Flower:

Clusters of stalked flowers at the top of the stem and arising from upper leaf axils, each unbranched or sometimes few branched, the flowers in pairs and mostly loosely arranged on the stem. Flowers are pea-shaped, about 1/3 inch long, pink to purplish with 2 green to yellowish spots near the base of the broad upper lobe. The stamens and pistil form a curving tube that protrudes from the center, between the 2 small lateral petals.

The calyx behind the flower is green to reddish, hairy, and cup-shaped with 4 triangular lobes, the lower lobe longest. Flower stalks are slender and hairy. Some hairs on the calyx and stalks may have a tiny hook at the tip (magnification required to see).

Leaves and stems:

Leaves are compound in 3s, alternately attached. All leaflets are toothless, lance to egg-shaped, rounded to slightly wedge-shaped at the base and pointed at the tip, often sharply so. The upper surface is hairless to sparsely hairy, the lower surface more densely hairy, slightly more so along the midrib. The terminal leaflet is largest, 2 to 5 inches long and 1 to 3 inches wide, on a stalk about ½ inch long. The 2 lateral leaflets are similar to the terminal leaflet but smaller and minutely stalked. The compound leaf stalk is 2 to 4 inches long, about as long as or longer than the width of the terminal leaflet and much longer than the stalk of the terminal leaflet.

At the base of the leaf stalk is a pair of leafy appendages (stipules) that are narrowly triangular to lance-shaped with a long taper to a pointed tip, up to about 2/3 inch long, and fringed with long, straight hairs. The stipules usually persist after the leaves have withered and fallen off the stem. Stems are unbranched except in the flower clusters, ridged or grooved, mostly hairless except in the flower clusters, and erect to ascending.

Fruit:

Fruit is a flat pod 1 to 2½ inches long with 3 to 7 sections, the sections nearly triangular, only slightly convex on the upper edge and tapering to a rounded point on the lower. Each section contains a single seed.

Seeds are also somewhat triangular with rounded corners, about 4 mm long, maturing to golden yellow. The pod is densely covered in tiny hooked hairs that latch onto anything that passes by.

Notes:

Large-bract Tick-trefoil is quite rare in Minnesota, where it reaches the northwest edge of its range. The species has been in decline over the past 30 years both here and in New England. One possible cause is climate change, but not for the reason you might think. Desmodium cuspidatum, like other members of the Pea family, is a nitrogen-fixing species, converting gasses in the air into soil nutrients through microbes that live on its roots. It is thought this ability gives it an advantage in competitive habitats, but with climate change comes increased levels of nitrogen deposition (acid rain), which reduces the competitive advantage. According to the DNR, in Minnesota nearly 90% of the mesic forest where this species lives has been degraded or destroyed by logging, agriculture and development, with additional threats from invasive species such as buckthorn, garlic mustard, and white-tailed deer (which eat it). When D. cuspidatum was listed as a Special Concern species in 1996 there were only 9 known sites in 3 counties and it was believed to be extirpated from 4 other counties. It was elevated to Threatened in 2013 after biological surveys failed to find any additional populations.

The leaves and flowers are similar to the related Showy Tick-trefoil (Desmodium canadense), which has compound leaf stalks shorter than the terminal leaflet stalk, stipules only up to 1/3 inch long, more densely packed flower clusters, and is more often found in open habitats, less often in open woods. Also similar is Illinois Tick-trefoil (Desmodium illinoense), which typically has a single raceme, has hooked hairs on the stem and leaves, pods with up to 9 sections that are well rounded on both edges, leaflets with prominent network of veins (like leatherette), and the leaflets tend to be narrower and more blunt at the tip. There are 2 recognized varieties of D. cuspidatum: var. cuspidatum is present in the eastern part of the range and has mostly hairless leaves, var. longifolium is in the western part of its range, including Minnesota, and has hairy leaves.