Aquatic invasive weeds in the United States: identification guide
Nine invasive aquatic plants cause the majority of waterway damage across the United States. Here's how to identify each one before treatment.
Misidentification is the most common reason aquatic vegetation treatment fails. A herbicide that kills hydrilla won't touch alligator weed. A grass carp that grazes on duckweed won't touch water hyacinth. Identify first, treat second.
1. Water hyacinth (Eichhornia crassipes)
- Form: free-floating, no rooted attachment to the bottom
- Leaves: thick, glossy, oval, with bulbous inflated petioles that act as flotation
- Flowers: showy lavender-purple with a yellow blotch, six petals
- Spread: doubles every 8–12 days in warm water; mats drift with wind
- Range: Southeast US year-round; summer-only as far north as Maryland and southern Illinois
The single fastest-spreading aquatic plant in North America. See our water hyacinth removal guide for treatment specifics.
2. Hydrilla (Hydrilla verticillata)
- Form: rooted submerged, stems reach the surface and form mats
- Leaves: whorls of 3–8 around the stem, sandpaper-rough edges with visible teeth
- Roots: small white tubers — these are the regrowth bank that makes hydrilla so persistent
- Spread: stem fragments form new plants; tubers viable in sediment for 4+ years
- Range: entire Southeast, Mid-Atlantic, California, Pacific NW reservoirs
The hardest invasive to eradicate because of its tuber bank. See our hydrilla vs. eelgrass identification post and hydrilla removal services.
3. Eurasian watermilfoil (Myriophyllum spicatum)
- Form: rooted submerged, feathery canopy at the surface
- Leaves: whorls of 4, each leaf finely divided into 12+ pairs of thread-like leaflets
- Stem: reddish-brown, snaps easily — fragments form new plants
- Range: dominant invasive in Northern lakes (Great Lakes, Northeast, Pacific NW)
- Tell: more leaflet pairs than the native northern milfoil (which has 5–10 pairs)
The "milfoil" in lake-property listings up north is almost always this species.
4. Water lettuce (Pistia stratiotes)
- Form: free-floating rosette, looks like a head of romaine on the water
- Leaves: thick, ribbed, light green, fuzzy texture
- Roots: dangling feathery roots up to 12 inches long
- Spread: ramets break off and form new rosettes; doubles weekly in summer
- Range: Florida, Gulf Coast, Lower Mississippi, lower Texas
Mistaken for a native by homeowners. It's not — it's a federal noxious weed.
5. Giant salvinia (Salvinia molesta)
- Form: free-floating fern, paired oval leaves with stiff hairs
- Leaves: the egg-beater hairs on mature leaves are diagnostic — distinguishes from common salvinia
- Spread: can double in 4–10 days; one of the worst invasives globally
- Range: Texas, Louisiana, parts of Florida, Hawaii
If you see paired floating fern leaves with hairs that split at the tips into a whisk shape, you have giant salvinia. Report it to the state — most states track it actively.
6. Alligator weed (Alternanthera philoxeroides)
- Form: mat-forming, can grow rooted in mud or floating on water
- Leaves: opposite, lance-shaped, smooth-edged, 2–4 inches long
- Stems: hollow, jointed, with a single white clover-like flower at the leaf axil
- Range: Southeast US, Texas, California
The flower distinguishes it from native pickerelweed and arrowhead. Treatment-resistant — biological control with the alligator weed flea beetle is the only consistently effective method.
7. Parrot feather (Myriophyllum aquaticum)
- Form: rooted submerged with emergent feathery foliage above the waterline
- Leaves: whorls of 4–6 above water, bluish-green, finely divided
- Spread: female-only in the US (sterile), spreads only by fragmentation
- Range: Southeast, Pacific NW, California
Often planted in water gardens and escapes — the single most common pathway of new infestations.
8. Curly-leaf pondweed (Potamogeton crispus)
- Form: rooted submerged, lasagna-noodle leaves
- Leaves: wavy crinkled edges, alternate along the stem (not whorled), 2–3 inches long
- Cycle: unique — peaks in May–June, dies back in summer when others peak
- Range: Northern US, Great Lakes, Pacific NW
The early-season peak is the diagnostic feature. By July most curly-leaf has decayed and fueled an algae bloom.
9. Duckweed (Lemna minor) and watermeal (Wolffia spp.)
- Form: free-floating tiny green specks
- Duckweed: 2–5 mm oval frond, single root visible underneath
- Watermeal: under 1 mm, no root, looks like grass clippings on the water
- Spread: prolific in nutrient-loaded ponds — duckweed cover can double in 2 days
Native species, but invasive in behavior under the right conditions. Both indicate excess nutrients — see our filamentous algae post for the underlying nutrient framework.
Why species ID matters before treatment
- Fluridone kills hydrilla but does nothing to alligator weed
- Grass carp eat hydrilla and pondweeds but ignore hyacinth and lettuce
- Mechanical harvesters work on floating mats but fragment hydrilla into more hydrilla
- 2,4-D works on hyacinth but damages native lily pads
- Imazapyr kills shoreline emergents but lasts 6+ months in soil
Spending $400 on a treatment for the wrong species is the most common waste in aquatic vegetation management. If you can't identify what you're treating, photograph it and email it to your state's aquatic plant management program before you spend the money. Florida residents can also reach out through our contact page for ID help in our service areas.
Frequently asked questions
What is the most invasive aquatic weed in the United States?
By total acreage affected, hydrilla (Hydrilla verticillata) is the most damaging submerged invasive in the US. By visible coverage and rate of spread, water hyacinth (Eichhornia crassipes) is the most aggressive floating invasive. Both are federal noxious weeds.
How can I tell hydrilla from Eurasian watermilfoil?
Hydrilla has whorls of 3–8 leaves with rough, toothed edges and tubers on the roots. Eurasian milfoil has feather-like leaves arranged in whorls of 4 with 12+ leaflet pairs per leaf — no toothed edges, no tubers. Both grow in submerged dense mats; treatment differs.
Are duckweed and watermeal the same thing?
No. Duckweed (Lemna minor) has visible oval fronds 2–5 mm across with a tiny root. Watermeal (Wolffia) is the smallest flowering plant in the world — fronds under 1 mm with no root, looks like green grit on the water surface. Watermeal is harder to control because it slips through most filtration.
Which aquatic invasives are illegal to possess?
All nine plants in this guide are listed under the US Federal Noxious Weed Act or as state-prohibited species in most southern and western states. Possession, transport, or sale across state lines is a federal violation. State-level rules vary — check your state's Department of Agriculture or Department of Natural Resources before moving any aquatic plant.
Founder of Aquatic Cleanup. Florida-licensed aquatic-vegetation operator working private lakes, HOA retention ponds, and waterfront properties across Volusia, Lake, Seminole, and Orange counties.
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