Utricularia macrorhiza is a submerged, floating aquatic, carnivorous herb; stems long, branched, slender, overwintering as buds, initially rooted but often becoming detached and floating freely (Magee 1981).
Leaves alternate, 1-5 cm long, branched in two from base, forking repeatedly into finer segments becoming filamentous with elongate, pointed tips, segment forks bearing small bladder-like traps ca 0.3 cm to capture aquatic invertebrates. Empty trap opens rapidly when trigger hairs are disturbed, sucking in water and prey animal, which is digested and the nutrients absorbed within 15 minutes to 2 hours (Fertig 2001; Rook 2004).
Flowers yellow, bilaterally symmetrical, to 2.5 cm wide, corolla 2-lipped, calyx lobes, petal lobes nearly the same size, the lower one bulges upward over a horn-like basal appendage; inflorescences few, of 6-20 flowers on a stout stalk to 20 cm long above water surface; blooms June-Aug. pollinated by bees (Fern 2004). Another visitor is Helophilus conostomus, a long-tongued syrphid fly, at least in Maine (Lovell 1918).
Fruit a capsule with numerous seeds.
Wetland status: OBL.
Frequency in NYC: Very infrequent.
Habitat: Quite water (Hough 1983).