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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.
Source: Washington State
Department of Ecology |
