The Most Invasive Aquatic Weed in America
Hydrilla (Hydrilla verticillata) is a submerged aquatic plant widely regarded as the single most problematic aquatic invasive species in the United States. Capable of growing an inch per day under optimal conditions, forming dense surface-to-bottom mats that crowd out all native vegetation, and persisting through drought and herbicide treatment via underground tubers that remain viable in sediment for more than four years, hydrilla presents management challenges that no other aquatic weed matches. It has been documented in more than 30 U.S. states and Puerto Rico, and its range continues to expand through recreational boating.
Hydrilla belongs to the family Hydrocharitaceae and is the only species in genus Hydrilla. It is a monoecious or dioecious submersed herb — meaning plants may carry either separate male and female flowers on different plants, or both on the same plant, depending on biotype. Two distinct biotypes are established in the United States: a monoecious biotype primarily found in the Pacific Northwest and some eastern states, and the more widespread dioecious biotype concentrated in the South and East. These biotypes differ in their reproductive biology and herbicide sensitivity, which affects management strategy.
Origin and Introduction History
Hydrilla is native to a broad region spanning Africa, Asia, and Australia. It arrived in the United States in the 1950s and 1960s through the aquarium and water garden trade — most likely introduced via aquarium water dumped into canals or natural waterways in Florida. By the 1970s it had established permanent populations in Florida's waterways, and by the 1980s it had spread to Georgia, Texas, Virginia, and the Carolinas. Today it is found as far north as Washington state (Pacific Northwest biotype) and Connecticut, and as far west as California and Arizona.
The federal government listed hydrilla as a federal noxious weed under the Federal Noxious Weed Act, and it is prohibited in most states. Sale, transport, and possession are illegal in the majority of U.S. jurisdictions, though enforcement challenges have not prevented continued spread through contaminated boating equipment and bait bucket releases.
Why Hydrilla Is So Problematic
Several biological traits make hydrilla uniquely difficult to manage:
Exceptional Low-Light Tolerance
Hydrilla can photosynthesize at light levels as low as 1% of surface irradiance — far below the threshold for most native submerged plants. This allows it to colonize turbid water where natives cannot compete and to grow to depths of 10 meters or more in clear lakes, dramatically exceeding the colonization depth of native vegetation.
Rapid Growth Rate
Under optimal summer temperatures (25–30°C) and adequate nutrients, hydrilla stems elongate up to 2.5 cm (1 inch) per day. This rate allows infestations to recover rapidly after mechanical harvesting and can overwhelm early-season herbicide treatments if application timing is delayed.
Multiple Propagule Types
Hydrilla reproduces via four mechanisms: stem fragments (which root at every node), axillary buds (turions) that detach and overwinter in sediment, underground tubers that persist for 4+ years and are metabolically dormant during herbicide-vulnerable growth windows, and (in the monoecious biotype) seeds. No single treatment eliminates all propagule types simultaneously, which is why multi-year management programs are always required. See the detailed growth habit page for more on each propagule type.
Economic Impact
The USDA and various university studies have estimated that hydrilla causes hundreds of millions of dollars in annual economic losses in the United States — through reduced property values on infested lakes, costs of mechanical harvesting (typically $300–600 per acre per treatment), herbicide treatments ($150–500 per surface acre depending on chemical and dosage), impacts on recreational boating and fishing, damage to water intake structures and hydroelectric facilities, and effects on commercial fisheries.
Florida alone spends more than $30 million annually on hydrilla management in public waterways. Lake Guntersville in Alabama, Potomac River tidal waters, and the Santee Cooper lakes in South Carolina each represent multi-decade hydrilla management programs with ongoing annual costs in the millions.
Ecological Role in Native Range
In its native Asian and African range, hydrilla is typically kept in check by herbivory (including native grass carp, insects, and waterfowl), competition from other plant species, and natural pathogen pressure. In the United States, it lacks most of these natural controls, allowing populations to grow to sizes and densities not observed in its home range. The biological control program targeting hydrilla has released two tuber weevils (Bagous affinis and B. hydrillae) that reduce tuber banks over time, and a fly (Hydrellia pakistanae) that mines hydrilla leaves, but none of these agents alone provides the level of suppression that native-range herbivory achieves.
Identification at a Glance
Hydrilla is most easily identified by four features: (1) whorls of 4–8 leaves around the stem, (2) distinctly serrated (toothed) leaf margins visible with a hand lens, (3) a small raised tooth on the underside of the leaf midrib, and (4) the presence of white tubers at the sediment surface near roots. No other common U.S. aquatic plant has all four of these features. See the full hydrilla identification guide for detailed comparison to look-alikes.
References and Further Reading
- Langeland, K.A. (1996). Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), "The Perfect Aquatic Weed." Castanea 61(3):293–304.
- USDA NRCS PLANTS Database: Hydrilla verticillata. plants.usda.gov
- Cuda, J.P., et al. (2002). Overview of Biological Control of Hydrilla. Journal of Aquatic Plant Management 40:7–12.
- Center for Aquatic and Invasive Plants, University of Florida/IFAS. plants.ifas.ufl.edu