Mike Naylor, MD DNR
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Hydrilla
Hydrilla verticillata
Origin: Central Africa
Background
Hydrilla first appeared in the Crystal River system of
Florida in 1960. Imported by the aquarium trade, its
presence on the Delmarva Peninsula was confirmed in 1981.
Hydrilla attracted national attention when infestations
were found in the Potomac River in Washington, D.C. in the
early 1980s.
Distribution and Ecological Threat
Hydrilla is a federal noxious weed that is documented
throughout the southern United States from California to
Delaware. In the mid-Atlantic, hydrilla now exists in most
southern Delaware ponds, in the Delaware portion of the
Nanticoke River, in Virginia and Maryland freshwater
tributaries of the Chesapeake Bay, and several sites in
eastern Pennsylvania. Hydrilla out-competes native
submerged aquatic vegetation and can quickly fill a pond
or lake, thus choking off the water body for boating,
fishing, swimming and other recreational uses. It does
provide good habitat for fish and shellfish as well as
water quality benefits.
Mike Naylor, MD DNR
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Description and Biology
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Plant: rooted aquatic plant; a member of the frogbit
family (Hydrocharitaceae).
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Leaves: up to 3/4 inch long; usually in whorls of five
oblong leaves on the stems; fine teeth visible to the
naked eye on leaf edges and mid-ribs.
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Flowers, fruits and seeds: tiny, translucent to white
flowers produced on the upper branches in late summer
and fall; tubers grow from the roots; winter buds
(turions) are produced in the leaf axils.
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Spreads: vegetatively through fragments of stems,
stolons, or rhizomes, turions, or tubers which are
carried on boat livewells, motors and trailers, bait
pails and other items, and by ingestion of tubers and
turions by waterfowl.
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Look-alikes: common waterweed (Elodea
canadensis), Nutall's waterweed (E.
nuttallii), Eurasian watermilfoil (Myriophyllum
spicatum) and other aquatic plants with whorled
leaves.
Note: Most elodeas and watermilfoil species have smooth
leaf margins and leaves in whorls of three.
Mike Naylor, MD DNR
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Prevention and Control
Physical, chemical and biological controls have all been
used on hydrilla. Each control method has advantages and
liabilities. Water level drawdowns have generally been
ineffective in our area. Mechanical aquatic weed
harvesters provide temporary relief and open boating
lanes, but resulting plant fragments can help spread the
vegetation faster. Contact herbicides provide temporary
control, but systemic herbicides provide more long-term
control. Herbivorous fish such as sterile grass carp have
been used for hydrilla control where allowed by law. Other
biological controls are still being investigated. Each
control method has its drawbacks and liabilities. On the
Potomac River and other parts of the Chesapeake Bay
system, resource managers who are struggling with hydrilla
also recognize the beneficial impacts of submerged aquatic
vegetation (including hydrilla) for water quality and fish
and shellfish habitat.
Native Alternatives
Some aquatic nurseries carry native and non-invasive
alternatives. However, due to the similarity in appearance
among aquatic plants to the untrained eye, they are easily
confused. Contact your state natural resource agency,
native plant society or other resource (see
reference section) for assistance in locating species
appropriate to your location and site conditions.
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