Introduced Pests Outreach Project


(Click on an image below to see the captioned full-size version)
Figure 1
Figure 2
Figure 3
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Figure 5
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Figure 8

Scientific Name: Hydrilla verticillata
Common Names: Hydrilla, Water Thyme

Hydrilla is a noxious water weed that can quickly form an impenetrable mat. In the Northeast this weed has been found in one pond on Cape Cod, MA and one pond in Stonington, Connecticut. Figures 1 and 2 illustrate the amount hydrilla grew over 18 days in a CT pond. See the invasive alert on the Invasive Plant Atlas of New England website:

Almost any body of fresh water: lakes, ponds, rivers, canals, marshes

Key ID Features:
A submersed water plant with long stems (up to 25 feet in length) which branches profusely at the surface. Plant forms a dense mat. (Figure 3 and 4)
Leaves are small (3/4 inch) and pointed with toothed margins. (Figure 5)
4 to 8 leaves are arranged in a whorl (leaves radiate from a single point on the stem) (Figure 5)
The midrib of each leaf is often reddish with one or more sharp teeth.
¼ inch long turions are vegetative buds produced in the axils of the leaves.
Potato-like tubers are attached to the roots in the mud (Figure 6)
This plant can reproduce from seed, turions, tubers, or small fragments of the vegetation that may be transported on boats and trailers. (Figure 7)

Similar species:

Brazilian elodea (Egeria densa)
Non-native invasive waterweed (Figure 8)
4 leaves per whorl
Each leaf is at least 2 cm long.
Does not produce tubers
American waterweed (Elodea canadensis)
Native water plant
3 leaves per whorl
Each leaf is less than 1 cm long
Does not produce tubers
Native Elodea (Elodea nuttalli)
Native water plant
Characteristics very similar to the American waterweed (E. canadensis)

The following website contains photos and descriptions of hydrilla and similar species:
Nonnative Fresh Water Plant: Hydrocharitaceae- Washington State Department of Ecology

Fact sheets and references:
Hydrilla Fact sheet: Massachusetts Dept. of Conservation and Recreation Lakes and Ponds Program

Invasive Plant Atlas of New England
Invasive Alert with details of New England finds of Hydrilla verticillata

Center for Aquatic and Invasive Plants, University of Florida and Sea Grant
Excellent web page about hydrilla with many photos

Early Detection and Distribution Mapping System.
Map of hydrilla distribution in the United States

The Nature Conservancy: Element Stewardship Abstract for Hydrilla verticillata

Northeast Aquatic Nuisance Species Panel: Hydrilla Profile Page

Smithsonian Marine Station at Fort Pierce

last reviewed December 22, 2014

Massachusetts Department of Agricultural Resources
The Massachusetts Introduced Pests Outreach Project is maintained by staff at the Massachusetts Dept. of Agricultural Resources. This website was made possible, in part, by a Cooperative Agreement from the United States Department of Agriculture's Animal and Plant Health Inspection Service (USDA APHIS). It may not necessarily express APHIS' views.