Laxmann's Milkvetch

Astragalus laxmannii var. robustior

Laxmann's Milkvetch (Astragalus laxmannii var. robustior), often called Prairie or Standing Milkvetch, is a native perennial of the pea family that grows in upright, leafy clumps. Mostly in June and July it carries dense, upright spikes of small lavender to blue flowers on stalks that rise from the leaf joints, on dry, open grassland, meadows and rocky slopes across the Black Hills.

Free photos from other sources

Laxmann's Milkvetch: Upright head of lavender pea flowers, close up — Prairie Milkvetch (Astragalus laxmannii var. robustior). Ryan Donnelly, CC BY 4.0, resized. Get it at the source

Laxmann's Milkvetch: Clump with many purple flower spikes among leafy stems — Laxmann's Milkvetch (Astragalus laxmannii). Jo Roberts, CC BY 4.0, resized. Get it at the source

Laxmann's Milkvetch: Leafy clump with flower spikes growing among pale limestone rocks — Prairie Milkvetch (Astragalus laxmannii var. robustior). Matt Berger, CC BY 4.0, resized. Get it at the source

Laxmann's Milkvetch: Two lavender flower heads on stalks rising from the leafy stems — Laxmann's Milkvetch (Astragalus laxmannii). marek, CC0 1.0. Get it at the source

Laxmann's Milkvetch: Leaf of narrow, gray-green leaflets in pairs — Prairie Milkvetch (Astragalus laxmannii var. robustior). Ryan Donnelly, CC BY 4.0, resized. Get it at the source

Laxmann's Milkvetch: Spent flower spikes turned pale tan above the leaves — Laxmann's Milkvetch (Astragalus laxmannii). Jo Roberts, CC BY 4.0, resized. Get it at the source

Quick facts
Family	Fabaceae · Pea
Status	Native perennial
Blooms	Late May into August, mostly June and July
Height	10–40 cm (4–16 in)
Where	Dry, open grassland, mountain meadows, rocky and limestone slopes, roadsides and open pine
Also called	Prairie Milkvetch, Standing Milkvetch, Lavender Milkvetch
Synonym	Astragalus adsurgens var. robustior, Astragalus adsurgens subsp. robustior, Astragalus laxmannii subsp. robustior, Astragalus striatus, Astragalus sulphurescens, Astragalus nitidus

Roots, seeds & germination

Root: A tough, woody taproot topped by a knobby, branching crown that sends up many stems each spring, so the plant forms a dense clump up to about 60 cm (2 ft) across. It has no runners. Like other legumes it carries nitrogen-fixing bacteria in root nodules.

Seed: Upright pods 6–12 mm (¼–½ in) long, three-sided, covered in flat-lying hairs and tipped with a short beak, pack the old flower spike and split along their seams when ripe. The seeds are 1.5–2.6 mm long, smooth and dark brown to nearly black.

Germination: The seed must be scarified, for example by rubbing it between sheets of sandpaper to scratch its surface, and then sown in fall so winter's cold, moist conditions bring it up in spring. At planting, the seed is coated with an inoculant of root-nodule bacteria made for its genus.

How do you identify Laxmann's Milkvetch?

Look for a dense clump of leafy stems from one crown, with 15–50 small pea flowers crowded into an upright, round to cylinder-shaped spike, all pointing upward, on a stout, ribbed stalk that rises from a leaf joint and is usually as long as the leaf or longer. The flowers are lavender to blue, sometimes pink or nearly white, and the cup behind each one has a salt-and-pepper mix of short black and white hairs. Field Milkvetch has similar flowers but grows as scattered stems from underground runners in moister ground, and its pods are shorter and broader, with long white hairs. Lambert's Crazyweed has leaves only at the base, a leafless flower stalk and a keel that ends in a sharp point.

When does it bloom?

Late May into August, mostly June and July.

Where does it grow?

Dry to moderately moist, open, sunny ground: mixed-grass prairie, the high meadows of the central Hills, rocky and limestone slopes, roadsides, burns and openings in ponderosa pine, from about 910 to 1,950 m (3,000–6,400 ft). The 1986 Jewel Cave survey lists it as Astragalus adsurgens var. robustior, and National Park Service lists record it at Wind Cave, Jewel Cave, Mount Rushmore and Devils Tower. Herbarium specimens come from all seven Black Hills counties, as far back as 1892.