Submerged Aquatic Vegetation (SAV) Identification Key

​​The bay grass key was designed to allow you to identify most species of bay grasses found in Maryland. Although bay grasses are notoriously difficult to identify using standard taxonomic keys, the flexible format of the Internet allows us to combine detailed pictures, simple line drawings and text messages in a stepwise sequence that makes identifying bay grasses simple. You may find it useful to have a clear metric ruler with millimeters marked, a magnifying glass, and a Ziploc plastic bag to help you in the process of identifying your plant.

I don't know the species

Use the SAV Key
I know the species
​If you already know the identity of a particular bay grass use the drop down boxes below.

Common Name:  
Scientific Name:

Common Name:Hydrilla

Scientific Name:Hydrilla verticillata

Native or Non-native:Non-native

Illustration:Hydrilla collage

Link to larger illustration:

Printable Version:


Hydrilla is an exotic species introduced from southeast Asia which first appeared in the United States in the 1960's. Today it is found in most of the southeastern United States, westward to California. Hydrilla was first observed in 1982 in the Potomac River near Washington D.C. and by 1992 grew to cover 3,000 acres in the Potomac. Fears that hydrilla would take over and permanently alter the Potomac ecosystem proved to be unfounded, as many of the areas that hydrilla expanded into were subsequently colonized by a host of native species. Hydrilla is now cosmopolitan in the mid-Atlantic, and is found in streams, ponds, lakes and both tidal and non-tidal rivers throughout Maryland. Hydrilla is predominantly a freshwater species, but has been found in waters of 6-9 ppt salinity. Hydrilla prefers silty to muddy substrates and tolerates lower light than other bay grass species.

Stems are freely branching with whorls of 3-5 linear to lanceolate leaves. Leaves have strongly toothed or serrated margins and a spinous midrib. Roots are adventitious, forming along nodes of rhizomes that grow horizontally atop or just below sediment surface. Tubers are also commonly found at the end of runners that branch from the buried rhizome.

Ecological Significance:
Hydrilla is an introduced species first identified in the Chesapeake Bay region in 1982. It is often considered a nuisance plant because of its habit of forming dense impenetrable beds that impede recreational uses of waterways. Because of the substantial populations of hydrilla found in the Potomac River when it first appeared, a mechanical harvesting program was instituted to keep the many marinas along the river open to boat traffic. Since that time hydrilla has gone through boom and bust cycles in many rivers, usually falling in abundance to the point where it's no longer a significant impediment to water contact recreation within a few years. Hydrilla is an excellent food source for waterfowl and provides excellent fish habitat.

Similar Species:
Common waterweed (Elodea canadensis) has a similar appearance, however, leaves of waterweed are in whorls of 3 and are not as markedly toothed as those of hydrilla. Common waterweed also lacks the tubers that hydrilla forms in late summer or early fall.

Hydrilla reproduces sexually and asexually. The strain found in Chesapeake Bay is monoecious, with male and female flowers occurring together near the growing stem tips. Small, white female flowers are born on a hypanthium at the water surface. Male flowers detach from stem tips and float to the water surface. The pollen they release must settle directly on the female flower for pollination to occur. Seed set has never been documented in hydrilla in Chesapeake Bay. Asexual reproduction occurs through fragmentation, production of new stems from rhizomes, turions (resting plant buds that develop in leaf axils or tips of branching stems) which break off then sink to the substrate and form a new plant, and tubers (another type of resting plant bud) that develop at ends of buried runners that branch off from rhizomes. Tubers and turions can over-winter and are the major form of reproduction in the mid-Atlantic region.

Print out a complete version of the key in PDF format (Adobe Acrobat file 18MB)

For permission to reproduce individual photos, please contact Mike Naylor

The text and photos used in this key were produced through a collaborative effort among the following partners.

SAV Key Sponsor Logos