Hydrilla: Hydrilla verticillata
Hydrilla is a submersed, much-branched, perennial herb, usually
rooted but frequently with fragments seen drifting in the water.
Stems can be more than 35 feet long. Hydrilla once was used as an
aquarium plant, and has become a weed of economic importance.
Hydrilla verticillata is the only species in this genus.
Look for first
- submersed leaves in whorls of 4 to 8
- saw teeth on leaf margins
- leaf underside usually with one or more bumps on midrib
||Leaves: submersed; in whorls of 3 to 8, 2-4 mm
(0.1-0.2 in) wide and 6-20 mm (0.2-0.8 in) long, bearing coarse
(visible) teeth along the margins and usually 1-4 small conical
bumps along the underside of the midrib, which is often red.
||Stems: slender with much branching and up to
10.6 m (35 ft.) long; eventually form dense tangled masses at the
||Flowers: female flowers solitary, on long
stalks, with three sepals and three petals, each about 4 mm (0.3
in.) long, whitish or translucent, floating at the water's
||Other characteristics: two types of special
vegetative propagules: turions - fattened leaf buds at stem nodes,
green, about 1.25 cm (1/2 in) wide; tubers - small (to 1cm long),
off-white, swollen ends of underground stems.
This submersed plant native to Africa and Southeast Asia is a
major aquatic weed throughout most of the world's warmer climates.
Hydrilla was introduced into Florida in the early 1950s and by the
early 1990s occupied more than 140,000 acres of public lakes and
rivers. Intensive interagency management has reduced the above
ground portions of hydrilla to under 50,000 acres.
However, once established, hydrilla produces
reproductive tubers numbering in the millions per acre in the soils
of Florida waterways. These tubers still impact nearly 140,000
acres and represent hydrilla's regrowth potential, if not
continually managed immediately after sprouting. Researchers have
not discovered methods to prevent or minimize tuber formation
Because of its aggressive growth rate, never transplant hydrilla
from waterway to waterway, and please clean all boats and trailers,
live wells, and diving gear of plant material before entering or
leaving a waterbody. Possession of hydrilla is illegal in Florida
without a special permit.
Origin in Old World, widely distributed in warmer regions of
Africa and Asia, found locally in Northern Europe, introduced into
South and Central America, U.S., and Australia.
Hydrilla can grow an
inch or more per day and can be found in water only a few inches
deep to the deepest parts of Florida's lakes and rivers. In
Florida, hydrilla produces dense canopies covering entire surfaces
of waterbodies within one or two years after it becomes
established. Hydrilla disperses quickly throughout a waterway by
stem fragments, buds, runners and tubers.
Why hydrilla must be managed
Hydrilla blocks waterways and limits boat traffic, recreation,
flood control and wildlife use. Almost 80 percent of hydrilla's
biomass is in the upper 2 feet of the water column producing a
dense canopy near the water surface. This exotic pest plant shades
out native submersed plant species, reduces oxygen levels and
degrades water quality.
Environmental damage caused by hydrilla
- Hydrilla canopies lower dissolved oxygen concentrations,
reducing aquatic life.
- Hydrilla decay doubles the amount of sediments that accumulate
in a water body.
- Dense hydrilla infestations can restrict water flow resulting
in flooding along rivers and canals.
- Hydrilla canopies produce ideal breeding environments for
- Dense hydrilla canopies shade out native submersed vegetation
- Hydrilla infestations restrict recreational activities such as
boating, swimming and fishing.
Image Credit: Center for Aquatic and Invasive Plants, University of Florida