Hydrilla verticillata

Name and classification

Hydrilla verticillata (L.f.) Royle
Family Hydrocharitaceae

Common names

hydrilla

Disseminule

tubers, turions, stem fragments, or stolons

Description

An annual or perennial submersed freshwater herb; rooted in the soil, or sometimes free-floating; underwater branches bear sessile whorled leaves with toothed margins. Tubers develop in soil at ends of stolons; turions (specialized buds enclosed by scales) form in leaf axils. Both monoecious and dioecious biotypes exist

Identification considerations

Leaf number, stem elongation and leaf shape are highly variable; difficult to distinguish from Egeria and Elodea species.

Distinguishing characteristics of Hydrilla verticillata:

  • in mature plants, midveins on lower surface of leaves have sharp spicules;
  • teeth on leaf margins more widely spaced than those on Elodea leaves; because of these features, Hydrilla feels scratchy when drawn through the hands;
  • leaf axils bear pairs of minute scales, to 0.5 mm long, fringed with orange-brown hairs.

Distribution

worldwide; in ca. 60 countries in Africa, Asia, Europe, Australia, Pacific Islands, and North and South America

probably native to southern Asia and possibly Africa

Habitat

wide range of ecological habitats: tropical to temperate climates, in still or slow-flowing water 0.5–3(12) m deep and 10º–35º C; found in lakes, ponds, rivers, canals, rice fields, ditches, and can tolerate polluted water

General information

Hydrilla verticillata is one of the most serious and troublesome aquatic weeds in the world's waterways. Its massive spread is largely due to human activities, such as boating and fishing and the aquarium trade. It is a severe problem in several areas in the U.S.; in Florida it has infested a large percentage of the states waters. Large subsurface masses of underwater branches form that impede navigation, interfere with hydroelectric plants, clog irrigation canals, affect water supplies, and block light, displacing native plants. This species produces four distinct vegetative disseminule types, undoubtedly one of the reasons for its prodigious spread. The only known seed production is in monoecious populations in North Carolina.

photo: © S.L. Winterton

photo: © S.L. Winterton

photo: © S.L. Winterton

photo: © S.L. Winterton

photo: © S.L. Winterton

photo: © S.L. Winterton

photo: © California Department of Food & Agriculture/Integrated Pest Control

photo: © California Department of Food & Agriculture/Integrated Pest Control

turions; photo: R.J. Helton © California Department of Food & Agriculture/Integrated Pest Control

turions; photo: R.J. Helton © California Department of Food & Agriculture/Integrated Pest Control

tubers and turions; photo: John Clayton © National Institute of Water & Atmospheric Research, New Zealand (NIWA)

tubers and turions; photo: John Clayton © National Institute of Water & Atmospheric Research, New Zealand (NIWA)

A, seed; B, longitudinal section of seed; C, transverse section of seed; drawing by Lynda E. Chandler

A, seed; B, longitudinal section of seed; C, transverse section of seed; drawing by Lynda E. Chandler