Cup plant (Silphium perfoliatum, L. 1759)

Identity

Cup plant (Silphium perfoliatum L., 1759) is a perennial in the Asteraceae family, described by Linnaeus and accepted under that name by POWO/WCVP. The genus Silphium contains around twenty North American species; this one is the most widely cultivated outside its native range. It is also known as cup rosinweed and Indian cup. ^{1 3}

Linnaeus adopted the Greek word silphion for the genus in 1753. It originally referred to the ancient silphium of Cyrenaica, a now-extinct resinous plant celebrated on ancient Greek coins. The two plants have no real botanical relationship: a resemblance between their resinous gums prompted the comparison. ⁷

The epithet perfoliatum describes a precise leaf feature: opposite leaves join at their bases around the stem, making the stem appear to pass through them. This detail matters more than flower color when identifying the species outdoors. ^{7 3}

Recognition

Cup plant forms a tall, upright perennial with a smooth, square stem, usually 1.2–2.5 m (4–8 ft) tall depending on growing conditions. NC State and the Royal Horticultural Society give a typical height within this range; plants may approach 3 m (10 ft) in very fertile soil. ^{3 2}

Unlike height, which changes with soil fertility, the leaf cup is a stable identification feature. Broad, rough, opposite leaves join at their bases around the stem, forming a reservoir for rainwater. Birds drink from these cups, while small insects sometimes drown in them accidentally. ^{7 2}

Golden-yellow flower heads measure 7–10 cm (3–4 in) across, with 20–30 ray florets around a darker central disk. They open on branching stems from midsummer into early fall, providing food for bees, bumblebees and butterflies for several weeks. ^{2 3}

Useful taxonomy

For cup plant, the accepted POWO and WCVP name is Silphium perfoliatum L., in the family Asteraceae and tribe Heliantheae. Three infraspecific taxa are recorded: the typical form, variety connatum and a historical form now treated as a synonym. This limited synonymy causes little practical confusion for gardeners. ¹

The principal infraspecific names are:

• Silphium perfoliatum var. perfoliatum, the widespread typical form;
• Silphium perfoliatum var. connatum (L.) Cronquist, with hairier stems, restricted to a few Mid-Atlantic states around the New River Valley;
• Silphium perfoliatum f. hornemannii (Schrad. ex DC.) Voss, a historical form now treated as a synonym.

Use Silphium perfoliatum as the reliable accepted name. Variety connatum concerns only a geographically limited population. ¹

Habitat and range

Southeastern Canada, including Ontario and Quebec, marks the northern part of its native range, which continues through the central and eastern United States, from Iowa and Kansas to Georgia. NC State describes floodplains, streambanks, wet prairies and open woodland edges as its habitats. ^{1 3}

European growers first valued cup plant as a nectar-rich garden perennial before adopting it as an energy crop. In Germany it occupies approximately 10,000 hectares (24,700 acres) as a perennial alternative to silage corn. A planting remains in place for several years, whereas conventional corn is sown annually. ⁸

French records describe naturalized populations in two departments, Aube and Haute-Saône, without inclusion on the EPPO invasive alien plant list. A recent study of seed dispersal by wind, water and rodents nevertheless supports monitoring near waterways, where high biomass production could help the plant expand locally. ^{6 9}

Life cycle and reproduction

Cup plant germinates slowly and unevenly, with a natural germination rate of about 15–20%. Seedlings are raised in a nursery or after cold stratification. Without flowers in the first season, the young plant directs its energy toward deep rooting. ³

Established plants develop a deep taproot together with short rhizomes. Botanical garden clumps have been documented over several decades. Division is most successful on young plants and becomes risky after the first two years. ⁴

From midsummer into early fall, cup plant flowering depends entirely on pollinating insects: bees, bumblebees and butterflies fertilize the flower heads. Its seeds, flat, winged achenes, then disperse by wind, water and, to a lesser extent, rodents that carry them away. ^{4 9}

Uses and risks

At the back of a naturalistic border or in a flowering meadow, cup plant provides a long flowering season and water-holding leaf cups that benefit pollinators and birds. The Royal Horticultural Society has given it an Award of Garden Merit for its performance in the British climate. ²

Central European agriculture uses cup plant on a larger scale, driven by the German bioenergy sector. A University of Bayreuth study shows that after five years of cultivation, cup plant equals or exceeds silage corn in root biomass while improving soil carbon storage, microbial activity and earthworm activity. ⁸

For cats, dogs and humans, no toxicity is documented; horse status remains uncertain because specific data are lacking. Historically, some Native American nations, including the Chippewa, used the root as a tonic, diaphoretic and diuretic for respiratory or joint complaints. These uses are documented history rather than common current practice. ^{7 4}

Seed dispersal is the main concern. Because cup plant produces abundant biomass and spreads its achenes by several routes, remove faded flower heads before the seeds fully mature when growing it near a waterway or sensitive natural area. ⁹

Varieties, look-alikes and related plants

Selected horticultural cultivars are not normally offered for cup plant. Commercial seed almost always represents the typical species, with variety connatum sometimes available for specialist collections. Diversity comes mainly from wild populations and regional seed lots. ³

Other members of Silphium cause the most frequent identification problems:

• Silphium laciniatum, compass plant, has deeply divided leaves that never join around the stem;
• Silphium terebinthinaceum, prairie dock, has large basal leaves and stems almost without stem leaves;
• Silphium integrifolium has stalkless leaves that are not perfoliate.

Among these four species, the cup formed by fused leaves is the most reliable clue. Only Silphium perfoliatum has it among the species commonly encountered in gardens or along prairie margins. ^{7 3}

Main sources

• Plants of the World Online / Kew, Silphium perfoliatum L.
• Royal Horticultural Society, Silphium perfoliatum - cup plant
• NC State Extension Gardener Plant Toolbox, Silphium perfoliatum
• Plants For A Future, Silphium perfoliatum
• INPN, Silphium perfoliatum L., 1759, CD_nom 123701
• Tela Botanica / eFlore BDTFX, Silphium perfoliatum
• Eloise Butler Wildflower Garden, Four Silphiums
• Universität Bayreuth, Cup plant outperforms silage maize as a sustainable bioenergy crop
• Ende, Hummel & Lauerer 2024, Dispersal and persistence of cup plant seeds, Plant Ecology and Evolution