Starry rosinweed is an upright perennial herb with short, stout rhizomes. The stalk is usually solitary (though in colonies they can appear clustered) and usually branch only at the top of the stalk as flowerheads form. Flowerheads loose, open clusters at the top of the plant, with 15–21 yellow rays. Blooms May–September. Stems and leaves are roughened and hairy, but the leaves are not leathery. Leaves mostly alternate. Lower leaves, which have often withered away by bloom time, have long petioles and are elliptic to narrowly ovate to lanceolate, unlobed, tapered at the base and tapering to a usually sharp-pointed tip. The leaves become smaller at the top of the stem and have progressively shorter leaf stalks and have progressively less tapered bases.
Similar species: There are 6 Silphium species recorded for Missouri. Of these, starry rosinweed, prairie dock (S. terebinthinaceum), rosinweed (S. integrifolium), 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) florets in rosinweeds are essentially 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.
Height: usually 1 to 3 feet, but can reach 5 feet.
Scattered, mostly in the Ozarks. Missouri is on the northwest edge of this plant’s overall range.
Habitat and Conservation
Occurs in glades, tops of bluffs, sinkholes, and openings of dry upland forests; also occurs in pastures and ditches and along railroads and roadsides.
Human Connections
Starry rosinweed isn’t the most breathtaking member of its genus, but it contributes to the legions of sunshiny yellow composite flowers that adorn our Ozark hills in midsummer.
Rosinweeds exude a gummy resin when cut (hence the name), and Native Americans and pioneers used this exudate as a kind of chewing gum. Most species had folk medicinal uses, too — for example, for pain relief or for treating urinary tract infections.
Ecosystem Connections
Plant-insect interactions are a fertile area of study. Researchers in Illinois found that several types of insects spend most of their lives within the stems of certain types of rosinweeds. They found that certain species of tiny wasps (genus Antistrophus) are so connected to specific types of rosinweeds that their survival depends on being able to deposit their eggs in the stems of these species. In response, the rosinweed develops a swollen-looking gall around the larvae. The larval wasps feed on a specialized lining of nutritious cells within their chamber before maturing and exiting their natal home. In a fascinating twist, other types of wasps deposit their eggs into existing stem galls, and their larvae parasitize the larvae of the Antistrophus wasps. Complicating the situation further, those secondary wasps may themselves be parasitized by yet other wasps!
