Identifying Hydrilla in the Field
Accurate hydrilla identification is critical because it is often confused with several native and non-native submerged plants, including elodea, coontail, and native naiad species. Misidentification can lead to inappropriate management that harms native ecosystems while leaving hydrilla untreated. The following guide covers the definitive features used to confirm hydrilla and distinguish it from look-alikes.
Key Identification Features
1. Leaf Arrangement: Whorls of 4–8 Leaves
Hydrilla leaves are arranged in whorls (rings) around the stem. Each whorl typically contains 4–8 leaves, with 5 being most common. This contrasts with elodea, which consistently has whorls of exactly 3 leaves, and with coontail, which has leaves arranged in dense, brush-like whorls that differ in texture and branching pattern. Leaf whorls in hydrilla are spaced roughly 0.5–1.0 cm apart along the stem.
2. Serrated (Toothed) Leaf Margins
The most diagnostic feature of hydrilla is its distinctly serrated leaf margins — the edges of the leaves bear small, sharp teeth visible under a 10x hand lens. This serration is present on all leaves and is consistent across the entire plant. Elodea leaves, by contrast, have smooth or very finely toothed margins visible only under magnification. Leaf length in hydrilla ranges from 6–20 mm, with width 1–4 mm. The shape is lance-like, tapering to a pointed tip.
3. Midrib Tooth on the Underside
The underside of each hydrilla leaf bears one or more small, raised teeth along the central vein (midrib). This feature is unique to hydrilla among commonly encountered U.S. submerged plants. To check for this feature, bend a leaf gently and run your fingertip from the tip toward the base — the midrib teeth produce a rough, scratchy sensation. This feature is absent in elodea and all native naiad species.
4. White Tubers at Roots
In established infestations, small white or cream-colored tubers (typically 5–15 mm diameter) can be found attached to the roots at or just below the sediment surface. These tubers are unique to hydrilla among submerged plants in North America. They are starch-storage organs produced in late summer and fall that serve as overwintering propagules. Presence of tubers confirms hydrilla identity even in the absence of above-ground plant material.
5. Turions (Axillary Buds)
In late summer and fall, hydrilla produces small, greenish, cone-shaped axillary buds (turions) in the leaf axils along the stem. Turions are typically 2–5 mm long and compact. They detach from the stem, sink to the sediment, and overwinter before germinating in spring. Turions resemble small pine cones or green pellets and are visible without magnification on stems during the late growing season.
Stem Characteristics
Hydrilla stems are long, branching, and slender — often 1–3 mm in diameter. They are green to reddish-green, becoming whitish toward the base. Stems break easily at nodes and fragment readily when disturbed by boats or currents, each fragment being capable of establishing a new plant. Unlike milfoil, hydrilla stems are not pinnately compound — the leaves are simple, undivided blades arranged in whorls, not feathery divisions.
Flower Characteristics (Seasonal)
Hydrilla flowers are tiny, white, and float to the surface at the end of long, slender peduncles from the leaf axils. Male flowers (in the dioecious biotype) detach at maturity and release pollen at the water surface. Female flowers remain attached while pollination occurs. Flowering typically occurs in late summer (July–September in most of the U.S.). Flowers are rarely seen in practice and are not necessary for field identification.
Comparison to Common Look-Alikes
Hydrilla vs. Elodea (Elodea canadensis)
Elodea is the most frequently confused species. Both form dense underwater stands with whorled leaves. Key differences: Elodea has exactly 3 leaves per whorl (hydrilla has 4–8); Elodea leaf margins are smooth or finely toothed without the prominent serration of hydrilla; Elodea leaves are longer and strap-like (10–35 mm) compared to hydrilla; Elodea lacks the midrib tooth. See the detailed hydrilla vs. elodea comparison.
Hydrilla vs. Coontail (Ceratophyllum demersum)
Coontail has forked, stiff leaves arranged in dense whorls that give the stem a bottlebrush appearance. Coontail has no roots and no tubers. Its leaves are not strap-like but are divided (bifurcated). Texture is stiff and scratchy throughout. Coontail typically forms dense tangles near the surface rather than the upright stem growth pattern of hydrilla. Coontail is native across North America and generally does not require management unless it reaches nuisance density.
Hydrilla vs. Naiad Species (Najas spp.)
Naiad species (slender naiad, southern naiad) have opposite or whorled leaves but are much more slender and delicate than hydrilla, with leaves typically less than 1 mm wide. Naiad leaves often have toothed margins but lack the midrib tooth of hydrilla. Naiads are native and ecologically valuable; they should not be treated as hydrilla.
Seasonal Identification Tips
Hydrilla is easiest to identify at peak summer growth (June–August), when plants are actively growing and have full leaf development, turions may be visible, and surface mats are present. In winter and early spring, hydrilla growth may be sparse or confined to tuber-germinated seedlings near the sediment, making identification harder. Year-round identification is possible by carefully examining sediment near prior infestation sites for tubers.
Field Collection Protocol
When collecting samples for confirmation: wear gloves; place plant material in a sealed plastic bag (never wash fragments into natural waterways); collect stems showing whorled leaf arrangement and, if possible, root material with tubers; provide samples to your state's department of natural resources or university extension service for confirmation. Do not rely on online photos alone for regulatory reporting — confirmed identification by a trained specialist is required in most states before regulatory action can be taken.
References
- Langeland, K.A. (1996). Hydrilla verticillata. Castanea 61(3):293–304.
- Haynes, R.R. (2000). Hydrocharitaceae. In: Flora of North America. Oxford University Press.
- UF/IFAS Center for Aquatic and Invasive Plants. Hydrilla Identification. plants.ifas.ufl.edu