Alligator Weed Identification & Treatment
Scientific Name: Alternanthera philoxeroides Category: Emergent Tier: 2 — Regionally Significant — Common in multiple EPA ecoregions
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Identification
Alligator weed has opposite, lance-shaped leaves on hollow stems that can float on water or creep across moist ground. Stems are thick, fleshy, and often reddish. Produces small, round, white, papery flower heads on short stalks resembling white clover balls. Key distinguishing features:
- Opposite, lance-shaped leaves
- Hollow, fleshy stems (sometimes reddish)
- Small round white "clover-like" flower heads
- Forms dense floating mats or shoreline mats
Treatment
Recommended Natural Waterscapes Products:
- Glyphosate 5.4 — Systemic herbicide for emergent foliage. Glyphosate 5.4
- Clearcast (imazamox) — Systemic herbicide. Clearcast
Biological control: The flea beetle Agasicles hygrophila has been introduced as a biocontrol agent and has been effective in reducing alligator weed in some southern regions.
Full Profile
- Native Range: South America
- US Distribution: Southeastern US, Gulf Coast. EPA Ecoregions: Southern Coastal Plain, Southeastern Plains, Mississippi Alluvial Plain. USDA Zones 7–11.
- Regulatory Status: Federal Noxious Weed. Listed as invasive in multiple states.
- Habitat: Ponds, lakes, rivers, canals, ditches, and wet terrestrial areas. Grows in both aquatic and terrestrial forms.
- Reproduction: Fragments root extremely easily. One node fragment can start a new colony.
- Ecological Benefits: Limited. Dense mats impede navigation, block light, and displace native vegetation.
Sources: Texas A&M AquaPlant – Alligator Weed | USDA APHIS
Related Species
- Cattails (Typha latifolia (broadleaf), T. angustifolia (narrowleaf), T. × glauca (hybrid)) — Emergent, Tier 1
- Phragmites (Common Reed) (Phragmites australis subsp. australis (invasive) vs. P. australis subsp. americanus (native)) — Emergent, Tier 1
- Creeping Water Primrose (Ludwigia spp. (L. peploides, L. grandiflora, L. repens)) — Emergent, Tier 1
- Dollar Weed (Pennywort) (Hydrocotyle spp. (H. umbellata, H. ranunculoides, H. verticillata)) — Emergent, Tier 2
- Flowering Rush (Butomus umbellatus) — Emergent, Tier 3
- Purple Loosestrife (Lythrum salicaria) — Emergent, Tier 3
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