Wholeleaf rosinweed is not quite so tall as most of our other rosinweeds and is commonly misidentified as a sunflower. Stems can be solitary or clustered and vary in their amount of hairiness, but hairs, if present, are short. The stem usually doesn’t branch except at the very top. Stems often turn reddish in bright sun. The sunflower-like flowerheads are in loose, open clusters at the top of the plant, usually with 20–35 yellow rays. Blooms July–September. Basal leaves often have withered away by flowering time. Stem leaves are thick, sometimes leathery, sometimes roughened, sometimes smooth, sometimes hairy. Leaves are usually well distributed along the stems; they are opposite, and each pair is turned 90 degrees from the pair above and below it. Stem leaves are sessile (stalkless) and become slightly smaller at the top of the plant; shape varies from lance-shaped to oval to heart-shaped to clasping, tapering to a sharply pointed tip.
Two varieties of wholeleaf rosinweed occur in our state. Var. integrifolium is a roughened, hairy plant and is scattered nearly statewide. Var. laeve is glabrous (smooth) and strongly glaucous (white-waxy coated) and is scattered along the western edge of the state, including the loess hill prairies of northwestern Missouri.
This species sometimes hybridizes with cup plant (S. perfoliatum), and where the two plants occur together, their offspring, exhibiting traits of both parents, may be present.
Similar species: There are 6 Silphium species recorded for Missouri. Of these, wholeleaf rosinweed, prairie dock (S. terebinthinaceum), starry rosinweed (S. asteriscus), compass plant (S. laciniatum), and cup plant (S. perfoliatum) are relatively common. The sixth species, rough-leaved rosinweed (S. radula), is known only from a single collection from Vernon County in 1965.
Rosinweeds (genus Silphium) generally resemble sunflowers (genus Helianthus). But here’s a big difference: The disk (center) of the flowerheads is usually smaller than in true sunflowers. Indeed: the disk florets in rosinweeds are staminate (male) and therefore don’t create seeds, just pollen; but the disk florets in sunflowers, as most of us know, create seeds. In rosinweeds, it’s the petal-like ray florets that are pistillate (female) and turn into seeds, while those in sunflowers produce only pollen.