General Information About Hydrilla
Hydrilla (Hydrilla verticillata) is considered the most problematic aquatic plant in the United States. This plant is native to Africa, Australia, and parts of Asia but was introduced to Florida in 1960 via the aquarium trade. In the 1990s hydrilla is now well-established in the southern states where control and management costs millions of dollars each year. Florida spent $56 million dollars for hydrilla control during a ten-year period and, during this time, the acreage of hydrilla doubled. On the West Coast, hydrilla has been introduced into California and Washington. California has an eradication policy for hydrilla infestations because hydrilla can severely impact water delivery systems. The Washington hydrilla infestation, discovered in 1995, is the only known occurrence of hydrilla in the Pacific Northwest and eradication efforts are ongoing.
Growth Habit
Hydrilla forms dense mats of vegetation that interfere with recreation and destroy fish and wildlife habitat. Unlike other problem aquatic plants, like Brazilian elodea, that reproduce only by fragmentation, hydrilla spreads by seeds, tubers, plant fragments, and turions (overwintering buds). One square meter of hydrilla can produce 5,000 tubers. Once hydrilla becomes established, it is readily spread by waterfowl and boating activities.
Hydrilla has several advantages over other plants. It will grow with less light and is more efficient at taking up nutrients than other plants. 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 waterbody to waterbody.
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 which distinguishes this plant from similar looking plants.) Turions are compact "buds" produced along the leafy stems. They break free of the parent plant and drift or settle to the bottom to start new plants. They are 1/4 inch long, dark green, and appear spiny. Tubers are underground and form at the end of roots. They are small, potato-like, and are usually white or yellowish. Hydrilla produces an abundance of tubers and turions in the fall. 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 nondormant turions throughout the growing season. Tubers and turions can withstand ice cover, drying, herbicides, and ingestion and regurgitation by waterfowl.
There are two varieties of hydrilla in the United States. Many of the plants in the southern United States are all one sex (female). The plants in Washington are monoecious (having both male and female flowers on the same plant). In New Zealand, where hydrilla has also been introduced, the hydrilla plants are all male. Generally the northern-most populations of hydrilla in the United States are monoecious. Monoecious hydrilla looks and grows somewhat differently than the southern female populations. It tends to have a delicate appearance and sprawl along the lake bottom. The tubers from these monoecious plants are smaller than tubers produced by their southern female relatives.
Management
Hydrilla is not being sold today, but it was recently introduced to California as a contaminant of water lily rhizomes. The hydrilla discovered in Washington was growing in two interconnected privately-owned lakes near Seattle. Because introduced water lilies are common in these two lakes, we suspect that that water lilies may have been the method of hydrilla introduction to these lakes. A diver survey of nearby lakes showed that hydrilla remained confined to the two-lake system, and since the hydrilla discovery in 1995, no other hydrilla populations have been discovered in Washington.
Hydrilla can be controlled by the application of aquatic herbicides and it is also highly preferred by grass carp. State and local governments are working together in an attempt to eradicate the hydrilla infestation in Washington by using an aquatic herbicide called Sonar. This is a multi-year effort (2001 is our seventh year of management since its discovery) because the tubers are long-lived and they do not all sprout at once. However, the number of plants in the lakes in 2000 was much reduced over the 1995 plant population. Before 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.
Identification
Hydrilla closely resembles two other plants found in Washington: The nonnative plant Brazilian elodea (Egeria densa) and native American waterweed (Elodea canadensis).
Hydrilla can be distinguished from these two plants by the presence of tubers (0.2 to 0.4 inch long, off-white to yellowish, potato-like structures buried in the sediment).
Other characteristics to look for include:
- Leaves in whorls around the stem (generally five leaves per whorl).
- Serrations or small spines along the leaf edges.
- The midrib of the leaf is often reddish when fresh.
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 (kham461@ecy.wa.gov) or Jenifer Parsons (jenp@ecy.wa.gov) 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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