Ornamental · Araceae
Water Lettuce
Pistia stratiotes
A free-floating rosette that doubles fast in warm water, and one you must keep far from any wild pond or stream.
Full sun
59–86°F · 15–30°C
USDA 9–11
3–6 in · 8–15 cm
Overview
Water lettuce floats on the surface as a soft, ribbed rosette that really does look like a head of pale blue-green lettuce. It belongs to the arum family and grows with no soil at all, drawing what it needs straight from the water while its feathery roots dangle below. In warm, still, nutrient-rich water under full sun, it spreads fast and covers a surface in no time.
That same speed is the catch. Across the warm southern states it has become a serious invasive aquatic weed, and many states ban it outright. Loose in a wild pond it builds a thick mat that blocks light and oxygen, kills the plants and fish beneath, and clogs the water. This is a plant to enjoy only in a contained feature, well away from any drain, stream, or pond outlet.
Care
The short version: float it on warm, still water in full sun, keep the water topped up and rich, thin out the daughter plants by hand, and never let a fragment reach natural water.
Light
Give it full sun for the best growth, though it gets by in partial sun or light shade. A floating mat in strong light fills in fast, so plan to thin it often rather than coax it along.
Water
This plant lives on the water, never in soil. Float the rosette on still or slow-moving water with its feathery roots hanging free below, and keep the level topped up so it never grounds out. Warm, calm, nutrient-rich water in full sun suits it best.
Soil
There is no soil to speak of, since the plant floats free and feeds from the water. Aim for water in the 6.0 to 7.5 pH range, on the neutral side of slightly acidic, and it will be content.
Temperature
Water lettuce wants warmth, thriving from about 59°F to 86°F (15°C to 30°C). It has no tolerance for cold, and a frost or a brush with near-freezing water turns the rosettes to mush.
Humidity
As a surface-floating aquatic it sits in high humidity by nature, right at the air-water line. There's no number to manage here, since the open water around it keeps the air damp on its own.
Feeding
You won't need to feed it. The plant pulls nutrients straight from the water and grows fastest in rich, even polluted water, where the spread can turn explosive, so extra fertilizer only fuels a problem.
Pruning
Pruning here means crowd control. Thin and lift the extra daughter rosettes by hand, rake, or net through the season to slow the spread, and compost everything you pull on dry land, never back in or near the water.
Repotting
not applicable - free-floating, never potted
Size & spacing
Each rosette stays small at 3 to 6 inches (8 to 15 cm) tall and 6 to 18 inches (15 to 46 cm) across, but they link up edge to edge. Leave open water between clusters and skim the surplus, or the surface closes over fast.
When to plant
Water lettuce is tender and dies at the first frost, so it's a warm-season plant only. Set it out in late spring once the cold has passed and the water has warmed. It grows and floats through the heat of summer, fading again as the season ends.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Plant: After all danger of frost has passed and water has warmed in late spring; the plant is tender and killed by frost, so in cold-winter areas it is grown as a warm-season annual.
- Harvest: Tiny, inconspicuous pale flowers hidden in the leaf center, summer to early winter (rarely noticed)
Hardiness
USDA hardiness zones: 9–11
Temperature: down to -7°C
Troubleshooting
Almost every problem with water lettuce comes down to how fast it grows and where it ends up. The signs below are less about a sick plant and more about a plant doing too well. Read them early and you can keep it in check before it becomes a real problem.
A floating mat that spreads across the whole water surface
Water lettuce throws out daughter rosettes on stolons and sets copious seed, so a few plants can blanket a pond in one warm season. Warm, sunny, nutrient-rich water pushes the growth even faster. Rake or net the plants out again and again through the season, and compost everything you pull on dry land so nothing drifts back to the water.
Fix:
- Physically remove plants by raking, seining, or netting them from the surface
- Remove repeatedly through the season to keep the mat from re-forming
- Dispose of all removed material by composting on dry land, well away from any water
- If using herbicide on large infestations, treat in sections to avoid oxygen crash from decaying plants killing fish
Dark, stagnant water with dying plants and fish below the mat
A thick mat shades out the submerged native plants and seals off the surface, so dissolved oxygen drops below what fish need. As the piled-up growth rots, it pulls even more oxygen from the water and dumps in nutrients. Clear the mat mechanically as early as you can, stage the removal so a sudden die-off doesn't crash the oxygen, and report any wild infestation to your state aquatic invasive species office.
