The Identification Problem That Matters Most
In the world of submerged aquatic plant management, no identification confusion has more serious consequences than mistaking hydrilla for elodea or vice versa. Both are whorled-leaved submerged plants that grow in the same water bodies. But hydrilla is a destructive federal noxious weed requiring aggressive management, while elodea is a native plant providing ecological habitat value. Treating elodea as hydrilla wastes management resources and destroys native habitat. Treating hydrilla as elodea allows a devastating invasive plant to spread unchecked.
This guide provides a definitive, field-ready comparison of these two species. The identification tests described here are reliable and can be performed with no equipment beyond your own hands and eyes (a hand lens is helpful but not required).
The Four Field Tests
Test 1: Count Leaves Per Whorl
This is the single fastest and most reliable field test. Find a node (leaf attachment point) on the stem and count how many leaves emerge from that point. Do this on 5–10 nodes from different parts of the plant:
Result — Exactly 3: You have elodea. Elodea canadensis and all North American Elodea species have exactly 3 leaves per whorl, invariably. This is not just typical — it is constant across the genus.
Result — 4, 5, 6, 7, or 8: You have hydrilla (or possibly another species, but not elodea). Hydrilla most commonly has 5 leaves per whorl but ranges from 4–8 at different positions on the same stem.
Do this test before any other examination — the whorl count alone resolves the identification in the great majority of cases.
Test 2: Run Your Fingernail Along the Leaf Margin
Remove a single leaf and run your fingernail from the leaf tip toward the base along one margin:
Result — Smooth: You have elodea. No resistance, no scratching sensation.
Result — Bumpy, toothed, or scratchy: You have hydrilla. The distinct serrations (teeth) along hydrilla leaf margins produce a clearly rough sensation.
Test 3: Run Your Fingernail Along the Leaf Midrib Underside
Turn the same leaf over and run your fingernail from tip to base along the central vein (midrib) on the underside:
Result — Smooth: Elodea. No bump or catch.
Result — Scratchy, catches fingernail: Hydrilla. The raised midrib tooth is unique to hydrilla and produces a distinct bump or scratch sensation.
Test 4: Look for Tubers at the Root Base
If you can collect the plant with some attached sediment, examine the base of the roots:
White globular structures (5–15 mm) at the sediment surface: Hydrilla. Tubers are unmistakable and uniquely diagnostic for hydrilla among North American submerged plants.
No tubers — only roots: Consistent with elodea, though this test alone doesn't confirm identity.
Decision Summary
If the whorl count is exactly 3 AND the leaf margin is smooth AND the midrib underside is smooth: Elodea — no management action required (in most U.S. states).
If the whorl count is 4–8 AND/OR the leaf margin is clearly toothed AND/OR the midrib underside is scratchy AND/OR tubers are present at the roots: Hydrilla — contact your state DNR for management guidance and permit information.
If you are unsure after performing all tests: collect a sample in a sealed bag (do not release fragments) and submit to your state DNR identification service or cooperative extension aquatic plant specialist. Do not apply herbicide on uncertain identification. For hydrilla management details, see hydrilla control methods.
Why Correct Identification Is a Legal Matter
In every U.S. state where hydrilla has been documented, it is regulated as a noxious weed. Transport, sale, and intentional introduction of hydrilla are illegal. If you correctly identify hydrilla in a water body where it has not been previously documented, you are legally and ethically obligated to report it to your state department of natural resources. Failing to report a new hydrilla detection — and especially failing to report it after having confirmed the identification — can delay rapid response programs that depend on early detection. State agencies have rapid response protocols for new hydrilla detections that can potentially contain or eradicate small infestations before they expand; these responses are vastly more cost-effective when triggered early. In contrast, elodea requires no such reporting — it is a native plant in nearly all U.S. states (with the exception of Alaska) and its presence does not trigger any regulatory response. The identification test described in this guide directly determines whether a regulatory obligation applies. When in doubt, submit a sample for expert verification before taking any action or making any report.
References
- Langeland, K.A. (1996). Hydrilla verticillata. Castanea 61(3):293–304.
- Cook, C.D.K. & Urmi-König, K. (1985). Revision of Elodea. Aquatic Botany 21:111–156.
- UF/IFAS Center for Aquatic and Invasive Plants. plants.ifas.ufl.edu