
* Stem Leaf: Auriculate-clasping, mostly sessile.
* Inflorescence: Terminal racemes do not considerably elongate in fruit (opened flowers crowded at tip).
* Flower: Petals lavender to purple, 2.5-5 mm wide. Anthers 1-2 mm long.
* Fruit: Siliques often straight, 1.3-2.3 mm wide.
* Seed: Plump.
* Habitat: Wet meadows and streambanks. In the montane zone of Montana.

Entire-leaved Thelypody - Thelypodium integrifolium subspecies integrifolium
* Stem Leaf: Bases are cuneate to attenuate, not auriculate, sagittate, or auriculate clasping; sessile.
* Flower: Petals white to purple, 4-7 mm wide
* Fruit: Siliques ascend, 1-3 cm by 0.7-1.1 mm long with a 0.3-2.0 mm long stipe.
* Habitat: Moist to wet, alkaline or saline meadows in the plains, valleys, and montane zones of Montana.

Moist, alkaline meadows, often with greasewood or shrubby cinquefoil, in the valley to montane zones. Thelypodium sagittatum, grows in alkaline meadows that are often dry, but may be wet in the early part of the season (Al-Shehbaz 1973).

STATE THREAT SCORE REASON
Reported threats to Montana's populations of Slender Thelypody are currently assigned as unknown. Potential threats due to livestock trampling and grazing are negligible according to reported data (MTNHP Threat Assessment 2021).