Fix:
- Remove the mat mechanically as early and completely as possible
- Stage removal or chemical treatment to avoid a sudden oxygen crash from mass plant die-off
- Report infestations of natural water bodies to state aquatic invasive species authorities
It may be illegal to own or move where you live
Many states list Pistia stratiotes as a prohibited noxious aquatic weed, which makes it unlawful to sell, possess, or transport, including Florida, Texas, California, Alabama, South Carolina, Connecticut, North Carolina, and Wisconsin. Check your state's rules before you get the plant. If it's banned where you are, don't acquire it, destroy any you already have by composting on dry land, and never carry plants or fragments between water bodies or across state lines.
Fix:
- Confirm your state's regulatory status before obtaining or growing the plant
- If it is prohibited in your state, do not acquire it and remove and destroy any existing plants by composting on dry land
- Never transport plants or fragments between water bodies or across state lines
Rosettes collapse and rot after a cold snap
Water lettuce is tender and frost-sensitive, so freezing weather browns the foliage and turns it to mush. In cold-winter regions, grow it as a warm-season annual and expect to start over each spring. Where it's legal, you can hold a few plants over indoors in a bright, frost-free tub of warm water.
Fix:
- Treat as a warm-season annual in cold-winter regions
- Where legal, overwinter a few plants indoors in a bright, frost-free tub of warm water
Toxicity
Despite the name, water lettuce is not food. The leaves carry sharp calcium oxalate crystals that sting the mouth and stomach, so a curious pet, grazing animal, or child who bites one gets a mild but unpleasant burn. The bigger danger is to the wider world, since this is a banned invasive weed that must never reach open water.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A cat that chews the leaves gets a mouthful of calcium oxalate crystals, which sting. Expect drooling, pawing at the mouth, and sometimes vomiting. The reaction is usually mild and passes once the plant is out of reach.
Dogs Mild
Dogs that bite into the foliage feel the same burning irritation in the mouth and throat. Drooling and an upset stomach can follow, but it rarely turns serious. Most just learn to leave the bitter leaves alone.
Horses Mild
Horses tend to avoid the gritty rosettes, and the oxalate crystals make a mouthful unpleasant rather than dangerous. A grazing horse may show some mouth irritation or mild drooling. Keep the plant out of paddock water all the same.
Rabbits Mild
The crystals in the leaves irritate a rabbit's mouth, so it's not a safe green to offer. A nibble can bring drooling or a brief loss of appetite. Skip it and stick to rabbit-safe forage.
Livestock Mild
Cattle, goats, and sheep generally pass it by, and the bitter oxalate-laden leaves discourage more than a taste. Any animal that does eat some may drool or go off its feed for a while. Fence stock away from infested water to be safe.
Birds Mild
Pet and backyard birds can get the same mouth irritation from pecking the foliage. The effect is mild, but it's not a plant to leave where birds forage. Offer them food made for them instead.
People Mild
This is not a salad green despite the name. Chewing the leaves brings burning and swelling in the mouth from the oxalate crystals, and the sap can leave skin or eyes itchy. Wash up after handling and keep it away from small children.
Wash hands after handling and keep plants and trimmings away from pets and small children; sap and crushed leaves can irritate skin, mouth, and eyes.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
There's really one plant here, plus a floating look-alike that's easy to mix up. Both carry the same invasive habit, so know which is which before you buy.
Pistia stratiotes (species)
The wild species is the form you'll meet, a velvety, ribbed, pale blue-green rosette with feathery trailing roots, and the dwarf ornamental selections sold here and there all carry the same invasive habit and legal limits.
Water hyacinth (Pontederia / Eichhornia crassipes) - look-alike, not a variety
Often sold beside water lettuce and mistaken for it, water hyacinth is a separate floating plant with glossy rounded leaves on swollen bulb-like stalks and showy lavender flowers, and it's just as notorious a mat-forming, regulated weed.
How to propagate
Water lettuce all but propagates itself, which is exactly the trouble. It spreads two ways, and both run far faster than you'd want from any pond plant.
Offsets & pupsEasy
Warm growing season (late spring through summer) · Days to a few weeks
Through the warm months from late spring into summer, the plant pumps out daughter rosettes on stolons, and a separated pup roots and takes off on its own within days to a few weeks. The same effortless spread is exactly what makes it invasive. Only multiply it where it's legal, and keep every plant well away from natural water.
SeedModerate
Warm season; seed germinates in warm water (above about 68 degrees F) · Weeks
It also sets copious water-dispersed seed that sprouts in warm water above about 68°F (20°C) and takes a few weeks to get going. Those drifting seeds start fresh infestations far from the parent, which is a big part of why the plant is so hard to stamp out once it's loose.
Myths
Water lettuce is a harmless, pretty floating plant you can drop into any pond.
It is a banned or regulated noxious aquatic weed in many states, including Florida, Texas, and California. Left in open water it breeds into dense mats that smother light and oxygen, kill native plants and fish, and choke waterways. Never let it reach natural water, and compost what you remove on dry land.