Alternanthera philoxeroides aka Alligator Weed

Alternanthera philoxeroides is a fast-growing perennial that can spread extensively. In aquatic environments, stems grow to about 60cm (2 feet) tall, while terrestrial forms are shorter. The creeping stems can extend up to 10 meters long, forming dense mats over 30 feet wide along shorelines or across water bodies. The plant has a prostrate, spreading growth habit and produces masses of creeping and layering stems from nodes.

Alternanthera philoxeroides is native to South America, specifically the temperate and subtropical regions including Argentina, Brazil, Paraguay, Uruguay, Bolivia, Peru, Venezuela, Colombia, and the Guyanas. Its original geographic range was centered around the Paraná River region. The species has since spread to over 30 countries across North America, Europe, Asia, and Oceania, primarily introduced through ship ballast water in the late 1800s.

Alternanthera philoxeroides thrives in tropical to warm temperate climates and is hardy in USDA zones 7-10. The plant is not cold-tolerant; frost and ice will kill exposed above-ground portions. However, the root system can survive winter temperatures in zone 7 and regrow in spring. The species performs best in warm conditions during mid-summer when growth rate is maximum.

Alternanthera philoxeroides reproduces almost entirely through vegetative means, as it rarely produces viable seeds outside its native range. Each stem node has two axillary buds, and new plants can develop from fragments of roots, stems, or nodes. Even small plant pieces can establish new populations when dispersed by water, flooding, or human activities. This remarkable regenerative ability makes it extremely difficult to control once established.

Yes, Alternanthera philoxeroides produces small, white, papery, clover-like flowers from spring through fall (or November to March in the Southern Hemisphere). The flower heads are 8-10mm in diameter and appear on short stalks (peduncles) in dense axillary spikes. This stalked inflorescence distinguishes it from the closely related Alternanthera sessilis. However, the flowers rarely produce viable seeds outside the plant's native South American range.

Alternanthera philoxeroides grows outdoors in USDA zones 7-10, but cultivation is strongly discouraged or illegal in many regions due to its highly invasive nature. The plant has been banned from sale in the United States since 1956 and is listed as a Weed of National Significance in Australia. It is on the EU list of invasive alien species of Union Concern. If you encounter this plant, contact local authorities for proper removal guidance rather than attempting to cultivate it.

Several biological control agents have been intentionally released to combat Alternanthera philoxeroides. The most successful is Agasicles hygrophila (alligator weed flea beetle), which has been used in Australia, China, Thailand, New Zealand, and the United States. Amynothrips andersoni (alligator weed thrips) and Vogtia malloi (alligator weed stem borer) have also been released. Fungal pathogens including Fusarium species are being researched for additional biological control options.

Two recognized biotypes of Alternanthera philoxeroides exist. The narrow-stemmed alligatorweed (NSA) biotype has relatively slender stems and longer internodes. The broad-stemmed alligatorweed (BSA) biotype has broader stems and longer internodes. The plant also shows distinct morphological differences between aquatic and terrestrial growth forms, with aquatic plants having hollow stems and adventitious roots, while terrestrial plants develop shorter stems and thickened taproots.

Yes, the young leaves and tops of Alternanthera philoxeroides are edible and consumed as a vegetable in Bangladesh, Burma, and other parts of Southeast Asia, where it's known as 'Malancha'. The leaves can be eaten raw in small amounts (reportedly tasting better than many salad greens) or cooked. However, the plant contains calcium oxalates, so consumption should be limited. Cooking helps reduce the oxalate content. Avoid consuming more than about half a pound fresh weight to prevent excessive oxalate intake.

Alternanthera philoxeroides has extensive traditional medicinal uses. In China, it treats viral diseases including measles, influenza, and hemorrhagic fever. In India, extracts address women's health issues. In Thailand, it's used for blood conditions, fever, wounds, post-natal depression, and to stimulate milk secretion. In Bangladesh, it treats diarrhea and dysentery. Scientific research has identified approximately 60 secondary metabolites including flavonoids and saponins with documented antiviral, antibacterial, antioxidant, and anticancer properties.

Beyond food and medicine, Alternanthera philoxeroides has been used for water filtration to treat domestic sewage due to its ability to absorb nutrients from water. The biomass can be composted. It has been used as animal feed in some regions. POWO notes environmental applications for this species. However, due to its highly invasive nature, intentional cultivation for any purpose is strongly discouraged or prohibited in most countries.

Alternanthera philoxeroides shows maximum growth during mid-summer when conditions are warm. In temperate regions (USDA zones 7-8), frost and ice kill the above-ground portions in winter, but the root system can survive and regrow in spring. The plant flowers from spring through fall in the Northern Hemisphere. In tropical regions, it grows year-round. Stems may turn pink or red when stressed by cold or other environmental factors.

Caring for Alligator Weed is a breeze, as it is highly adaptable and forgiving to minor neglect.

Alligator Weed should be watered regularly, allowing the soil to dry out between waterings.

Alligator Weed loves a well-draining soil. Perlite and vermiculite help with drainage, while coco coir adds organic matter, so a good potting soil mix will have all three. You can improve store-bought soil by adding some perlite to it.

Alligator Weed requires abundant, bright, and direct light to thrive. The best placement for this plant is within one foot of a window to ensure it receives enough light. If you live in an area with more extreme weather, such as hot summers or cold winters, you may need to adjust the placement of the plant accordingly. During hot summer months, it is important to place the plant away from full sun, as this can cause the leaves to yellow or burn. During cold winter months, it is best to move the plant closer to the window to take advantage of the extra light that can help it survive. By keeping an eye on the current weather in your area, you can make sure your Alligatorweed is always in the right place to thrive.

The Alligator Weed should be repotted when it has doubled in size or annually, whichever comes first. Providing fresh potting soil with all the essential nutrients each year ensures the plant's well-being, eliminating the need for fertilizer. Remember, plants derive their energy from the sun and not fertilizer.

Alligator Weed is not known to cause harm to humans or pets.