7. Iris pseudacorus L. (yellow iris, yellow flag)

Pl. 91 c; Map 348

Rhizomes lacking conspicuously thickened areas. Aerial stems 50–120 cm long, about as long as or somewhat shorter than the leaves, erect to arching. Leaves basal and on the aerial stems, 40–120 cm long, 10–30 mm wide, erect to more commonly arching or nodding near the tips. Clusters of flowers terminal and axillary, each with 2–3 flowers, the spathelike bracts unequal, 3–6 cm long, herbaceous, green. Sepals 5–8 cm long, spreading outward to arching downward, broadly obovate, with a longitudinal ridge, yellow, sometimes with irregular brown or purple markings along the ridge. Petals much shorter and narrower than the sepals, erect, yellow. Capsules 4–8 cm long, oblong-elliptic in outline, 3-angled, with a single rib at each angle. 2n=24, 30, 32, 34, 40. May–July.

Introduced, escaped from cultivation and uncommonly naturalized in the southern half of the state (native of Europe and northern Africa, widely naturalized in eastern North America). Emergent aquatic, usually in shallow water along margins of streams and ponds.

This attractive species is relatively commonly cultivated in Missouri, and is expected to become naturalized sporadically at other sites in the state. It is an aggressive colonizer with dense, branching rhizomes and can form large colonies. Vegetatively, I. pseudacorus is nearly impossible to distinguish from the native I. virginica.