Common moonseed is a rather slender, twining vine with stems to 16 feet long or more, that climbs or sprawls. It occurs nearly statewide. It bears clusters of bluish-black fruits. The seeds are flattened, with a raised edge shaped like a crescent moon.
Leaves are alternate, simple, triangular or kidney-shaped, 2–6 inches long and wide, unlobed or with 5 or 7 shallow lobes; tips of lobes blunt or with an abrupt, minute sharp point; base heart-shaped to flattened; the point of the leaf stalk’s attachment is not on the margin but is on the lower surface of the leaf, near the margin; 7 or 9 main veins originate at the leaf stalk’s point of attachment; margin lacks teeth; upper surface dull green, smooth; lower surface much paler green, pale gray or silvery gray, smooth to thinly hairy; leaf blade firm and rather thick; leaf stalk slender, elongate, about as long as the blade.
Stems are slender, twining, reddish brown or greenish brown, shiny with fine grooves and ridges, slightly hairy to smooth later; tendrils absent.
Bark is reddish brown to greenish brown, smooth on young stems; brown to grayish brown, scaly or warty with short ridges of corky bark near the base of old stems; vine herbaceous above, somewhat woody near the base; wood soft, white.
Flowers May–June, in loose, drooping clusters along the leaf axils of new growth; both male and female flowers are present in separate clusters; flowers numerous, often 40–50, greenish white to white, smooth; petals 4–12, white, broadest above the middle; stamens 18–20, slightly extending beyond the flower, stamens not well developed in the female flowers.
Fruits September–October, in a grapelike cluster, about ¼ inch in diameter, dark blue to black with a whitish coating that can be rubbed off, globe-shaped, somewhat flattened on the sides; flesh thick, juicy, the skin tough; seeds solitary, yellowish, circular, crescent-shaped, flattened; fruit somewhat poisonous if eaten.
- Carolina moonseed is a close relative, but it has clusters of bright red (not bluish-black) berries, and the leaf stem attaches to the edge of the leaf blade (not on the lower surface of the leaf blade).
- Moonseed fruits may easily be confused with our various species of wild grapes (Vitis spp.). Because wild grapes are edible and moonseeds are toxic if eaten, it is important to be able to distinguish between these plants.
- Key characters for identifying grapes are their toothed leaves, the curling tendrils by which they climb, and their seeds, which are not disk-shaped or bowl-shaped.
- Unlike grapes, common moonseed has the leaf lobe tips with an abrupt, minute sharp point, but the leaf margins are not toothed.
- Bur cucumber (Sicyos angulatus) is another native vine with lobed leaves something like moonseed’s, but it is an herbaceous annual (not a woody vine), and its spiny, burlike fruits are quite different.