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Invaders SearchResources Materials Related Sites Hydrilla
(Hydrilla verticillata (L.f.) Royle) [Profile revised October 3,
2012] Hydrilla (Hydrilla verticillata), also commonly called
water thyme, is a submersed perennial herb. The plant is rooted
in the bed of the waterbody and has long stems (up to 25 feet in
length) that branch at the surface where growth becomes
horizontal and forms dense mats. Small (2 - 4 mm wide, 6 - 20 mm
long), pointed, often serrated leaves are arranged around the
stem in whorls of 3 to 10. Southern populations are
predominantly dioecious female (plants having only female
flowers) that overwinter as perennials. Populations north of
South Carolina, including populations in New York, are
essentially monoecious (having both male and female flowers on
the same plant) that set some fertile seed, and depend on tubers
for overwintering. These monoecious plants produce female