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Dense H. verticillata stands can overtake and choke water bodies as seen in this 1998 photograph from Wakulla Springs, Florida. Photo courtesy UF/IFAS Center for Aquatic and Invasive Plants. Photographer Vic Ramey.

Species Name: 
Hydrilla verticillata (L. f.) Royle

Common Name(s):
Hydrilla, Water Thyme, Florida Elodea, Indian Star Vine

Species Description:
Hydrilla, Hydrilla verticillata, is an aquatic weed not native to Florida but widespread within the state. It is a submerged, rooted (usually) perennial with long, sinewy branching stems that often reach the surface and form dense mats. The stems are covered in small, pointed, often serrate leaves arranged in 4-8 whorls. Leaf midribs are often reddish with one or more sharp spines. Small pale subterranean tubers occur attached to the roots. Small white flowers grow above the water line on stalks (Godfrey and Wooten 1979, Illinois-Indiana Sea Grant College Program undated).

H. verticillata exhibits a degree of penotyoic plasticity (variable physical appearance) in response to age, habitat conditions, and water quality (Kay 1992). Branching is generally sparse in submerged portions of the plant, tending to become profuse at the surface (Langeland 1996).

Potentially Misidentified Species:
In Florida, Hydrilla may be easily mistaken for a related (confamilial) non-native aquatic weed Egeria densa with which it co-occurs in much of the state. The leaves of E. densa occur in whorls of 3-6 and have very fine serrations that can only be discerned under magnification.

Hydrilla can also be confused with a native aquatic plant, Elodea canadensis, although this species only occurs in northernmost Florida.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Two distinct introduced Hydrilla verticillata biotypes exhibiting different reproductive traits (see below) occur in the United States. The dioecious southern form found in Florida appears to have originated from the Indian subcontinent, while the monoecious northern form (occurring north of North Carolina) appears to be derived from stock originating in Korea (Schmitz et al. 1991, Madeira et al. 1997).

Hydrilla occurs in freshwater throughout most of peninsular Florida.

IRL Distribution:
Florida Exotic Pest Plant Council (FLEPPC) collection records confirm the presence of Hydrilla verticillata in Volusia, St. Lucie, Martin, and Palm Beach counties. Although not reflected in the FLEPPC records, collection records from the UF/IFAS Lakewatch water quality monitoring program and elsewhere suggest hydrilla is also widespread in freshwater systems within Brevard and Indian River counties.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
The sinewy branching stems of Hydrilla verticillata regularly reach 2 m length and can attain lengths of more than 7.5 m (Cook and Luond 1982, Langland 1996).

Hydrilla is an herbaceous perennial that experiences seasonal winter dieback (Carter et al. 1994).

Abundance:
Schardt (1994) refers to Hydrilla verticillata as the most abundant aquatic plant in Florida public waters, and hydrilla-infested waterbodies occur in seventy percent of the state's freshwater drainage basins. Schardt (1997) reported hydrilla was present in 43% of Florida's public water bodies in 1994, representing an estimated coverage of 38,500 ha.

Reproduction:
The southern Hydrilla verticillata biotype that occurs in Florida is comprised mostly of dioecious (one sex) female plants. Although extensive flowering may occur, the lack of male plants indicates that sexual reproduction is essentially non-existent. The northern (non-Florida) biotype consists of monoecious individuals (male and female flowers occur on the same plant), and at least some sexual reproduction resulting in the setting of fertile seed occurs in this population (Langland and Smith 1984, Madiera et al. 2000). Where reproduction through flowering does occur, wind-pollination is the means of fertilization (Steward et al. 1984).

Reproduction of hydrilla in Florida is predominantly if not exclusively through vegetative means. Vegetative strategies include regrowth from stem fragments and clonal reproduction via runners, rhizomes, and tubers (Pieterse 1981, Hurley 1990). Sutton et al. (1992) indicate that one tuber can give rise to as many as 6,000 new tubers per square meter, and Van and Steward (1990) notes tubers can remain viable longer than 4 years.

Vegetative reproduction also occurs via specialized axillary buds called turions which reside on the stems in the water column and are somewhat smaller than tubers (Yeo et al. 1984, Spencer et al. 1994). Turions, which are actually underground tubers, become detached from the parent plants to disperse through water movement to new location where they can grow vegetatively into new plants (Hofstra et al. 1990). In colder climates, turions are important as overwintering organs.

Embryology:
Where seed production occurs, seeds are poorly dispersed and usually sink to the sediment with perhaps some water current transport occurring as well. Seed set occurs around September and germination commences the following April-May. Maturation is rapid, and plants are capable of flowering just two months after germination (Steward et al. 1984). Hurley (1990) reports seed germination rates are generally less than 50%. Germination of turions occurs at around 18°C (Hurley 1990, Pieterse 1981).

IV.  PHYSICAL TOLERANCES

Temperature:
The U.S. distribution of Hydrilla verticillata extends into temperate regions where prolonged freezing winter temperatures occur. Freezing temperatures result in dieback of hydrilla stem, but overwintering turions and sub-sediment biomass including vegetative tubers survive to grow new shoots in the spring.

Barko and Smart (1981) report the following responses of hydrilla to various temperature treatments. At 20-24°C, optimum rates of photosynthesis were attained. At 16°C, photosyntheis was diminished but some growth still occurred. At 0°C, the water column biomass dies back but sub-sediment biomass survives.

Although the majority of hydrilla in the temperate United States is derived from the monoecious strain, genetic studies by Les et al. (1997) concluded that an established population in Mystic, CT, is actually derived from the dioecious Indian strain. With additional testing, the dioecious strain may be revealed to be more capable of temperate range extension than previously believed.

Salinity:
Hydrilla verticillata exhibits moderate salinity tolerance, persisting in a laboratory environment at 7 ppt when transitioned in one step from fresh water, and at up to 12 ppt when the transfer was gradual (Haller et al. 1974, Twilley and Barko 1990).

Moderate halotolerance allows H. verticillata to occupy the upper reaches of estuaries such as Chesapeake Bay (Carter et al. 1994) and the Lower St. Johns River.