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 flowers with three translucent petals 10 - 50 mm long by 4 - 8 mm wide, and male flowers with three white to red narrow petals about 2 mm long. The dioecious form of Hydrilla is believed to originate from the Indian subcontinent, specifically the island of Sri Lanka, although random DNA analysis also indicates India's southern mainland as a possible source location. The monoecious form is believed to have arrived on our shores from Korea.
- Langeland, K. A. 1996. Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), "The Perfect Aquatic Weed". Castanea 61:293-304.
- Menninger, H. 2011. Hydrilla verticillata in the Cayuga Inlet: A science‐based review to guide management actions. NY Invasive Species Research Institute, Cornell University, Ithaca, NY. 11 pp.