Silphium terebinthinaceum var. terebinthinaceum

Native to northeastern, north-central, and southeastern United States, including Iowa and Wisconsin, and to Ontario, Canada.

Adventive

Moderate moisture to wet. Prairies. Full sun.

July to September

Yellow ray florets, yellow disk florets

40″ to 98″

This is a 40″ to 98″ tall, erect, long-lived, perennial forb that rises on a bassal rosette and usually a single stem from a woody taproot. The taproot may extend up to 12′ deep.

The stem is erect, round, hairless, and usually branched. It is sometimes slightly covered with a whitish, waxy coating (glaucous).

A basal rosette of large, thick, leathery basal leaves appears early in the season. Later, a single flowering stalk bolts from the center of the rosette and reaches full height in just a few days.

Basal leaves are on long leaf stalks and may be triangular, lance-shaped, or egg-shaped. The blades are unlobed, 6″ to 20″ long, and up to 12″ wide. They are usually broadly heart shaped at the base and rounded, angled, or tapered to a point at the tip. The upper surface is hairless and shiny when young, moderately covered with spreading hairs, dull, and rough to the touch when mature. The lower surface is hairy, especially along the midrib. The margins may be coarsely toothed or untoothed. Sometimes there are one or more similar long-stalked leaves attached to the stem near the base.

The flowering stems are nearly naked, with usually a few much-reduced, bract-like leaves. Stem leaves are alternate, egg-shaped, stalkless, and ⅜″ to 1¼″ long. They clasp the stem at the base but do not wrap completely around the stem. The margins are untoothed.

The inflorescence is an open, branched cluster (panicle) of one to several flower heads at the end of the stem and branches.

Each flower head is 2″ to 3½″ wide with disk ½″ to 1½″ wide. There is a whorl of 23 to 33 bracts (involucre) in 2 or 3 overlapping series at the base of the flower head. The involucre is ½″ to 1″ in diameter. The outer bracts are short, broadly elliptic, and tightly appressed. The inner bracts are longer, oblong-elliptic, loose, and usually ascending at the tip. The outer surface is hairless, and sometimes there is a dense fringe of minute hairs along the margin.

There are 17 to 29 yellow ray florets and 120 to 140 yellow disk florets. The ray florets are ¾″ to 1¼″ long. The disk florets are sterile. The flower heads are not fragrant.

The fruit of the ray floret is a dry cypsela with no fluffy tuft of hairs attached. The cypsela is black, egg-shaped, strongly flattened, winged, ¼″ to ½″ long, and ⅛″ to ⅜″ wide.

Sources: 7.