Why This Comparison Matters
Hydrilla (Hydrilla verticillata) and elodea (Elodea canadensis) are among the most commonly confused submerged aquatic plants in North America. The confusion is understandable: both form dense stands of stems with whorled leaves in the same lakes and rivers, and both look green and bushy underwater. But the management implications could not be more different. Hydrilla is a federally listed noxious weed — invasive, destructive, and legally regulated. Elodea is a native North American plant — ecologically valuable, legally protected in most states, and important fish and waterfowl habitat. Treating elodea as hydrilla wastes management resources and damages native ecosystems. Failing to recognize hydrilla allows infestations to expand unchecked.
Side-by-Side Identification: The Four Key Tests
Test 1: Count the Leaves Per Whorl
This is the fastest field test. Examine a mid-stem section of the plant carefully. Count the number of leaves radiating outward from a single point on the stem (a whorl). Elodea consistently has exactly 3 leaves per whorl. Hydrilla has 4–8 leaves per whorl, most commonly 5. This single feature is nearly diagnostic. If you count 3, you likely have elodea (or a related native like Elodea nuttallii). If you count 5 or more, examine further for hydrilla.
Test 2: Examine the Leaf Margin
Remove a single leaf and examine its edges against a light source, using a 10x hand lens if available. Hydrilla leaves have prominently serrated (toothed) margins — small, sharp teeth visible along the entire leaf edge. These teeth are large enough to feel by running your fingernail along the margin. Elodea leaves have smooth or very finely toothed margins — the edges appear smooth to the naked eye and only show extremely fine ornamentation under high magnification. This is one of the clearest distinguishing features in good specimens.
Test 3: Check for the Midrib Tooth
Turn a leaf over and gently run your fingernail from tip to base along the underside. Hydrilla has a raised tooth (or multiple small teeth) on the underside of the midrib — you will feel a distinct bump or scratchy sensation. Elodea lacks any midrib tooth — the underside is smooth. This feature is unique to hydrilla among all commonly encountered U.S. submerged plants.
Test 4: Look for Tubers at the Roots
If you can collect plant material with roots and some sediment, examine the root base carefully. Hydrilla produces small, white or cream-colored globular tubers (5–15 mm diameter) at or just below the sediment surface — these are unmistakable once you know what to look for. Elodea does not produce tubers. Presence of tubers is a definitive confirmation of hydrilla.
Other Differences
Leaf Size and Shape
Elodea leaves are typically longer (10–35 mm) and more strap-like than hydrilla leaves (6–20 mm). Elodea leaves are wider in proportion to length, with a more oblong appearance. Hydrilla leaves are narrower and more lance-like. These size differences are not reliable as a standalone feature because leaf size varies with water quality, light, and growth stage in both species.
Stem Texture
Hydrilla stems feel rougher when you run them between your fingers — the leaf margins contribute a slightly scratchy texture. Elodea stems feel smoother. This is a rough tactile test and should not be used in isolation.
Ecological Context
Elodea (E. canadensis) is native across North America and is most common in slower-moving rivers, lakes, and ponds in the northern and eastern U.S. It tends to grow in moderate densities and coexists with other native vegetation. Hydrilla forms much denser, often monospecific stands and typically dominates the entire water column from sediment to surface where established. If the plant forms a solid mat from bottom to surface with no other plant species visible, suspect hydrilla. For detailed management guidance, see the hydrilla control methods guide.
When You Are Unsure
Do not apply herbicide based on uncertain visual identification alone. If you cannot confirm identification with high confidence using the above tests, collect a sample (seal in a plastic bag, do not rinse in waterways), and submit it to your state department of natural resources or university cooperative extension service. Most states have aquatic plant identification services, and some offer photographic ID through online portals. The consequences of misidentification — treating native habitat or leaving an invasive infestation unaddressed — are serious in both cases.
References
- Langeland, K.A. (1996). Hydrilla verticillata. Castanea 61(3):293–304.
- Catling, P.M. & Wojtas, W. (1986). Waterweed flora of Canada. Canadian Journal of Botany 64:1257–1270.
- UF/IFAS Center for Aquatic and Invasive Plants. plants.ifas.ufl.edu