Distribution: The northern two-thirds of the temperate zone in North America; also in Eurasia.
Habitat: Wet places, from the lowlands to moderate elevations in the mountains.
Flowers: May - September
Conservation Status: Not of concern
Usually glabrous perennial from shallow, creeping rhizomes, the stem erect to ascending, 1-4 dm. long.
Leaves all opposite, sessile, linear to lanceolate, 2-8 cm. long and 2-15 mm. wide, usually entire.
Inflorescence of open racemes on long peduncles arising from the leaf axils, each raceme 5-20 flowered, the flowers on filiform pedicels 6-17 mm. long; sepals 4; corolla bluish, 6-10 mm. wide, rotate, 4-lobed, the upper lobe the largest; style 2-4 mm. long; stamens 2.
Capsule flattened, 2.5-4 mm. high, noticeably wider than high, broadly notched.