Called carpenter's weed for its remarkably square stems, cup plant is also notable for its large, opposite leaves that fuse around the stem to form a leafy cup that holds rainwater.
Cup plant is a tall perennial herb with stout stems that are square in cross-section.
The flowerheads are numerous, to about 3 inches wide, yellow, with 18–35 ray flowers to 2 inches long.
Blooms July–September.
The leaves are mostly opposite, to 1 foot long, with wavy, coarsely toothed margins (not lobed), rough on both sides, oval to triangular, covered with dots. The basal and lowest stem leaves are usually withered by flowering time; the leaves are progressively smaller the higher on the stem.
- The bases of opposite leaves are perfoliate: they join together around the stem, looking as if they were a single leaf being pierced by the stem. Together, they form a leafy cup that holds water.
Similar species: There are 6 Silphium species recorded for Missouri. Of these, cup plant, starry rosinweed (S. asteriscus), rosinweed (S. integrifolium), compass plant (S. laciniatum), and prairie dock (S. terebinthinaceum) are relatively common.
- Cup plant is identified by its square stems and unlobed, perfoliate leaves that form cups around the stem.
To separate rosinweeds from the similar-looking sunflowers (genus Helianthus), see Ecosystem Connections.
Height: 4 to 8 feet.
Scattered nearly statewide; uncommon in the Bootheel lowlands.
Habitat and Conservation
Occurs on banks of streams and rivers, bottomland forests, and margins of ponds and lakes; also along edges of crop fields, railroads, and roadsides.
Look for it in low areas near water and waste places. Note that our other rosinweeds tend to live in drier, open, upland habitats, native prairies, and glades.
Status
Native Missouri wildflower.
Sometimes considered a weed. In Connecticut, it is considered a noxious plant, but in Missouri it is not problematic.
Some people cultivate cup plant as a coarse, tall background plant in native wildflower gardens.
In Michigan, it is classified as threatened.
Human Connections
Silphiums are called rosinweeds because of the gummy resin that oozes from damaged tissues, which was used by Native Americans and pioneers as a kind of chewing gum.
Rosinweeds were also used for a variety of medicinal applications.
The name carpenter’s weed, or carpenter weed, alludes to the square stems, which can be conspicuous even in winter, when they turn dry and tan, long after the leaves have withered.
Cup plant’s species name, perfoliatum, means “perfoliate.” That’s a botanical term for when two opposite leaves clasp the stem and fuse together, so that the stem seems to pierce through the leaf tissue. It almost looks like a single leaf with a stem poking through it.
Ecosystem Connections
Many bees, butterflies, and other insects visit and pollinate the flowers.
Birds, including goldfinches, eat the seeds and sip water collected in the leaf cups.
Many animals find shelter in this plant’s dense colonies.
A variety of wasps are obliged to deposit their eggs in rosinweeds; their gall-forming larvae grow in the stems. See Ecosystem Connections on the starry rosinweed page for a deeper dive on these fascinating wasps.
Cup plant and other rosinweeds are in their own genus, Silphium, and not in genus Helianthus (sunflowers). Yet the two groups look so much alike! How can you tell the difference? It has to do with which parts of the compound flowerheads produce seeds:
- The disk (center) florets in rosinweeds are staminate (male, producing only pollen) and therefore don’t create seeds; meanwhile, the disk florets in sunflowers are pistillate and create seeds. Then, in rosinweeds, it’s the petal-like ray florets that are pistillate (female) and turn into seeds, while those in sunflowers produce only pollen.
- Due to the above fact, the disks in rosinweeds tend to be smaller in diameter than the disks of sunflowers.
- Looking beneath the flowerhead, the green, leafy outer involucral bracts in rosinweeds are comparatively large and broad compared to those in sunflowers.
