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Alligatorweed (Alternanthera philoxeroides)

Alligatorweed

Photo credit: Melissa McMasters/Flickr

(Alternanthera philoxeroides)

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If you believe you have found this species anywhere in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

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Species at a Glance

Alligatorweed is a fast-growing perennial plant considered to be one of the world’s worst weeds because of its ability to grow both over land and in water. In aquatic environments, it forms dense mats that begin at the shoreline and grow across the surface. In some cases, mats can reach completely across slow moving rivers and can be robust enough to support the weight of a man.

Identification

Leaves: Simple leaves are opposite, elliptical to lance-shaped, shiny, and up to 10 cm (4 in) long with a prominent midrib and smooth margins. Soft, whitish hairs are found in the leaf axis.

 

Flowers: Distinctive flowers are white, papery, fragrant, and resemble a white clover. While appearing to be a single flower, they are actually a cluster of 6-10 small florettes that are about 1.3 cm (0.5 in) in diameter and grow on a stalk that can be up to 7.6 cm (3 in) long. Flowers generally appear from April through October.

 

Fruits/Seeds: Very small fruits develop a single seed.

 

Stems/Roots: Smooth, hollow stems can be single or branched, and sprawl out along the ground or out across the water. Each node on the stem can produce a new stem or root, and mats of creeping stems can be up to 10 m (33 ft) long. Stems are particularly large and hollow when growing in water.

Similar Species

Alligatorweed is often confused with other members of the Alternanthera family such as sessile joyweed (A. sessilis) and lesser joyweed (A. denticulata); however, alligatorweed is the only aquatic member of this family. Lesser joyweed is smaller in size and has a small, stalkless cluster of white flowers. Water primrose (Ludwigia peploides) is another look-alike; however, its leaves are alternatively arranged, and it has a large yellow flower. One of the main identifying features of alligatorweed is the hollow stems on mature plants.

Habitat

While alligatorweed is usually found in aquatic habitats such as lakes, ponds, rivers, or along shorelines, it has the ability to grow in a variety of areas including many terrestrial habitats. When growing on land, this plant forms smaller, tougher leaves. When growing in water, it may be rooted in the substrate, or exist as a free-floating mat or tussock.

Spread

Growing and spreading quickly, alligatorweed reproduces asexually by fragmentation. Since new stems and roots can develop from a single node, a short fragment can regenerate into a new plant. Fragments are moved by water currents, and through human activities such as boating and fishing.

Distribution

Native to South America, alligatorweed is believed to have been introduced to the United States as a stowaway in the ballast water of transoceanic ships. It was first documented in Alabama in 1897, and is now found in coastal states from Virginia to Texas, the Tennessee Valley, Puerto Rico, and California.

 

Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.

Environmental Impacts

The dense, tangled mats formed by alligator weed can be up to 1 m (3 ft) thick and have extensive root systems. Excessive growth of this plant clogs waterways, smothers native plants, prevents light and oxygen from entering the water column, and impedes water flow, which can lead to flooding damage. While some invertebrates may use these mats for habitat, this plant has no known direct food value to wildlife.

Video
Note

Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).

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