Knobweed (Hyptis capitata) is an aromatic long-lived (perennial) herb or sub-shrub, with several upright branching stems growing up to 2.5 m high that arise from a short, robust rootstock. The stems are square in cross section. Bright green leaves are arranged in opposite pairs that are widely spaced along the stem. The leaf blade is usually ovate (egg-shaped) to narrowly ovate or narrowly rhombic (diamond shaped), up to 15 cm long and 7.5 cm wide, sparsely hairy, with irregularly toothed to serrated margins and dotted with numerous oil glands on the undersurface. The leaf stalk is up to 3 cm, rarely up to 6 cm, long. Spherical flower-heads (about 10-15 mm in diameter, increasing up to 25 mm when in fruit) occur on the stalks that are generally up to 6 cm long, or sometimes up to 10 cm long, and borne in the forks of the leaves (resulting in pairs of leaves and corresponding flower-heads along the stems). The compact ball-shaped heads are composed of many small, white, tubular, 2-lipped flowers with the upper lip violet-dotted. The fruit is a 4-lobed capsule dividing into 4 seed-like nutlets. The seeds are small and 1-2 mm diameter, dark brown to black, and rattle in the capsules when shaken. The seeds are held inside the rigid, persistent fruiting calyx.