Acmispon americanus (Nutt.) Rydb.

Prairie Trefoil


CC = 6
CW = 3
MOC = 14

© DETenaglia

Family - Fabaceae/Faboideae

Habit - Taprooted annual forb.

Stems - Ascending to erect, often branched and bushy, pubescent with fine spreading to ascending hairs.

Acmispon_americanus_stem.jpg Stem.

© DETenaglia

Leaves - Alternate, pinnately trifoliate or the upper leaves simple, petiolate, the petiole 0.6-2.0 mm long. Stipules minute, sessile, circular to bean-shaped glands. Leaflets 6-20 mm long, 2.5-5.0 mm wide, elliptic to lanceolate, angled to a short-stalked base, sharply pointed, the margins entire, finely hairy, the surfaces moderately to densely and finely hairy. Terminal leaflet with the stalk 1-4 mm long, symmetric at the base; lateral leaflets with the stalk 0.2-1.5 mm long, often asymmetric at the base.


© DETenaglia

Inflorescences - Axillary, of solitary flowers in most or only the upper axils, the stalks 5-20 mm long, hairy, with a simple, leaflike bract and a pair of minute glands at the tip, the flower with a stalk 0.2-1.0 mm long above the bract.

Acmispon_americanus_inflorescence.jpg Inflorescence.

© DETenaglia

Flowers - Calyces finely hairy, the tube 1.5-2.0 mm long, conic, the lobes 4-6 mm long. Corollas papilionaceous, white to pale pink, often with a dense pattern of darker pink nerves, the keel often yellowish toward the tip, the banner 5-7 mm long, 3.5-5.0 mm wide, the wings 4-6 mm long, 2-3 mm wide, obovate, the keel 4-6 mm long, 1.5-2.0 mm wide, boat-shaped, fused nearly to the tip. Stamens 10, all of similar length, 9 of the filaments fused and 1 free, the fused portion 3.5-4.0 mm long, the free portion 1.0-1.5 mm long. Ovary 2-3 mm long, the style 1-2 mm long, slender, slightly curved, the stigma small and terminal.

Acmispon_americanus_flower.jpg Flower.

© DETenaglia

Acmispon_americanus_calyx.jpg Calyx.

© DETenaglia

Fruits - Legumes to 4 cm long, 2-3 mm wide, linear, beaked, terete, dehiscing by 2 valves, these green at maturity, twisting spirally after dehiscence, mostly 5- or 6-seeded. Seeds 2.5-3.2 mm long, 1.5-2.0 mm wide, more or less kidney-shaped, the surface olive green to reddish brown, mottled with darker brown or black, smooth, somewhat shiny.

Acmispon_americanus_fruit.jpg Developing fruit.

© DETenaglia

Flowering - May - September.

Habitat - Prairies, glades, limestone slopes, bluff-tops, roadsides, railroads.

Origin - Native to U.S.

Other info. - This species can be found in a handful of Missouri counties and is most common in the prairie areas of the southwestern portion of the state. The plant can be identified by its hairy, much-branched stems, trifoliolate leaves, and small pinkish flowers. The flowers are whitish when they first open and then fade to a dark pink by late afternoon. Each flower only lasts one day and then quickly sets a fruit.

Photographs taken at Homestead National Monument, Beatrice, NE., 8-14-06.