(Prairie Rosinweed). The tallest composite in the Upland Garden, growing from 4 to 10 feet high with numerous flower heads. Stems are smooth, bare, branching in the inflorescence, green, but turn reddish in autumn. Flowers: The inflorescence is a large open spreading branched panicle. The yellow flower heads are 2 - 3 1/2" wide and on long stalks. They have 13 - 21 fertile pistillate ray flowers that have pointed tips; they surround a central disk of 1/2 - 1 1/2" width, composed of numerous sterile disk flowers. The disk flowers have a deep yellow color 5-lobed corolla with spreading tips at the apex. The phyllaries of the flower head (bracts) are broad, smooth, not spreading, rounded (obtuse) at the top. Seed is a dry narrowly winged achene without pappus. Leaves are large, triangular shaped, basal only, stalked, unlobed, with large pointed teeth, rough with a heart shape base. They can be up to 18" long and 12" wide. Young leaves have fine hair on the leaf and the stalk.

There are four Silphiums in the Garden, only Prairie Dock has basal leaves only. The genus name, Silphium, is from the Greek referring to resinous juice. The plant’s roots do contain resin but the name actually refers to an ancient plant of North Africa that was named “Silphium” and was said to have resinous juice that was medicinally sought after. The species name, terebinthinaceum, means "like turpentine" which may refer to the odor of the resinous juice.