Bird's-foot Trefoil is an introduced invasive perennial legume growing 6 to 24 inches high on stems that may be erect to drooping. Stems are smooth with some branching, usually hairy. Many may grow from the same root.
The leaves are alternate, 5-parted with 3 larger sections at the top of the leaf and two smaller leaflets separated from the others and placed on the leaf stalk near where it meets the stem making these look like large stipules. The upper leaflets are oblanceolate (more pointed at the base) while the lower leaflets are more triangular shaped. All have smooth edges, but with edge and fine surface hair.
The inflorescence is a rounded umbel of 4 to 8 stalked flowers that rises from the upper leaf axils. From above the arrangement resembles a whorl.
The flowers have a hairy tubular green calyx with 5 long pointed teeth. The yellow flower petals are arranged in pea fashion with a standard (banner) above, two lateral petals and 2 projecting forward forming a keel. The keel houses the stamens and pistil. Flower petals may show some brick-red nectar guide marks in bright sun. Flowers are about 1/2 inch long. Insects must penetrate the keel petals to pollinate the flower.
Seed: Mature flowers produce brown linear seed pods that project outward from the flower cluster and look like bird's feet. The pods are about 1 inch long and split in two when ripe with a twisting motion that ejects the seeds. Each pod half contains a number of shiny light brown kidney shaped seeds, about 2 mm long, each in its own compartment.
Habitat: Bird's-foot Trefoil grows from a branching taproot, with laterals near the surface. When not grown in cultivation (see below) it is found in disturbed sites, path edges and uncut areas where there is mostly full sun and mesic to dry conditions. It spreads via the seeds.
Names: The genus Lotus is from the classical Greek and is applied to a large number of species that have ' trefoil' as part of a their name. The species corniculatus is from the Latin meaning 'with small horns' and probably refers to the slightly curved beak on the end of the seed pods. The author name for the plant classification - 'L.' - is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: While the flower resembles others in the pea family, the yellow petals may remind one most closely of Butter & Eggs, Linaria vulgaris Mill., but the leaves there are much different. Prairie Trefoil, Lotus unifoliolatus, resembles L. corniculatus, but the flowers there are single and the leaf is only 3-part. Finally, the whorl of flowers is similar to Crown Vetch, Coronilla varia, but there the flowers are pinkish.
Above: The inflorescence is a rounded umbel of 4 to 8 stalked flowers that rises from the upper leaf axils. Illustration by Dr. Otto Thomé, courtesy of Kurt Stüber's Online Library.
Below: 1st photo - Flowers in bright sun frequently show these red vein lines. 2nd photo - From above the arrangement of flowers in the inflorescence resembles a whorl.
Below: 1st photo - The green hairy calyx has 5 long pointed teeth. 2nd photo - When the two keel petals are removed the stamens and styles and the pistil can be seen. Insects must penetrate those petals to pollinate the flower.
Below: 1st photo - Looking like a bird's foot, are the fully developed upper lobes of the leaf, while the two base lobes (2nd photo) are triangular in shape.
Below: New leaf just forming - The leaf has five lobes, the upper three, widest near the top and separated from the two lower lobes which are right at the base of the leaf stalk where it meets the stem
Below: The long seed pods spread out from the stem like a bird's foot. They split into two halves when mature.
Below: The shiny brown kidney shaped seeds, are lined up in separate compartments in each half of the seed pod and are ejected when the pod splits open with a twisting motion.
Notes: Bird's-foot trefoil has naturalized itself throughout North America except the Canadian far north and Alaska. It is known in half the counties in Minnesota, mostly absent in those with extensive agricultural cropland. It is listed in Minnesota on the "non-native invasive terrestrial plant" list and is considered a noxious weed.
Use: Bird's-foot Trefoil was imported to North America for agricultural forage. It is a non-bloating legume when eaten in moderation and in dryland pasture grows better that other grasses. Unfortunately, when it escapes the pasture or cropland area it becomes difficult to eradicate. In prairie areas, periodic burns increase seed germination. Repeated cutting reduces seed production but also weakens native plants. Spot herbicide application is the usual method of elimination.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"