1. Armoracia rusticana (Lam.) P. Gaertn., B. Mey. & Scherb. (horseradish)

Pl. 313 c–e; Map 1315

Plants perennial herbs with thick, fleshy roots, lacking rhizomes, terrestrial. Stems (40–)50–120(–200) cm long, erect, unbranched below the inflorescence, glabrous. Leaves alternate and basal. Leaf blade (5–)10–45(–60) cm long, glabrous, the basal and lower stem leaves with petioles to 60 cm long, the median and upper leaves progressively reduced and short-petiolate or sessile, the bases not clasping, the leaf blades of the lower leaves broadly oblong or ovate with irregularly toothed or scalloped margins, those of the upper leaves lanceolate to linear, entire to lobed. Inflorescences panicles, the lower branches subtended by small, leaflike bracts. Sepals 2–4 mm long, ascending, elliptic to obovate, glabrous. Petals 5–8 mm long, unlobed, white. Styles absent or to 0.5 mm long. Fruits ascending, often aborting prior to maturity, 5–8 mm long, about as long as wide or less than 2 times as long as wide, globose to ovoid, circular in cross-section or slightly flattened at a right angle to the septum, dehiscing longitudinally, each valve with a faint midnerve. Seeds in 2 rows in each locule, often aborting prior to maturity, 1.0–1.5 mm long, nearly circular in outline, the surface with a netlike or honeycomb-like pattern of ridges and pits, brownish orange to brown. 2n=32. May–July.

Introduced, widely scattered in the state, mostly in and around the Missouri River floodplain (native of Europe and Asia, introduced sporadically in North America). Roadsides, margins of cultivated fields, and disturbed areas; sometimes a weed in flower beds.

Horseradish is widely cultivated for its roots, which are used as a flavoring and ingredient in various dishes.