Flower:

Flat clusters (umbels) 3 to 6 inches across made up of 10 to 25 groups (umbellets) of up to 25 flowers each. Flowers are about 1/8 inch across with 5 white petals notched at the tip, on slender, hairless stalks up to ¾ inch long. 5 white stamens radiate from the greenish white ovary in the center.

Up to 3 thread-like bracts are at the base of an umbellet, though not all umbellets have bracts. Likewise, umbels may have up to 3 narrow bracts at the base, or none.

Leaves and stems:

Leaves are few, alternately attached, compound with 5 to 11 leaflets, 12 inches long or more on the lower stem becoming somewhat smaller as they ascend the stem. Leaflets are 1½ to 4 inches long, 1/8 to 1 inch wide, lance-linear to oblong-elliptic, hairless, and stalkless. Leaflet edges may be toothless, have a few, widely spaced teeth, or have a few narrow, tooth-like projections (dentate) toward the tip.

Leaf stalks form a narrow sheath that rings the stem. Stems are stout or slender, hairless, lined or weakly ridged, and unbranched or few branched.

Fruit:

Fruit is about ¼ inch long, flattened, oval, ribbed, and splits into 2 seeds.

Notes:

Cowbane is one of the several white-flowered carrot species found in moist to wet places. It is most similar to Water Parsnip (Sium suave), which has 6 to 10 bracts at the base of each umbel and leaflets that are finely serrated all around the edges. Water Hemlock (Cicuta maculata) also has serrated leaflets, and its leaves are twice compound. Cowbane is far less common than these two species; while not currently listed as a rare species, it is not often encountered in the wild and is tracked by the DNR.