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.
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).
2006-11-09 22:16:29 補充：
www.ecy.wa.gov/programs/wq/plants/weeds/hydrilla.htm 
