St. Andrew's cross is a small shrub with smooth, reddish bark that peels away in thin strips or flakes.
Flowers are bright lemon yellow; the 4 petals form an oblique cross; there are 4 sepals, of which 2 are large and 2 extremely small; with many stamens. Blooms July–October.
Leaves are opposite, sessile, lighter green below, narrowed at the base, to 1 inch long.
Fruits are ovoid capsules, widest at or below the middle, tapered to a short beak, flattened, with numerous seeds.
Of the two varieties in the state, the more common, var. multicaule, grows only to about 1 foot tall; it is bushy and branches at the base. Var. hypericoides is taller and doesn’t branch at the base, though it branches well above ground level.
Similar species: Missouri has 14 species of Hypericum. All have opposite, simple, entire leaves with small resin dots, flowers with 4 or 5 yellow petals, 4 or 5 sepals, many stamens, and 1 pistil. Many are small shrubs, with twigs often angled or 2-winged. The capsules have numerous seeds.