Hydrilla Identification & Treatment
Scientific Name: Hydrilla verticillata Category: Submerged Tier: 1 — High-Priority — Most prevalent and/or most damaging across the US
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Identification
Hydrilla has small, pointed leaves arranged in whorls of 4–8 around the stem. Leaf margins are distinctly serrated (visible with a hand lens), and the underside of the leaf midrib often has one or more small spines. Stems are slender, branching, and can grow up to 25 feet to reach the surface. Hydrilla forms dense mats at the water surface that block light to native vegetation below. Key distinguishing features:
- Leaves in whorls of 4–8 (usually 5)
- Serrated leaf margins
- Small spines on leaf midrib underside
- Tubers (small, potato-like structures) on roots
Commonly confused with: Elodea (leaves in whorls of 3, smooth margins), Egeria (leaves in whorls of 4–6, larger leaves, no midrib spines)
Treatment
Hydrilla is one of the most difficult aquatic weeds to manage because it produces tubers that can remain dormant in sediment for several years and are unaffected by most herbicides.
Recommended Natural Waterscapes Products:
- Sonar A.S. or SonarOne (fluridone) — Systemic herbicide; the most effective long-term option. Requires sustained contact over 60–90 days. Best applied early in the growing season before hydrilla reaches the surface. A single application can provide season-long control. Sonar A.S. | SonarOne
- Aquathol K or Aquathol Super K (endothall) — Contact herbicide for spot treatments and quick knockdowns. Effective within 2–5 days but does not kill tubers. Aquathol K | Aquathol Super K
- Hydrothol Granular (endothall) — Granular formulation that sinks to the bottom, ideal for targeting submerged growth. Hydrothol Granular
Important: Always run an aerator after herbicide treatment to maintain dissolved oxygen as dead plant material decomposes.
Full Profile
- Native Range: Southeast Asia (India, Sri Lanka, Korea, Japan)
- US Distribution: Established across the Southeast (EPA Ecoregions: Southeastern Plains, Mississippi Alluvial Plain, Southern Coastal Plain, East Central Texas Plains) and expanding northward. Found in at least 30 states. USDA Hardiness Zones 5–11, though most problematic in Zones 7–10.
- Regulatory Status: Federal Noxious Weed (USDA APHIS). Listed as prohibited or noxious in most states where it occurs.
- Habitat: Thrives in freshwater lakes, ponds, rivers, and canals. Tolerates a wide range of water clarity, nutrient levels, and depths (up to 40 feet in clear water). Grows in both still and flowing water.
- Reproduction: Extremely aggressive. Spreads by fragmentation (small stem pieces root and grow), tubers (survive years in sediment), turions (overwintering buds), and stolons. A single tuber can produce over 6,000 new tubers per square meter in one growing season.
- Ecological Benefits: Despite its invasive nature, hydrilla provides cover for juvenile fish and invertebrates. Dense hydrilla beds can support largemouth bass populations in the short term, though long-term monocultures reduce overall species diversity. Some waterfowl species feed on hydrilla tubers and foliage.
- Seasonal Behavior: In southern regions, grows year-round. In northern areas, growth slows in winter but tubers survive. Peak biomass typically in late summer. Dioecious hydrilla (female only) is the most common form in the US.
Sources: Texas A&M AquaPlant – Hydrilla | UF/IFAS – Hydrilla Management | USGS NAS
Related Species
- Eurasian Watermilfoil (Myriophyllum spicatum) — Submerged, Tier 1
- Coontail (Hornwort) (Ceratophyllum demersum) — Submerged, Tier 1
- Curly-Leaf Pondweed (Potamogeton crispus) — Submerged, Tier 1
- Elodea (Common Waterweed) (Elodea canadensis) — Submerged, Tier 1
- Egeria (Brazilian Elodea) (Egeria densa) — Submerged, Tier 1
- Sago Pondweed (Stuckenia pectinata (formerly Potamogeton pectinatus)) — Submerged, Tier 2
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