A glabrous, aquatic perennial herb with horizontal rhizomes; scapes (30-) 50-90 (rarely-150) cm long, erect; roots fibrous. Leaves all radical, usually about as long as the scapes, linear, ensiform, sheathing at the base, 6-9 mm broad. Inflorescence umbellate, cymose with an involucre of lanceolate-acuminate bracts and bracteoles; bracts 2 or 3, upto 25 mm long, 6-8 mm broad; bracteoles numerous, less than 10 mm long, 2-3 mm broad. Flowers pink, pedicellate; pedicels (3-) 5-9 cm long. Perianth segments 6. biseriate. Sepal-like segments obovate. c. 17-veined, keeled, 9-13 mm long and 6-8 mm broad; petal-like segments broadly obovate, c. 13-veined, 9-15 mm long, 8 mm broad. Stamens usually 9, rarely 5 or numerous; filaments linear, usually dilated at the base, variable in length, free, slightly connate at the base; anthers basifixed, 2-celled, ovate to oblong (linear-oblong in young flowers), 2-4 mm long. Carpels 6-9 or many, each 3-5.5 mm long, 1.5-2 mm broad, superior, free or somewhat connate basally, obovate; style simple, short, terminal, persistent; stigma ventral, curved. Ovules numerous, anatropous, minute on reticulate parietal placentas. Fruit a many-seeded, beaked follicle, obovate, 7-9 mm long, 2-3.5 mm broad. Seeds numerous, minute, 0.2-0.4 mm long, oblong with thin testa and straight embryo.
B. umbellatus is widespread and abundant with apparently stable populations throughout much of its European, Mediterranean and SW Asian range. However, there are areas where it has declined, such as Norway, Croatia and Switzerland.
The species occurs throughout much of the Northern Hemisphere; it is native to North Africa and Europe north to Scandinavia and east through European Russia, Siberia, the Middle East, Kazakhstan, Mongolia and the Western Himalayas to the Amur region of the eastern seaboard of Russia. It is apparently naturalised in southern Canada and the northern United States, Ireland and some of the British Isles.