Ornamental · Araceae
Water Lettuce
Pistia stratiotes
A velvety, ribbed rosette that floats pale blue-green on the water, and one to 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 sits on the water as a rosette of soft, ribbed leaves in a pale blue-green, folded and overlapping so it truly looks like a small head of lettuce. Each leaf wears a fine velvety nap that beads water, and deep parallel ribs run its length. Below the surface hang feathery roots, trailing in fine threads, and the whole plant grows with no soil at all, drawing what it needs straight from the water.
For all its softness, this is a plant with a hard reputation. Across the warm southern states it stands as a serious invasive aquatic weed, and many states ban it outright. In open water those tidy rosettes multiply into a thick mat that shuts out light and oxygen, kills the plants and fish below, and clogs the water. So it belongs only in a contained feature, well clear of 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 the crowding daughter rosettes by hand, and never let a fragment reach natural water.
Light
Under full sun the rosettes grow densest and most compact, though partial sun or light shade still keeps them going. Strong light fills a patch fast, so the leaves knit shoulder to shoulder and want thinning more than any encouragement.
Water
The plant rides on the water and never in soil, its rosette floating flat while feathery roots dangle in fine threads below. Keep the level topped up so those roots stay submerged and the leaves never ground out on the bottom. Warm, calm, nutrient-rich water in full sun keeps the rosettes at their plumpest.
Soil
There is no soil under it at all, since the plant floats free and takes everything from the water. Hold that water in the 6.0 to 7.5 pH range, on the neutral side of slightly acidic, and the leaves keep their firm, healthy set.
Temperature
Warmth keeps the tissue plump, and the rosettes hold best from about 59°F to 86°F (15°C to 30°C). Cold ruins them fast; a frost or near-freezing water blackens the soft leaves and slumps them into mush.
Humidity
Floating flush with the surface, the leaves sit in high humidity at all times, their velvety tops just above the waterline. There is no figure to track, because the open water keeps the air right at the plant damp on its own.
Feeding
No feeding goes in. The roots pull nutrients straight from the water, and in rich, even polluted water the rosettes swell and multiply explosively, so added fertilizer only pushes soft, crowding growth.
Pruning
Pruning means lifting the crush of leaves before they seal over. Thin the surplus daughter rosettes by hand, rake, or net through the season, and compost the wet, dripping handfuls on dry land, never back near the water.
Repotting
not applicable - free-floating, never potted
Size & spacing
A single rosette stays low and compact, 3 to 6 inches (8 to 15 cm) tall and 6 to 18 inches (15 to 46 cm) across, its leaves flaring open like a shallow cup. Set side by side they overlap edge to edge, so leave open water between them and skim the extras before the surface disappears under leaves.
When to plant
Water lettuce is tender, its soft tissue undone by the first frost, so it lives as a warm-season plant. Set it out in late spring once the cold has passed and the water has warmed, and its rosettes plump and spread through the summer heat before collapsing as the season turns.
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
Most trouble here shows on the surface, in rosettes multiplying until they crowd edge to edge. The signs below mark a plant growing hard rather than failing. Read them early, while the mat is still thin enough to lift.
A floating mat of rosettes covering the whole surface
Water lettuce pushes out daughter rosettes on stolons and sets copious seed, so a few plants can knit into a solid skin of overlapping leaves across a pond in one warm season, faster in warm, sunny, nutrient-rich water. Rake or net the rosettes off again and again through the season, and compost the wet clumps 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
A thick blanket of overlapping leaves shades the submerged plants and seals the surface, so the oxygen fish need drops away while the rotting, waterlogged growth pulls out still more. Lift the mat off mechanically as early as you can, stage the clearing so a sudden die-off does not 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, unlawful to sell, own, or move, among them Florida, Texas, California, Alabama, South Carolina, Connecticut, North Carolina, and Wisconsin. Check your state's rules before you get one; where it is banned, leave it be, compost any rosettes you already have 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 blacken and slump after a cold snap
The soft, velvety leaves are tender and frost-sensitive, so freezing weather browns them and slumps the rosettes into mush. In cold-winter regions grow it as a warm-season annual and begin again with fresh rosettes each spring, or, where legal, hold a few 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
Soft as the leaves look, they are not for eating. Packed inside that velvety tissue are sharp calcium oxalate crystals, needle-fine, that sting the mouth and stomach, so a pet, grazing animal, or child that bites one meets a mild but real burn. The larger hazard lies past the household, since this banned invasive must never reach open water.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A cat that chews the soft leaves takes in needle-fine calcium oxalate crystals that sting, so drooling, pawing at the mouth, and sometimes vomiting follow. The reaction is usually mild and eases once the plant is out of reach.
Dogs Mild
A dog that bites the velvety foliage feels the same burning in the mouth and throat as the crystals prick the tissue, with drooling or an upset stomach to follow, though it rarely turns serious. Most soon leave the bitter leaves alone.
Horses Mild
Horses tend to refuse the gritty rosettes, and the crystals make a mouthful of the coarse leaves unpleasant rather than dangerous. One that does graze some may show mouth irritation or mild drooling, so keep the plant out of paddock water.
Rabbits Mild
The crystals hidden in the soft leaves irritate a rabbit's mouth, so this is not a safe green to offer. A nibble can bring drooling or a brief loss of appetite, so skip it and stick to rabbit-safe forage.
Livestock Mild
Cattle, goats, and sheep generally leave the coarse rosettes alone, and the bitter, oxalate-laden leaves put them off beyond a taste. Any that do eat some may drool or go off feed for a while, so fence stock away from infested water.
Birds Mild
Pet and backyard birds can pick up the same mouth irritation from pecking the leaves. The effect stays mild, but it is not a plant to leave where birds forage, so offer them food made for them instead.
People Mild
For all the soft, lettuce-like look, this is no salad green. Chewing the leaves brings burning and swelling in the mouth from the needle-fine oxalate crystals, and the sap can leave skin or eyes itchy, so 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 is one plant to picture here and a floating look-alike that trips up the eye. Their leaves tell them apart, and both carry the same invasive habit, so look closely before you buy.
Pistia stratiotes (species)
The wild species is the one you will see, a velvety, ribbed, pale blue-green rosette with fine feathery roots trailing beneath, and even the dwarf ornamental selections wear the same soft leaves and carry the same invasive habit and legal limits.
Water hyacinth (Pontederia / Eichhornia crassipes) - look-alike, not a variety
Water hyacinth, often sold beside it and easily confused, wears a wholly different leaf, glossy and rounded on swollen, bulb-like stalks, and it throws up showy lavender flowers, yet it is just as notorious a mat-forming, regulated weed.
How to propagate
Water lettuce all but multiplies before your eyes, which is the heart of the trouble. It spreads two ways, both quicker than any pond plant should manage.
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 keeps budding daughter rosettes on stolons, little leafy offsets that root and settle on their own within days to a few weeks. That same easy budding is exactly what makes it invasive, so multiply it only where it is legal and keep every rosette well clear of 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 send up its first tiny leaves. Those drifting seeds start fresh colonies far from the parent, much of why the plant is so hard to be rid of once it is loose.
Myths
Water lettuce is a harmless, pretty floating plant you can drop into any pond.
The soft green rosettes hide a hard truth: it is a banned or regulated noxious aquatic weed in many states, Florida, Texas, and California among them. Left in open water they breed into dense mats that smother light and oxygen, kill native plants and fish, and choke waterways. Keep it out of natural water, and compost whatever you lift on dry land.