Many aquatic weed scientists consider Hydrilla verticillata the most problematic aquatic plant in the United States. This plant, native to Africa, Australia, and parts of Asia, was introduced to Florida in 1960 via the aquarium trade. Hydrilla is now well established throughout water bodies in the southern states where control and management costs millions of dollars each year. From 1980 to 2005, Florida alone spent $174 million on hydrilla control. On the West Coast, California, Washington, and Idaho all have limited populations of hydrilla. Managers in all three states are serious about eradicating these infestations. Washington’s hydrilla infestation, discovered in 1995 in two interconnected lakes in King County, is the only known occurrence of hydrilla in Washington and eradication efforts are ongoing. Hydrilla is also increasingly being discovered in the northern tier states and in the Midwest.
Hydrilla forms dense mats of vegetation that interfere with recreation and destroy fish and wildlife habitat. Hydrilla has several advantages over other plants. It will grow with less light and is more efficient at taking up nutrients than native species. It also has extremely effective methods of propagation. Besides making seeds (seedlings are actually rarely seen in nature), it can sprout new plants from root fragments or stem fragments containing as few as two whorls of leaves. Recreational users can easily spread these small fragments from water body to water body.
However, hydrilla's real secret to success is its ability to produce structures called turions and tubers. (Presence of these structures is also a characteristic that distinguishes this species from similar looking plants.) Turions are compact and produced along the leafy stems. They break free from the parent plant and drift or settle to the lake bottom to start new plants. They are generally about a quarter inch long, dark green, and appear spiny. Tubers are underground and form at the end of roots. They are small, potato-like or pea-like, and are usually white or yellowish. Hydrilla produces an abundance of tubers and turions in the fall and the tubers may remain dormant for several years in the sediment. The hydrilla variety found in Washington will also make tubers in the spring and will produce non-dormant turions throughout the growing season. Tubers and turions can withstand ice cover, drying, herbicides, and ingestion and regurgitation by waterfowl. One square meter of hydrilla can produce 5,000 tubers!
There are two varieties of hydrilla in the United States. Many of the plants in the southern United States are all one sex (female) and are dioecious. Dioecious plants cannot produce seed. The plants in Washington are monoecious (having both male and female flowers on the same plant) and can produce seed. In New Zealand, where hydrilla is not native, the hydrilla plants are all male. Generally, the northern-most populations of hydrilla in the United States are monoecious. Although the hydrilla in Idaho is dioecious, all of Idaho’s dioecious hydrilla populations are associated with warmer geothermal-influenced waters. Monoecious hydrilla looks and grows somewhat differently than dioecious hydrilla. It tends to have a delicate appearance and sprawls along the lake bottom. The tubers from these monoecious plants are smaller than tubers produced by their southern female relatives.
Hydrilla is a federally listed noxious weed, listed as a Class A weed on Washington’s Noxious Weed List, and is on the Washington State Department of Agriculture’s Quarantine list. Weed scientists suspect that some of the hydrilla infestations in California resulted from hydrilla tubers hitch hiking on mail order water lily rhizomes. Plant managers also speculate that Washington’s only hydrilla infestation in Pipe and Lucerne Lakes near Seattle also resulted from contaminated water lilies. Non-native water lilies were once common in these two lakes (before lake managers started herbicide treatments for hydrilla).
Since the hydrilla discovery in 1995 in Pipe and Lucerne Lakes, there have been no other reports of hydrilla in Washington. State and local governments (King County and the cities of Covington and Maple Valley) are working together to eradicate the hydrilla infestation by using a combination of an aquatic herbicide called fluridone and diver and snorkeler hand removal. This is a multi-year ongoing effort because hydrilla tubers are long-lived and they do not all sprout at once. Prior to herbicide treatments (started in 1995) hydrilla densely covered the bottom of Pipe and Lucerne Lakes and had started to grow over the tops of Eurasian watermilfoil plants also in the lakes. As of 2009, surveyors have not detected any hydrilla plants in Lucerne Lake since 2004 and no hydrilla plants in Pipe Lake since 2006.
Hydrilla closely resembles two other aquatic plants found in Washington: The non-native plant Brazilian elodea - Egeria densa and the native plant American waterweed - Elodea canadensis. You can distinguish hydrilla from these look-alike species by the presence of tubers (0.2 to 0.4 inch long, off-white to yellowish, pea-like structures buried in the sediment). Neither Brazilian elodea nor waterweed has tubers.
Other characteristics to look for include:
We are especially concerned about new introductions of hydrilla in the Pacific Northwest. If you think that you have seen hydrilla growing in Washington, please contact Kathy Hamel (firstname.lastname@example.org) or Jenifer Parsons (email@example.com) immediately.
The hydrilla line drawing is the copyright property of the University of Florida Center for Aquatic Plants (Gainesville). Used with permission.
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