Acmispon americanus (American Bird's-foot Trefoil)
Also known as: | Prairie Trefoil, Spanish Clover, American Deerweed |
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Genus: | Acmispon |
Family: | Fabaceae (Pea) |
Life cycle: | annual |
Origin: | native |
Habitat: | sun; dry to moist sandy or rocky soil; prairies, shores, rock outcrops, along railroads |
Bloom season: | June - September |
Plant height: | 6 to 20 inches |
Wetland Indicator Status: | GP: FACU MW: FACU NCNE: FACU |
MN county distribution (click map to enlarge): | ![]() |
National distribution (click map to enlarge): | ![]() |
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Detailed Information
Flower:
Flowers are single in the leaf axils along branching stems, up to ~1/3 inch (5 to 8 mm) long, white or pinkish. The broad upper petal (the standard) is erect and rounded at the tip, the lateral wings are smaller and more oval, extend outward and flank the yellow-tipped keel below. The calyx cupping the flower has 5 narrow lobes that may be longer than the flower; a leaf-like bract sits at its base. Flower stalks are longer or shorter than the subtending leaf. The calyx, bract and stalk are all covered in long spreading hairs.
Leaves and stem:
Leaves are alternate, compound with 3 leaflets. Leaflets are up to ~1 inch (1 to 3 cm) long and to 3/8 inch (3 to 10 mm) wide, widest above, below or at the middle, toothless, hairy, pointed at the tip, tapering or rounded at the base, stalkless or nearly so.
At the base of the compound leaf is a pair of minute, gland-like stipules, commonly reddish brown. Stems are erect to spreading, freely branching, covered in long spreading hairs.
Fruit: 
Fruit is a slender, straight, bean-like pod, about 1 inch (15 to 35 mm) long and containing up to 8 seeds.
Notes:
American Bird's-foot Trefoil, formerly Lotus purshianus (or Lotus unifoliolatus), is uncommon in Minnesota, found primarily in dry sandy prairies, rock outcrops, and along railroads, occasionally shores or wetter areas. It is a relatively short plant, rarely getting knee-high, and the many branches can give it a bushy appearance. It somewhat resembles the invasive Bird's-foot Trefoil (Lotus corniculatus), which has small clusters of yellow flowers, lacks the long, spreading hairs, and has large leaf-like stipules at the base of the leaf stalk.
There are 2 vars of Acmispon americanus, though some references consider them separate species; var. helleri, restricted to the Piedmont region of the southeastern US, is hairless or short-hairy and its leaflets are proportionately narrower (4 to 5x long as wide); var. americanus, present mostly west of the Mississippi River (considered introduced farther east), is long-hairy with broader leaflets (3 to 3.5x long as wide).
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More photos
Photos by K. Chayka taken in Ramsey County. Photos by Peter M. Dziuk taken in Chippewa County.
Comments
Have you seen this plant in Minnesota, or have any other comments about it?
on: 2015-07-01 21:47:17
Just found this one last week while working out at Pipestone National Monument. Lovely little surprise - at first I thought it might be a diminutive tick trefoil (Desmodium). If this page didn't have it, I was about to pour through the Great Plains flora to look for it. Thanks for the ID!
on: 2022-07-29 16:36:03
Growing along periphery of UTV trails paralleling railway. apparently not supposed to be in this county. Many small plants, none over 8 inches. The tiny, singular white flowers lead me to believe this is the right ID.
on: 2022-07-29 16:51:38
Gabriel, seeing it along trail edges leads me to believe it was brought in on a vehicle's tires.