Alternanthera philoxeroides
Invasive aquatic weed forming dense mats worldwide.
Alternanthera philoxeroides, called alligator weed, is a herbaceous plant native to the temperate parts of South America—specifically Argentina, Brazil, Paraguay, and Uruguay. Argentina alone has about 27 species in the genus *Alternanthera*. Originally limited to the Paraná River region, it has since spread to more than 30 countries, including the United States, Japan, China, Australia, and New Zealand. It likely hitched a ride in sediment stuck to ship tanks and cargo from South America.
This plant can live in both dry and wet environments. It has whitish, papery flowers on short stalks, hollow stems that sprawl or grow irregularly, and simple leaves that sprout opposite each other at the nodes. Its shoots are short-lived. Horizontal stems called stolons can reach up to 10 meters (33 feet) long, and because the stems are hollow, they float easily. This lets them form thick, dense mats on the water’s surface. The small, white bisexual flowers grow in tight, axillary spikes (a type of raceme). A key difference from *Alternanthera sessilis* is that this species has a peduncle (a flower stalk), while *A. sessilis* has sessile inflorescences. Though seeds are produced in its native range, no viable seeds have been found where it has been introduced. Its roots can either hang freely in the water to absorb nutrients or anchor into soil or sediment to pull nutrients from below.
As an invasive species, alligator weed is a major threat because it harms both aquatic and terrestrial environments and affects human society. Since 2017, it has been on the European Union’s list of invasive alien species of Union concern, which bans its import and trade across the EU.
The dense mats it forms block natural water flow. They crowd out native plants by taking up space and blocking sunlight, reducing the light that submerged vegetation gets. These mats also interfere with gas exchange under the water, which affects aerobic processes like photosynthesis. Beyond harming native aquatic plants, the weed can lower crop yields in pastures and fields. Its mats disrupt water flow used for irrigation and worsen water quality by increasing sediment. Both factors are crucial for healthy crops and good harvests. Flooding becomes more likely because drainage is impaired, which can also damage crops.
Quick Facts
- Genus
- Alternanthera
- Species
- philoxeroides
Facts from the source article.
Lore & Background
Alternanthera philoxeroides is characterized by whitish, papery flowers on short stalks, hollow stems that float easily, and simple opposite leaves. It produces stolons up to 10 meters long, creating dense surface mats. The species has bisexual flowers on axillary spikes, with a peduncle distinguishing it from Alternanthera sessilis. In its native range, it produces viable seeds, but no viable seeds have been recorded in introduced ranges. Its root system can either hang freely in water or penetrate soil. The plant was accidentally introduced to non-native regions via sediments trapped in ship tanks and cargo from South America. It was banned for sale or shipment in the United States in 1956, a law repealed in 2019.
Reader's Guide
Alternanthera philoxeroides is a major invasive species listed on the European Union's list of invasive alien species of Union concern since 2017, making its import and trade forbidden in the EU. Its dense mats disrupt water flow, outcompete native vegetation for light and space, and impair gas exchange, affecting photosynthesis. This reduces native plant populations and harms herbivores that depend on them. The mats also block wildlife access to water and degrade water quality through increased sedimentation. For society, the mats impede boating and fishing, disrupt hydro-electric infrastructure, and create breeding grounds for mosquitoes, increasing the risk of mosquito-borne diseases. The plant spreads vegetatively via fragmentation, regenerating from small stem or leaf cuttings. Human dispersal occurs through ineffective manual removal and soil movement by machinery; natural dispersal happens via water currents and erosion. Control methods include early detection, barriers in shallow water, overplanting with native species, and biological agents like the alligator weed flea beetle (Agasicles hygrophila), though its effectiveness is limited below 11 °C. Chemical controls such as glyphosate and triclopyr require constant application.
Did You Know?
- Alternanthera philoxeroides can regenerate from small stem fragments or leaf cuttings.
- The alligator weed flea beetle (Agasicles hygrophila) is used for biocontrol but cannot survive below 11 °C.
- No viable seeds of this species have been recorded in its introduced ranges.
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