Hydrilla

(Click on an image below to see the captioned full-size version)
Figure 1		Figure 2		Figure 3		Figure 4		Figure 5		Figure 6
Figure 7		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: http://nbii-nin.ciesin.columbia.edu/ipane/invasivealerts/invasives_hydrilla.htm

Habitat:
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
http://www.ecy.wa.gov/programs/wq/plants/weeds/aqua008.html

Fact sheets and references:
Hydrilla Fact sheet: Massachusetts Dept. of Conservation and Recreation Lakes and Ponds Program
http://www.mass.gov/eea/docs/dcr/watersupply/lakepond/factsheet/hydrilla.pdf

Invasive Plant Atlas of New England
Invasive Alert with details of New England finds of Hydrilla verticillata
http://nbii-nin.ciesin.columbia.edu/ipane/invasivealerts/invasives_hydrilla.htm

Center for Aquatic and Invasive Plants, University of Florida and Sea Grant
Excellent web page about hydrilla with many photos
http://plants.ifas.ufl.edu/node/183

Early Detection and Distribution Mapping System.
Map of hydrilla distribution in the United States
http://www.eddmaps.org/distribution/usstate.cfm?sub=3028

The Nature Conservancy: Element Stewardship Abstract for Hydrilla verticillata
http://www.imapinvasives.org/GIST/ESA/esapages/documnts/hydrver.pdf

Northeast Aquatic Nuisance Species Panel: Hydrilla Profile Page
http://www.northeastans.org/hydrilla/hydrillaprofile.htm

Smithsonian Marine Station at Fort Pierce
http://www.sms.si.edu/irlspec/Hydrilla_verticillata.htm

last reviewed December 22, 2014