Hydrilla spp.

Asia, Africa, and Australia (widely naturalised elsewhere)

SunFull Sun – Part Shade – Full Shade

HardinessZones 5–11

Overview

Hydrilla spp. is a submerged aquatic genus in the frogbit family, Hydrocharitaceae, usually treated as the single species H. verticillata, native to warm fresh waters of Asia, Africa, and Australia and now naturalised across much of North America and Europe. It is a rooted perennial with slender, branching stems that can grow 25 feet (7.5 m) or more from the bed to the surface, where they branch and spread into dense canopies. The leaves are small, 0.2-0.8 inch (5-20 mm) long, strap-shaped, and arranged in whorls of three to eight around the stem, with toothed margins and small spines on the midrib underside that feel rough. Tiny flowers are produced at the water surface, but most spread is vegetative, through stem fragments, overwintering turions, and starchy underground tubers that survive drawdown and herbicide. Hydrilla spp. is listed as a noxious or prohibited weed in the United States and other countries, where dense growth blocks waterways, crowds out native plants, and impedes boating and water flow. It grows across a wide range of light, depth, and water-chemistry conditions, which aids its spread.

Native Range

Native to warm fresh waters of Asia, Africa, and northern Australia; widely naturalised in North America, Central America, and parts of Europe.

Suggested Uses

It is not grown ornamentally and is prohibited from sale and cultivation in many regions. In its native range it gives food and cover for waterfowl, fish, and invertebrates. Management programs study it as a model invasive aquatic.

How to Identify

Recognised as a submerged aquatic with long, branching stems and small strap-shaped leaves 0.2-0.8 inch (5-20 mm) long in whorls of three to eight. Leaf margins are toothed, and small spines on the underside of the midrib make the leaves feel rough to the touch. Small tubers and overwintering turions on the stems and roots separate it from similar pondweeds and Elodea spp..

Appearance

Size & Dimensions

Height1' - 25'

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

Summer

Small flowers appear at the water surface mainly in summer, the female flowers floating on thread-like stalks. Flowering is sparse and easily missed, and reproduction is mostly vegetative. Turions and tubers form from late summer into autumn.

Detailed Descriptions

Flower Description

Translucent white to greenish

Foliage Description

Green to dark green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade SunFull Shade

Requires 2-10 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.0 - 8.0(Neutral)

357912

Soil Types

silt loam

Drainagewet

Water & Climate

Water Needs

High

Frost Tolerance

hardy

Care & Maintenance

Care Guide

Hydrilla spp. grows in still and flowing fresh water from a few inches to many feet deep, in full sun to shade, across a wide range of temperature and nutrient levels. It roots in mud but also grows from free-floating fragments, and it tolerates low light better than most submerged plants. Because it is a regulated noxious weed in many regions, it is not sold or planted, and its movement on boats and equipment is restricted by law. In managed water bodies it is controlled by mechanical harvesting, drawdown, grass carp, and aquatic herbicides, though tubers in the sediment allow regrowth. Fragments as small as a single node can start a new colony. Spread is favoured by warm, nutrient-rich, slow-moving water.

Pruning

Management rather than pruning applies. Stems are cut or harvested mechanically below the surface, but cut fragments regrow, so loose material is removed from the water during control work. Repeated harvesting through the season reduces the canopy without clearing the tubers in the sediment.