Flower:

Large flat cluster (umbel) 3 to 6 inches across at stem tips and arising from upper leaf axils, made up of 9 to 20 smaller clusters (umbellets) each with 12 to 18 tiny 5-petaled yellow flowers. Umbellet stalks are up to 4 inches (1 to 10 cm) long, flower stalks to ½ inch (5 to 12 mm) long, giving the cluster an open, airy appearance.

Flowers are less than 1/8 inch across, the 5 petals typically folded inward, though are sometimes spreading. Alternating with the petals are long yellow stamens. Flowers in the center of an umbellet are typically sterile. Flower and umbellet stalks are hairless and there are no bracts at the base of umbels or umbellets.

Leaves and stems:

Leaves are alternate, the lower leaves 2 or 3 times compound, up to 12 inches long, on a long stalk, each branch usually with 3 leaflets, sometimes 5. Leaflets are up to about 1½ inches (1 to 4 cm) long, 1/3 to half as wide, hairless, toothless, mostly elliptic to egg-shaped, blunt to pointed at the tip, rounded to wedge-shaped at the base, and the lateral leaflets stalkless. 

Some leaflets may be cleft with 2 or 3 lobes, most often seen on the terminal leaflet of upper leaves. Upper leaves are 1 or 2 times compound and shorter stalked. Stems are erect to ascending, unbranched or few branched, hairless, green to reddish, often with a waxy sheen.

Fruit:

Fruit is oval-elliptic, 4 to 5 mm long, hairless, dark brown with 5 low ribs when mature, and splits into 2 seeds.

Notes:

Yellow Pimpernel is uncommon in Minnesota, where it reaches the northwestern fringe of its range. It's found primarily in the edges and openings of oak woods and savannas, wooded bluffs and slopes in our southeastern counties. According to the DNR, habitat loss is its greatest threat, as the oak ecosystems it depends on, once common in the state, have diminished significantly in both number and quality since the time of settlement. It was listed as a Special Concern species in 2013.

It is one of several members of the carrot family in Minnesota with yellow flowers, including the more common Alexanders (Zizia aurea and Z. aptera) and the invasive Wild Parsnip (Pastinaca sativa), but is readily distinguished by its more open, airy umbels, leaves 1 to 3 times compound, and toothless leaflets, where the other yellow-flowered carrots have toothed leaflets.