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Photo of a clump of hydrilla held in a hand
Hydrilla is probably the worst submersed aquatic weed in America. It harms aquatic communities in small ponds, lakes, and rivers. It hurts our economy by hindering fishing and other recreational uses in large reservoirs.
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
Invasive
Family

Hydrocharitaceae (frogbits)

Description

Hydrilla is a submerged aquatic plant rooted to the bottom with potato-like tubers attached to a root structure. Stems branch little until they reach the surface; just under the surface it branches profusely, forming thick mats. Leaves narrow, less than ¼ inch wide, ½-¾ inch long, not needlelike, finely toothed, in whorls of 5 (or 3-8). Leaf midrib is often red. The potato-like tuber attached to the root structure is a good way to identify it.

Similar species: Hydrilla looks quite a bit like the giant elodea or anacharis familiar to aquarists, and also like two species of elodea that are native to Missouri. All these plants are in the same family. Hydrilla, however, is the only one that grows from small potato-like tubers. Our native elodeas have leaves in whorls of 3 or in pairs, while hydrilla has leaves in whorls of 3-8 (usually 5):

  • Two species of Elodea are Missouri natives: Both lack tubers; the leaves are mostly in whorls of 3 or in pairs; and stems sparsely branched or unbranched.
  • The nonnative giant elodea or anacharis (Egeria densa), a popular aquarium plant, is sparsely branched, lacks tubers, has leaves in whorls of 4-6, and in Missouri occurs only sporadically and locally when it escapes from cultivation.
  • Coontail (Ceratophyllum demersum) has needlelike, forked leaves and is bushy and heavily branched throughout.
Size

Stem length: can exceed 26 feet.

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Photo of small hydrilla plants with ruler for scale
Hydrilla Plants with Ruler
There are many ways hydrilla spreads — by cuttings, tubers, seeds, and overwintering buds. Americans pay millions of dollars a year trying to stop or delay its spread. Once it’s in a body of water, eradication requires several years of season-long control.

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Photo of hydrilla closeup showing teeth on leaf margins
Hydrilla Leaves
Look closely at hydrilla's leaves. Even the narrowest leaves are definitely flattened, not needlelike. The leaf edges have teeth large enough to see with the naked eye. The leaves often have a reddish midvein. The leaves usually grow in whorls of 5.

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Photo of three potato-like tubers of hydrilla plant
Hydrilla “Potatoes”
Small, potato-like tubers are characteristic of invasive hydrilla. They form in the mud at the base of the plant and are attached to roots. A single one of these tubers, if accidentally transported to a new body of water, can start a new colony.

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Photo of several hydrilla stems just below surface of water
Hydrilla Below Water Surface
Hydrilla looks quite a bit like the giant elodea or anacharis familiar to aquarists, and also like two species of elodea that are native to Missouri. All these plants are in the same family. Hydrilla, however, is the only one that grows from small potato-like tubers.

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Photo of big clump of hydrilla stems tangled around a boat motor
Hydrilla on Boat Motor
This tangled wad of hydrilla shows one way it’s a problem, as well as one way it gets dispersed. Just a tiny fragment of stem, or a tuber, shoot, or seed, is enough to start a whole new colony someplace else.

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Photo of whole hydrilla plants out of water
Whole Hydrilla Plants Out of Water
Hydrilla has potato-like tubers attached to roots, and flattened (not needlelike) leaves in whorls of 3-8 (usually 5). Learn to identify it, and report its presence.

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Photo of hydrilla infestation
Hydrilla Infestation
Even though these particular hydrilla plants have rather narrow leaves, they are still flattened. Leaf shape distinguishes hydrilla from our native coontail, which has branching, needlelike leaves.

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Photo of hydrilla stem showing leaf whorl with five leaves
Hydrilla Leaf Whorl
This is what botanists mean when they say this plant has leaves in whorls of five. Each node of the stem is encircled by a number of leaves. In hydrilla, the number can vary from 3 to 8 per node, but 5 is most common.
Habitat and conservation

A native of Eurasia, this submerged, rooted aquatic plant forms large, branching masses close to the water surface. It can grow in deeper water than coontail.

It invaded Florida in the 1950s from introduced aquarium plants. It spreads via dumped aquarium water, hitching rides on boats, boat trailers, and other gear, and as a contaminant when people introduce other water plants. Waterfowl can transport it from infested states. Small fragments of stems can flow downstream to form new colonies.

This invasive nonnative plant grows quickly, branches profusely, and forms a dense mat just under the water surface, shading out the plants below. Because it stays below the surface, many people are unaware they have hydrilla until it’s too late.

image of Hydrilla distribution map
Distribution in Missouri

First discovered in Greene, Dallas, and Warren counties, but because it can spread quickly, potentially statewide. Eradication efforts are under way, but it’s critical for Missourians to learn to identify it and prevent further spread.

Status

Hydrilla now infests states in the south and southeast and the East and West coasts. Americans pay millions of dollars a year trying to stop or delay its spread. Eradication requires several years of season-long control. A single herbicide treatment won’t kill it, nor will a single year of several treatments. It grows rapidly and easily reproduces via cuttings. A single whorl of leaves is enough to start a new colony. The tubers and overwintering buds can survive in a dormant state for years.

Human connections

Hydrilla spoils fishing and aquatic recreation and blocks water intake systems. It’s far easier to prevent its spread than to get rid of it once it’s arrived. Clean and dry boating and fishing gear before moving to a new body of water. Never release an aquarium’s contents into natural waters.

Ecosystem connections

Hydrilla is more aggressive than our native aquatic plants such as coontail. When it covers the surface in a thick mass, it blocks sunlight below, harming the native aquatic plant and animal community, including fish populations and even the water chemistry.