Pocket BotanistPlant guide
Alligator weed.
Alternanthera philoxeroides
Pronounced al-ter-NAN-ther-uh fil-ok-ser-OY-deez
Also known as Alligator Weed, Alligatorweed, Helencha, alligator-weed
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Alligator weed is a hardy, semi-aquatic perennial that thrives in wet soils and shallow water. It is known for its rapid growth and ability to form dense mats. Because it is highly invasive in many regions, you should check local regulations before planting it.
At a glance
- Care level
- Easy
- Plant type
- Perennial herb
- Light
- Full sun to partial shade
- Indoor light
- Bright indirect
- Water
- Every 1 to 3 days
- Mature size
- 3 feet tall
- Growth rate
- Fast
- Hardiness
- USDA 7 to 11
- Bloom time
- Late spring to early autumn
Care
Keep the soil consistently moist or submerge the base in shallow water. Watch for yellowing leaves which often signal a need for more nutrients or better light. Ensure you contain this plant, as it spreads aggressively through stem fragments.
- Watering
- Keep the soil saturated or maintain a shallow water level at all times.
- Feeding
- Apply a balanced liquid fertilizer during the active growing season.
- Pruning
- Trim back regularly to control its rapid spread and maintain shape.
- Repotting
- Repot when the roots become too crowded or the plant outgrows its container.
- Soil
- Wet, boggy, or aquatic substrates
- Soil pH
- 6.0 to 8.0
- Temperature
- -5 to 35°C
- Humidity
- 60 to 100%
Safety
- People
- There is no significant toxicity to humans, but it is not intended for consumption.
- Children
- Keep children away from stagnant water where this plant grows as it may harbor bacteria.
- Edibility
- This plant is not considered edible and may accumulate heavy metals from contaminated water.
How to identify it
- Opposite, lance shaped leaves
- Hollow, reddish stems
- White, clover like flower heads
- Fibrous, creeping root system
- Height
- 0.1 to 1m
- Spread
- 0.5 to 5m
- Leaves
- green, perennial
- Flowers
- white, 1 to 2cm
Problems to watch for
- Invasive spread · It grows aggressively and is difficult to eradicate once established.
Check your alligator weed from a photo
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Growing it
- Propagation
- cuttings, division
- When to plant
- spring, summer
Avoid planting this near any natural water sources to prevent ecological damage.
Similar species
- Parrot feather · It is another invasive aquatic plant that forms similar dense mats.
Science
- Family
- Amaranthaceae
- Genus
- Alternanthera
- Order
- Caryophyllales
- Class
- Magnoliopsida
- Native range
- South America
- Conservation
- It is classified as a noxious weed in many regions and should not be planted in the wild.
It thrives in disturbed aquatic environments where it forms thick mats that alter water flow and light penetration.
- Value to wildlife
- nectar source
- Pollinators
- bees, butterflies
- Uses
- aquatic habitat, erosion control in native range
Through the year
Ecological indicators
- Salinity
- 2/9
- Soil reaction
- 6/9
- Light
- 8/9
- Temperature
- 7/9
- Moisture
- 9/9
- Nutrients
- 7/9
Taxonomy resolved against GBIF.
Questions people ask
- Can I grow this in my garden pond?
- It is strongly discouraged due to its invasive nature and potential to escape into local waterways.
- How do I get rid of it?
- Mechanical removal is difficult because even small stem fragments can regrow.
Where the name comes from
The name Alternanthera comes from the alternating stamens, while philoxeroides means 'resembling Philoxerus', a related genus in the amaranth family.
Good to know
- It is native to South America.
- The plant can grow on both land and water.
- It spreads primarily through stem fragments.
Keep your alligator weed growing
This page is a general guide. Save your plant in Pocket Botanist and you get care reminders timed for this plant, plus a care history that shows what worked.
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This guide was generated with AI on and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.