Flower:

Cluster of stalked flowers at the top of the stem, usually a single cluster, occasionally 2, with the flowers in pairs, mostly loosely arranged on the stem and usually only a few open at a time. Flowers are pea-shaped, about 1/3 inch long, pink to purplish, occasionally white, 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 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. Hairs on the calyx and stalks are usually a mix of long, straight hairs and shorter hairs with a tiny hook at the tip (magnification required to see); some hairs may be glandular.

Leaves and stems:

Leaves are compound in 3s, alternately attached. All leaflets are toothless, narrowly lance to egg-shaped, rounded to slightly wedge-shaped at the base and blunt to pointed at the tip, with a prominent network of veins especially noticeable on the underside. The upper surface is variably covered in hooked hairs, the lower surface with hooked hairs along major veins; edges are fringed in straight hairs. The terminal leaflet is largest, 2 to 4 inches long and up to 1 inch 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 1 to 4 inches long, longer than the width of the terminal leaflet and much longer than the leaflet stalk.

At the base of the leaf stalk is a pair of leafy appendages (stipules) that are egg-shaped with a long taper to a pointed tip, up to about ½ inch long, and fringed with long, straight hairs. The stipules persist after the leaves have withered and fallen off the stem. Stems are erect to ascending, unbranched, ridged or grooved, and moderately to densely covered in a mix of long, spreading hairs and shorter, hooked hairs.

Fruit:

Fruit is a flat pod 1 to 3 inches long with 3 to 9 sections, the sections oval-elliptic to egg-shaped, rounded on both the upper and lower edge. Each section contains a single seed.

Seeds are oval to somewhat kidney-shaped, 3 to 4 mm long, maturing to golden brown. The pod is densely covered in tiny hooked hairs that latch onto anything that passes by.

Notes:

Illinois Tick-trefoil is an uncommon species in Minnesota, where it reaches the northern edge of its range. It was listed as a state Threatened species in the 1980s but was subsequently delisted, presumably after biological surveys conducted in the 1990s found many populations in our southeast counties. While there hasn't been a new record of it since then, we've visited a number of these sites but have failed to encounter it in the wild. This suggests (to us) that it has probably seen a decline, likely caused by further destruction of its prairie habitat from development, agriculture and invasive species. That, sadly, is a very common story.

Illinois Tick-trefoil is most easily distinguished from other Desmodium species by its (usually) single, loosely arranged flower cluster, leaflets with a strong network of veins (like leatherette), hooked hairs on the upper surface and along veins on the underside, compound leaf stalk much longer than the stalk of the terminal leaflet, obvious and persistent stipules, and pod sections well rounded on both edges.

The leaves and flowers are similar to the related Showy Tick-trefoil (Desmodium canadense), which has compound leaf stalks mostly shorter than the terminal leaflet stalk, shorter and narrower stipules, multiple and more densely packed flower clusters, and pod sections straight to only slightly rounded on the upper edge. Also similar is Large-bract Tick-trefoil (Desmodium cuspidatum), which usually has several flower clusters, broader leaflets that are weakly veined and evenly hairy across the lower surface, and pod sections that are more triangular in shape.