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Alligatorweed Alternanthera philoxeroides

Other Common Name(s):

Previously known as:

  • Achyranthes philoxeroides
Phonetic Spelling
Al-ter-nan-thera philoxe-roides
This plant is an invasive species in North Carolina
Description

Alligatorweed in an invasive perennial plant in the Amaranth (Amaranthaceae) family usually found growing on or near water or farmlands. Native from Trinidad to Northern Argentina, alligatorweed was introduced to the southeastern United States in the 1880s, likely from ships’ ballast water. It has also invaded California, parts of Central America, Asia, and Australia. In North Carolina, it is widely found in the coastal plain, less common in the piedmont, and rare in the mountains.

Alligatorweed plants can grow upright, but typically sprawl, to form dense, prostrate masses. The small, white, papery, clover-like flowers appear on short stalks from spring through fall, rarely producing seed. Alligatorweed has 1.5 to 4-inch elliptical to ovate simple green leaves growing oppositely on smooth green stems. Stems may turn pink or red when stressed. These slender round stems are hollow in aquatic areas, but more solid in terrestrial plants. Roots, leaves, and stems all grow from leaf nodes, facilitating invasive spread from even small plant fragments, typically through water.

Alligatorweed can form mats over 30 feet wide along shorelines or across rivers. These are unsightly, interfere with fishing, swimming, navigation, and harbor mosquitoes. Floating mats caught in dams or other areas contribute to both flooding and erosion. Nursery stock may also carry alligatorweed.  Terrestrial infestations can affect row crops and it is allelopathic to some plants.

Insects, Diseases, and Other Plant Problems: No significant problems. Biological control insects, including flea beetles (Agasicles hygrophila) and stem-borer moths (Arcola malloi), that effectively manage Alligatorweed elsewhere do not reliably overwinter in North Carolina. Listed as invasive by the NC Invasive Plant Council.

See this plant in the following landscape:
Cultivars / Varieties:
Tags:
 
Cultivars / Varieties:
Tags:
  • Attributes:
    Genus:
    Alternanthera
    Species:
    philoxeroides
    Family:
    Amaranthaceae
    Life Cycle:
    Perennial
    Country Or Region Of Origin:
    South America
    Distribution:
    Southeastern United States, California, parts of Central America, Asia, and Australia
  • Whole Plant Traits:
    Plant Type:
    Water Plant
    Weed
    Habit/Form:
    Creeping
    Dense
    Prostrate
    Spreading
    Growth Rate:
    Rapid
    Maintenance:
    High
    Texture:
    Medium
  • Cultural Conditions:
    Light:
    Full sun (6 or more hours of direct sunlight a day)
    Partial Shade (Direct sunlight only part of the day, 2-6 hours)
    Soil Drainage:
    Frequent Standing Water
    Moist
    Occasionally Wet
    NC Region:
    Coastal
    Mountains
    Piedmont
    USDA Plant Hardiness Zone:
    7a, 7b, 8a, 8b, 9a, 9b, 10a, 10b
  • Fruit:
    Display/Harvest Time:
    Fall
    Summer
    Fruit Type:
    Achene
    Fruit Length:
    < 1 inch
    Fruit Width:
    < 1 inch
    Fruit Description:
    Plants rarely form viable seeds in this country. The utricle (an achene-like structure) rarely has seeds.
  • Flowers:
    Flower Color:
    White
    Flower Inflorescence:
    Raceme
    Spike
    Flower Bloom Time:
    Fall
    Spring
    Summer
    Flower Shape:
    Dome
    Flower Petals:
    Tepals
    Flower Size:
    < 1 inch
    Flower Description:
    Flowers are small, white, papery, and clover-like on short petioles borne on dense axillary spike-like racemes.
  • Leaves:
    Leaf Color:
    Green
    Leaf Type:
    Simple
    Leaf Arrangement:
    Opposite
    Leaf Shape:
    Elliptical
    Ovate
    Leaf Margin:
    Entire
    Hairs Present:
    No
    Leaf Length:
    1-3 inches
    Leaf Width:
    < 1 inch
    Leaf Description:
    1.5 to 4 inch elliptical to ovate simple green leaves grow oppositely with a distinct midrib.
  • Stem:
    Stem Color:
    Green
    Pink
    Red/Burgundy
    Stem Is Aromatic:
    No
    Stem Cross Section:
    Round
    Stem Description:
    Round green stems turn pink or red when stressed. Aquatic plant stems are hollow; terrestrial plants have woodier (lignified) stems.
  • Landscape:
    Resistance To Challenges:
    Wet Soil
    Problems:
    Invasive Species
    Weedy