Hydrilla
(Hydrilla verticillata (L.f.) Royle)
Origin | Habitat | Introduction | Impacts | Eastern US Occurrences | Linked Resources
NOTE: The following information is taken from the US Geological Survey Nonindigenous Aquatic Species website Hydrilla factsheet. You will find a link to the full factsheet elsewhere on this page.
Hydrilla has recently (late-August 2011) been discovered in the Cayuga Lake Inlet in Ithaca, NY. Click here for the
Cayuga Inlet
Hydrilla Brochure which talks about the plant, the threat, and how to tell
Hydrilla from common
native water weed, Elodea Canadensis.
Background
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, pointed,
often serrated leaves are arranged around the stem in whorls of 4 to 8. Southern populations are predominantly dioecious female (plants having only female flowers) that overwinter as perennials.
Populations north of South Carolina are essentially monoecious (having both male and female flowers on the same plant) that set some fertile seed, and depend on tubers for overwintering.
Hydrilla infestation of small lake. Image credit: Tim Murphy, University of Georgia, Bugwood.org
The dioecious type 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.
Emergent stems and leaves of Hydrilla. Image credit: David J. Moorhead, University of Georgia, Bugwood.org
Hydrilla can be found infesting freshwater lakes, ponds, rivers, impoundments
and canals.
The dioecious strain was imported as an aquarium plant in the early 1950s. Discarded (or intentionally planted ) hydrilla was found in canals in Miami and Tampa shortly after.
The monoecious strain was introduced separately decades later in the Potomac Basin.
The main means of introduction of hydrilla is as castaway fragments on recreational boats and trailers and in their live wells. New colonies can often
be found near boat ramps as such stem pieces become rooted in the substrate. Boat traffic through established populations can shatter and spread hydrilla throughout
the waterbody, similar to the spread of Eurasian watermilfoil. Both dioecious and monoecious hydrilla propagate primarily by stem fragments, although turions (buds)
and subterranean tubers also play an important role.
Hydrilla is often a contaminant on popular watergarden plants and may be unwittingly transported and established in private ponds in this manner. As with most invasive
species, hydrilla is a very opportunistic organism and can often be found taking over waters that have had populations of Eurasian watermilfoil chemically removed without
a management plan for reestablishing native vegetation.
Stem and leaves of Hydrilla. Image credit: Robert Vidéki, Doronicum Kft., Bugwood.org
Hydrilla results can invade deep, dark waters where most native plants cannot grow. The plant’s aggressive growth (hydrilla’s 25 foot stems can add up to an inch per day) can spread into shallower waters
areas and form thick mats that block sunlight to native plants below, effectively displacing the native vegetation of a waterbody.
Major colonies of hydrilla can alter the physical and chemical characteristics of lakes. Reduced sportfish weight and size occurs as open water space and natural vegetation are lost.
Stratification of the water column, decreased oxygen levels, and fish kills have been documented. Thick growth of hydrilla can obstruct boating, swimming and fishing and have negative
impacts on agricultural irrigation and other water withdrawal uses.
Waterbodies infested with hydrilla can be found in 70% of Florida's freshwater
drainage basins, making it the most abundant aquatic plant in that state’s
waters. Hydrilla is also widespread throughout Alabama; impoundments on the
Tennessee River; eastern Mississippi; southeastern Tennessee; southwestern
Georgia; South Carolina; eastern North Carolina; in Virginia’s Potomac,
Rappahannock, and Appomattox Rivers and into the piedmont, in the tidal
freshwater reaches of the Potomac River on the Virginia/Maryland border; along
the western and northeastern shores of the Chesapeake Bay, including the
Pautuxent River, where it is the most abundant plant species; Pennsylvania (in
the Schuylkill River near downtown Philadelphia); eastern Kentucky; in ponds in
Delaware; southeastern Connecticut; in a Cape Cod pond in Massachusetts; in
southwestern Maine; in New Jersey’s Lower Delaware drainage; Indiana's Lake
Manitou; Wisconsin; and since 2008, in three New York lakes in Suffolk and
Orange Counties.
Hydrilla can also be found at numerous sites west of the Mississippi River.
US distribution of Hydrilla,
pre-Cayuga Inlet Credit:
USGS NAS
Langeland, K. A. 1996. Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), "The Perfect Aquatic Weed". Castanea 61:293-304.
Sousa, W. T. Z. 2011. Hydrilla verticillata (Hydrocharitaceae), a recent invader threatening Brazil's freshwater environments: a review of the extent of the problem. Hydrobiologia 669:1-20.
C.C. Jacono, M.M. Richerson), and V. Howard Morgan. 2011. Hydrilla verticillata. USGS Nonindigenous Aquatic Species Database, Gainesville, FL. US Geological Survey, US Department of
the Interior.
http://nas.er.usgs.gov/queries/factsheet.aspx?speciesid=6
Hydrilla verticillata. Pennsylvania Sea Grant factsheet.