Silphium L. is a genus of coarse herbaceous perennial plants in the family Asteraceae (tribe Heliantheae), native to North America. Collectively known as rosinweeds, the roughly 20–25 accepted species range in height from about 20 cm to over 2.5 m, with erect stems that are often branched and sometimes coated with a resinous exudate that gives the group its common name.
The flowerheads are radiate, superficially resembling sunflowers, and typically bear yellow ray florets — rarely white. Unlike sunflowers, where ray florets are sterile, in Silphium the ray florets are female and the disc florets are male. Leaves vary by species from deltate and rhombic to elliptic or linear, arranged basally, alternately, oppositely, or in whorls depending on the species. The fruit is a flattened cypsela, black to brown at maturity, with or without awns.
Species are found across the eastern and central United States, from Texas north to Ontario, Quebec, and New Brunswick in Canada. They grow in a variety of open habitats including tallgrass prairies, open woodlands, meadows, river banks, and rocky glades. The genus was formally described by Carl Linnaeus in Species Plantarum in 1753, with Silphium asteriscus L. as the type species.
Etymology
The genus name Silphium derives from the Ancient Greek word sílphion, which referred to a now-lost North African plant — almost certainly not related to the modern genus — that was highly prized in antiquity as a medicinal herb and culinary condiment. Linnaeus borrowed the name when he described the North American genus in 1753, linking the resinous, medicinally valued New World plants to their ancient namesake. The common name "rosinweed" refers to the sticky, resinous sap found on the stems and foliage of most species.
Distribution
Silphium species are native to eastern and central North America. The genus reaches from Texas and the southeastern United States northward through the Midwest to the Great Plains and along the eastern seaboard to Maine, with range extensions into Canada in Ontario, Quebec, and New Brunswick. Species are most diverse and abundant in the tallgrass prairie region of the central United States. Habitats include native prairies, open woodlands, woodland edges, rocky glades, river banks, and thickets. Silphium perfoliatum in particular occupies moist, rich soils along stream margins and in woodland understories from Ontario and South Dakota south through multiple eastern states.
Ecology
Members of Silphium are adapted to open, often fire-maintained habitats such as tallgrass prairies and open oak savannas. They develop deep, extensive root systems that make individual plants long-lived and resistant to drought and fire; this same trait makes transplanting difficult. Species are important resources for pollinators — bees, butterflies, and other insects visit the flowerheads — and seed-eating birds feed on the achenes. Silphium perfoliatum (Cup plant) is named for its connate, perfoliate leaves, which clasp the stem to form a small water-holding cup that provides drinking water for insects and small birds. Many species spread via rhizomes and can form persistent clonal colonies in favorable habitats.
Cultivation
Rosinweeds are increasingly valued in native-plant horticulture and prairie restoration. Silphium perfoliatum is hardy to at least USDA zone 3 (tolerating temperatures to –25°C) and accepts a wide range of soil textures from light sandy to heavy clay, preferring moist, moderately fertile conditions. Full sun to dappled shade is tolerated. Plants are heat resistant, attract pollinators and birds, and spread gradually by rhizomes. Their imposing stature (up to 2.5 m) suits the back of borders or naturalized prairie plantings. Silphium integrifolium is widely used in prairie-garden mixes and has attracted interest as a perennial oilseed crop.
Propagation
Silphium can be propagated from seed or by division. Seeds are best sown fresh (when ripe) and germinated under greenhouse conditions before outdoor planting. Spring division is feasible but challenging due to the plants' deep, extensive root systems, which resist disturbance. Established plants are long-lived perennials that rarely need division.
Cultural Uses
Native American peoples used several Silphium species medicinally. The root of S. perfoliatum was employed as an alterative, antispasmodic, diaphoretic, emmenagogue, febrifuge, hepatic, pectoral, stimulant, styptic, and tonic. Root decoctions were prepared to treat the cessation of menstrual periods, morning sickness, lung hemorrhages, and disorders of the liver and spleen. Root poultices were applied to control bleeding from wounds. The plant is traditionally contraindicated in pregnancy due to its emmenagogue properties.
Taxonomy
The genus Silphium L. was published by Carl Linnaeus in Species Plantarum 2: 919 (1753) and is placed in the family Asteraceae, subfamily Asteroideae, tribe Heliantheae, subtribe Engelmanniinae. The type species is Silphium asteriscus L. The accepted species count varies by authority: GBIF records 62 descendant taxa, SEINet documents 38, and traditional manual treatments recognize approximately 15–25. The genus shares a name with, but has no taxonomic relationship to, the ancient plant "silphion" of Cyrene (likely an Apiaceae). The common name "rosinweed" is shared with several other genera and species in Asteraceae.
