Source: California Invasive Plant Council

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Invasive Plants of California's Wildland

Hydrilla verticillata
Scientific name   Hydrilla verticillata
Additional name information: (L.f.) Caspary
Common name hydrilla, water thyme, Florida elodea
Synonymous scientific names Hottonia serrata, Hydrilla angustifolia, H. dentata, H. ovalifolia, H. wightii, Leptanthes verticillatus, Serpicula verticillata, Vallisneria verticillata
Closely related California natives 0
Closely related California non-natives: 0
Listed CalEPPC Red Alert,CDFA A
By: Kris Godfrey

Distinctive features:

Hydrilla (Hydrilla verticillata) is a perennial, submersed aquatic plant, consisting of a series of individual green stems that bear tightly packed whorls of two to eight triangular leaves at each node. It has small leaf scales at the base of each leaf that can be seen with 10x magnification. The spear-shaped leaves are about five to seven times as long as wide, with serrated leaf margins and small spines on the lower surface of the leaf midrib. Hydrilla produces distinctive subterranean vegetative propagules (tubers) and swollen shoots (turions) in its leaf axils (Anderson 1987).


Hydrocharitaceae. Perennial aquatic. Stems: erect and elongate from rhizomes; may branch extensively; produces tuber-like rhizomes that spread horizontally. Leaves: small, oblong or spear-shaped, 0.38-0.75 in (1-2 cm) long and 0.06-0.08 in (1.5-2 mm) wide, arranged in whorls with generally 2-8 leaves per whorl; leaf margin serrate; midrib may have small spines or tooth-like conic bumps. Inflorescence: flowers solitary and inconspicuous. Flowers: there are two biotypes of hydrilla: dioecious (staminate and pistillate flowers on different plants) and monoecious (staminate and pistillate flowers on same plant); both occur in California.

Staminate flowers (male) are deciduous, free-floating, perianth in 2 whorls, each with 3 parts; 3 stamens present, 0.12-0.2 in (3-5 mm). The pistillate flowers are persistent, floating, perianth in 2 whorls each with 3 parts, 3 stigmas. Fruit: cylindrical; 0.2-0.5 in (5-13 mm) long, <0.25 in (3-6 mm) in diameter (Hickman 1993).< /FONT >



Hydrilla is capable of infesting any freshwater aquatic system in California. It has been observed in the Mojave and Colorado deserts, south and central coasts, San Francisco Bay Area, and Central Valley. Currently, isolated infestations of hydrilla are found in Shasta, Yuba, Lake, Calaveras, Madera, Mariposa, and Imperial counties. Typically, it is found in shallow(<11.5-16.5 ft or 3-5 m) water, but if the water is clear enough it may be found growing to depths of forty-eight feet (15 m) (Langeland 1990). It can tolerate some salinity and is sometimes found in upper estuaries. It grows better on mud than on sand. Growth is enhanced in water with agricultural runoff that raises nutrient levels (Parsons 1992).


Hydrilla is thought to have been brought into the United States from Eurasia, its native region, as an aquarium plant. It is endemic to Asia, southern Europe, and Africa and has naturalized in the South Pacific and Australia. It escaped from aquaria in the 1950s and 1960s (Balcuinas 1985) and was first found in California in Yuba County in fall 1976 (California Department of Food and Agriculture 1991). Hydrilla spreads mainly by stem fragmentation and sprouting from tubers and turions that break free of parent plants. Monoecious hydrilla, present but rare in the United States, can produce seed (Sutton and Van 1992).


Hydrilla forms large mats that fill the water column and can block or severely restrict water flow. Physical blockage reduces recreational quality (e.g., swimming and boating) of infested water systems, crowds out native plants, decreases habitat for fish and other wildlife, degrades water quality, and slows water flow in canals, thereby increasing sedimentation rates and impairing irrigation and drainage (Anderson 1987, Langeland 1990). Hydrilla is easily spread by people (e.g., by fragments on boats or fishing equipment) or by wildlife. Once established, it produces a bank of tubers and turions in the soil that may remain viable for three to five years (Van and Steward 1990, Anderson et al. 1992).



Hydrilla reproduces primarily by vegetative means in the United States. It can reproduce by fragmentation of stems, rhizomes (underground stems), and root crowns, and by the production of tubers and turions. A single viable node can produce stems and rhizomes, leading to production of an independent plant. Tubers are produced from stem tissue beneath the surface of the sediments. Turions are produced on the terminal portion of stems containing leaves (Anderson 1987). Monoecious hydrilla is also capable of producing seed, although seedlings have never been observed in a natural setting in the United States (Anderson 1987).


Although it is a perennial, hydrilla acts like an annual. Dieback of above-ground portions of the plant usually occurs in late fall and winter. In spring, when water temperatures exceed 59 degrees F (15 degrees C), hydrilla begins to grow, producing large amounts of biomass by late summer and early fall (Anderson 1987).

(click on photos to view larger image)


Tubers and turions also begin to sprout, forming new plants when sediment temperatures rise (Haller et al. 1976). Dioecious and monoecious hydrilla have different growth patterns. Dioecious hydrilla stems elongate rapidly to form a dense canopy in the water column; once the canopy is formed, plants spread horizontally by producing rhizomes. Monoecious hydrilla spreads horizontally across the sediments by producing rhizomes and new root crowns; stems then elongate and form a canopy in the water column (Anderson 1987).



Hydrilla is a CDFA A-rated noxious weed and is targeted for eradication whenever it is found in California. By law hydrilla eradication efforts are the responsibility of state and county governments. Suspected infestations of hydrilla should be reported to the local county agricultural commissioner’s office. Once its presence has been confirmed, eradication efforts will be coordinated by the CDFA Integrated Pest Control Branch. The first action is to quarantine the infested aquatic system to minimize risk of further spread. Where possible, this includes temporarily closing the area to all traffic (e.g. boaters, campers, hikers). When it is not possible to close the area, an intensive education and inspection effort is undertaken to minimize spread of hydrilla fragments, tubers, or turions.

Methods used to control and eradicate hydrilla vary with the size and condition of the infestation and the environmental sensitivity of the area. If the infestation consists of just a few plants, divers may be used to harvest and dispose of above-ground and below-ground plant parts. Herbicides may be applied to larger infestations or, where possible, the entire system may be drained and dredged. Dredged spoils and the sediments remaining in the system are then fumigated with a soil sterilant. Sterile, triploid grass carp (Ctenopharyngodon idella), a hydrilla-eating fish, were used along with harvesting, herbicides, and dredging to eradicate a large infestation in the Imperial Irrigation District. Grass carp are not available for general use in California because of concerns that they could displace native fish. Thus far they have been approved for use only in the Imperial Irrigation District.