Flower:

A somewhat loose cluster of 10 to 20 pea-like flowers on a stalk arising from the upper leaf axils, the cluster usually slightly surpassing the length of the attending leaf. Flowers are about ½ inch long, white to creamy colored, the upper standard erect, the two laterals petals smaller and horizontal, a small keel between them. The calyx surrounding the base is green to pinkish, the lobes awl-like and about half as long as the tube, and all covered in blackish, appressed hairs. Flowers are stalkless or nearly so and droopy.

Leaves and stems:

Leaves are alternate, compound with 11 to 25 leaflets, up to about 4 inches long. Leaflets are ¼ to 1 inch long, oval-elliptic to oblong, the tips rounded usually with a small abrupt point (mucronate), roughly hairy on the underside and hairless on the upper surface.

Where the compound leaf meets the stem is a pair of leafy appendages (stipules), 1/8 inch long, triangular to nearly diamond-shaped with a sharply pointed tip and not joined together around the stem. Stems are multiple from the base, branched, angled to round in cross-section, usually hollow, smooth to sparsely hairy, and erect to spreading.

Fruit:

Fruit is an erect, smooth, oval, broadly inflated, 1-chambered pod ½ to ¾ inch long, with the remains of the style forming a “tail” at the top.

Seeds are kidney-shaped, 2 to 3 mm long and wide, spotted golden brown.

Notes:

Cooper's Milkvetch is rare throughout most of its range, which is centered around the Great Lakes. Prior to 1988, fewer than 20 locations were known in Minnesota, most of which were historical records from the 1890s and early 20th century and all but 2 of those populations no longer existed. It was listed as a Minnesota Special Concern species, but in the 1990s biological surveys discovered about 150 new locations, most in Mahnomen County, and it was subsequently de-listed. It is currently Endangered in Wisconsin. Its preferred habitat is the prairie-forest transition zone, which is a well-defined area in NW MN but more fragmented elsewhere, which would explain its abundance here and scarcity in other areas of its range.

Cooper's Milkvetch is very similar to the more common Canada Milkvetch (Astragalus canadensis), with which it may grow side-by-side. Canada Milkvetch flowers are more numerous, more tightly packed, creamy to greenish and mostly spreading not droopy, its stipules are connected together around the stem, fruit is a 2-chambered pod not much inflated, and is rhizomatous where Cooper's Milkvetch has a taproot. Also similar is Cream Milkvetch (Astragalus racemosus), a western species now considered historical in MN, which has more numerous flowers in long-stalked clusters that rise well above the attending leaf, has stipules joined around the stem, and the fruit is a drooping, elongated pod like a green bean.