The Most Consequential Misidentification in Aquatic Plant Management
Confusing coontail (Ceratophyllum demersum) with hydrilla (Hydrilla verticillata) is one of the most consequential — and one of the most common — identification errors made by lake managers, anglers, and property owners. Both are submerged, both have whorled leaves, and both grow in the same lakes and rivers across much of the United States. But the management implications of this confusion are serious in both directions. Identifying coontail as hydrilla may lead to herbicide treatment of a native plant community that provides valuable ecological services, while identifying hydrilla as coontail means leaving a destructive federal noxious weed unmanaged and allowing it to spread. This guide provides a definitive comparison of the two species.
Side-by-Side Comparison
The Leaf Test: The Single Most Important Distinction
Examine a single leaf carefully. This is the most important diagnostic test:
Coontail: Leaves are repeatedly forked (dichotomously branched) — each leaf divides into two segments, which each divide again, creating a fan-like or brush-like structure of progressively narrower, rigid, pointed segments. The leaf is not a flat blade — it is a branching structure. This forked leaf form is unique to coontail among common U.S. submersed plants.
Hydrilla: Leaves are simple, undivided blades — flat, lance-shaped, 6–20 mm long, with an obvious midrib running from base to tip. Leaf margins are distinctly serrated (toothed). This is a simple leaf, not a branching one.
Once you see this comparison, coontail and hydrilla are not easy to confuse. Coontail's branching leaf structure looks nothing like hydrilla's simple leaf.
The Root Test: The Fastest Field Check
Coontail: No roots. Pull a coontail plant from the bottom — you will find nothing anchoring it in the sediment. The stem base is bare. There are no white thread-like roots, no root crown, no tubers.
Hydrilla: Has white or pale roots anchored in the sediment. Well-established hydrilla also has small, white, globular tubers (5–15 mm diameter) at or just below the sediment surface near the roots. If you can retrieve a root ball with tubers, the identification of hydrilla is confirmed.
The Texture Test
Coontail: Very rough and scratchy throughout — running the plant through your fingers feels like dragging your hand through course steel wool (less so, but similar sensation). This roughness comes from tiny teeth on the leaf segments and is consistent along the entire plant.
Hydrilla: Leaf margins have distinct serrations (teeth) that can be felt by running your fingernail along the margin — "bumpy" or slightly rough. The midrib tooth on the underside of the leaf (unique to hydrilla) creates a scratch sensation when you run your fingernail from leaf tip to base on the underside. But the overall stem is not as uniformly rough as coontail.
Growth Form Comparison
Coontail: Dense, bottlebrush-like clusters at stem tips; becomes more sparse toward stem bases. Often forms floating, loosely anchored masses in the water column. Plants may range from 30 cm to 3+ meters long. Does not form the same dense, surface-to-bottom monoculture mat as hydrilla.
Hydrilla: More evenly leafed along the stem, with whorls spaced roughly 0.5–1 cm apart. Forms dense, compressed mats from sediment to surface in established infestations. Grows in a more organized, upright pattern than the tangle-form of coontail.
What to Do If You Are Uncertain
If after careful examination you remain uncertain about which species you have:
- Collect a sample in a sealed plastic bag with water. Do not rinse fragments into any waterway.
- Photograph the sample with a scale object (ruler or coin) and close-up photos of leaves and any roots found.
- Submit to your state department of natural resources identification service, or contact your county/state cooperative extension aquatic plant specialist.
- Do not apply herbicide based on uncertain identification. The ecological and legal consequences of treating the wrong species are significant.
For complete information on coontail management, see coontail control methods. For hydrilla management, see hydrilla control methods.
References
- Les, D.H. (1988). Systematics of Ceratophyllum. Systematic Botany 13(3):359–388.
- Langeland, K.A. (1996). Hydrilla verticillata. Castanea 61(3):293–304.
- UF/IFAS Center for Aquatic and Invasive Plants. plants.ifas.ufl.edu