Federal Noxious Weed List Invasive

Hydrilla verticillata

Hydrilla

Introduction

Origin: Warmer regions of the Old World
Introduction to Florida: 1950-1951 (ornamental)

Hydrilla is a submersed plant. It can grow to the surface and form dense mats. It may be found in all types of water bodies.

Habit

  • Hydrilla is a herbaceous perennial.
  • Submersed, “obligate” (requiring a wet habitat).
  • Forming dense stands of very long stems (25 ft.) in the water.
  • Reproduces mainly by regrowth of stem fragments; also reproduces by growth of axillary buds(turions) and subterranean tubers; tubers can remain viable for more than 4 years (Van & Steward 1990).
  • A single tuber can grow to produce more than 6,000 new tubers per m2 (Sutton et al. 1992).

Habitat

  • Hydrilla can grow in almost any freshwater: springs, lakes, marshes, ditches, rivers, tidal zones.
  • Can grow in only a few inches of water, or in water more than 20 feet deep.
  • Can grow in oligotrophic (low nutrient) to eutrophic (high nutrient) conditions.
  • Can grow in 7% salinity of seawater (Haller 1974).
  • Temperature tolerance: hydrilla is somewhat winter-hardy; its optimum growth temperature, 20-27o C (68-81o F); its maximum temperature, 30o C (86o F) (Kasselmann 1995).
  • U.S. southern populations overwinter as perennials; northern populations overwinter and regrow from tubers.
  • Can grow in only 1% of full sunlight.
  • Low light compensation and saturation points and low CO2 compensation point make it a competitive plant because it can start growing in low light before other plants do (Van et al. 1976; Bowes 1977).

 

  • There is only one species of Hydrilla in the world.
  • Hydrilla verticillata’s dioecious type (plants having female flowers only) originates from southern India. Hydrilla’s monoecious type (plants having male and female flowers on the same plant) is probably from Korea. (Madeira et al. 1997).
  • Occurs in Europe, Asia, Australia and the Pacific, Africa, South America and North America.

 

Hydrilla verticillata was probably brought to the Tampa and Miami, Florida areas as an aquarium plant in the late 1950s; by the 1970s, it was established throughout Florida. The monoecious type was
introduced separately much later in the Potomac Basin (Enrionmental Laboratory 1985).

Hydrilla verticillata continues to be sold through aquarium supply dealers and over the Internet, even though the plant is on the U.S. Federal
Noxious Weed List.

Description

Hydrilla stems are slender, branched and up to 25 feet long. Hydrilla’s small leaves are strap-like and pointed. They grow in whorls of four to eight around the stem. The leaf margins are distinctly saw-toothed. Hydrilla often has one or more sharp teeth along the length of the leaf mid-rib. Hydrilla produces tiny white flowers on long stalks. It also produces 1/4 inch turions at the leaf axils and potato-like tubers attached to the roots in the mud.

Original description: Linnaeus 1782; Royle 1839

  • Monocot, forb, submersed.
  • Rooted in the hydro-soil, adventitious roots are white.
  • The plant is submersed, except when branches have reached and grown across the water surface; sometimes found as detached floating mats.
  • Appearance can vary substantially, depending on growth conditions (Pieterse et al. 1985).
  • Stems submersed, slender (about 1/32 in. thick) and sinewy, long (to 25 feet).
  • Profuse branching occurs near the water surface.
  • Leaves small (5/8 in. long), strap-like, pointed tips, conspicuous midrib; arranged in whorls of 4 to 8, joined directly to the stem, whorl internodes 1/8 to 2 in. long.
  • Leaf margins distinctly visible saw-toothed; often with one or more sharp teeth along the underside midrib.
  • Leaf color green; clean leaves are transluscent; topped out leaves, bleached by the sun and attacked by fungus and bacteria, may appear yellowish to brownish-green.
  • Flowers, male and female.
  • Female flowers solitary, tiny, white, floating on the surface; reaching to the surface on “long” (to 4 in.) threadlike stalks, stalks attached at leaf axils near the stem tips; at surface, flower
    opens to form a wide funnel into the water; petal rims hold flower to surface and prevent water from getting into flower; female petals 6, to 1/16 in. long; stamens 3, styles 3, stigmas 3.
  • Male flowers tiny, greenish, closely attached to leaf axils toward stem tips, until they break loose and rise to the surface where they free-float, sometimes in large numbers, where
    they fertilize the female flowers by randomly bumping into them; male flowers at surface like “inverted bells.”
  • Other reproductive parts
  • Turions (“buds” in some of the leaf axils), are dark green, cylindrical, to 1/4 in. round, occasional.
  • Subterranean turions (“tubers”), are yellowish, potato-like, attached to the root tips in the hydrosoil, to 1/2 in. long, 1/2 inch broad.
Impacts
  • Hydrilla verticillata is an invisible menace, invisible that is until it fills the lake or river that it infests, “topping out” at the surface. Hydrilla can grow an inch a day. When hydrilla invades,
    ecologically-important native submersed plants such as pondweeds (Potamogeton spp.), tapegrass (Vallisneria americana) and coontail (Ceratophyllum demersum) are shaded out by hydrilla’s thick
    mats, or are simply outcompeted, and eliminated (van Dijk 1985).
  • Millions of dollars are spent each year on herbicides and mechanical harvesters in Florida alone in an effort to place hydrilla under “maintenance control.”
  • Hydrilla verticillata greatly slows water flow and clogs irrigation and flood-control canals; in Florida, large mats of fragments collect at culverts and clog essential water control pumping stations.
  • Hydrilla seriously interferes with boating, both recreational and commercial, and prevents swimming and fishing; major infestations limit sportfish weight and size (Colle & Shireman 1980).
  • Dense hydrilla infestations can alter water chemistry and oxygen levels (Pesacreta 1988).

Management Plan


Preventative

First, clean your boat before you leave the ramp! Transporting plant fragments on boats, trailers, and in livewells is the main introduction route to new lakes and rivers.

Cultural Mechanical

In some cases, lake drawdowns may help manage hydrilla by letting the exposed plants die and decompose.

The action of mechanical harvestors and chopping machines remove hydrilla from the water and transport it to disposal on shore; chopping machines, unfortunately, fragment the hydrilla plants and may actually increase the plant’s distribution.

Biological

Biocontrol fish and insects have been introduced to control hydrilla. The herbivorous (plant-eating) biological control fish, the Chinese grass carp, has a good preference for hydrilla (Cassani). Learn much more about the Chinese grass carp on this page of this web site. Other biological control work has been done for this species, including tuber-feeding weevils and leaf-eating flies.

Chemical

Registered aquatic herbicides do provide temporary control of hydrilla.

From the University of Florida Aquatic Weed Management Guide, Vandiver 1999.

According to this Guide, … copper, diquat, endothall and fluridone; As always, comply with federal law by following the herbicide label instructions, permissible sites and application rates.