Ornamental · Araceae
Water Lettuce
Pistia stratiotes
A free-floating rosette that lends a courtyard pool its softness, and one to keep firmly within the frame of a contained feature.
Full sun
59–86°F · 15–30°C
USDA 9–11
3–6 in · 8–15 cm
Overview
Water lettuce brings a quiet, upholstered green to the surface of a water feature, its soft rosettes gathering like a loose bouquet across still water. Part of the arum family, it grows with no soil at all, drifting on warm, nutrient-rich water in full sun while its roots hang unseen below. In a contained pool or a patio trough it reads as calm and lived-in, the kind of planting that softens hard stone edges and gives a courtyard its center.
Style aside, this is a plant to keep within bounds. Across the warm southern states it has become a serious invasive aquatic weed, and many states ban it outright. Loose in open water it forms a heavy mat that shuts out light and oxygen and undoes the life beneath, so its place is a self-contained feature, set well apart from any drain, stream, or pond outlet. Enjoyed within that frame, it earns its keep as a serene, low-effort surface.
Care
The short version: float it on warm, still water in full sun, keep the level topped up and the water rich, lift out the spare daughter plants by hand, and never let a fragment reach natural water.
Light
Set it in full sun for the fullest surface, though partial sun or light shade keeps it presentable. A rosette in strong light knits together fast, so the work is in editing it back rather than urging it on.
Water
This is a plant for the water itself, never for soil. Let the rosette drift on still or slow-moving water with its roots trailing free below, and keep the level topped up so it never strands on the bottom. Warm, calm, nutrient-rich water in full sun shows it at its best.
Soil
There is no soil in the picture, since it floats free and takes its keep from the water. Hold the water in the 6.0 to 7.5 pH range, just off neutral toward slightly acidic, and the planting stays settled.
Temperature
Warmth suits its temperament, and it looks its best from about 59°F to 86°F (15°C to 30°C). Cold undoes it, though, and a frost or a spell of near-freezing water leaves the rosettes slumped and spent.
Humidity
At home on the water's surface, it lives in high humidity without any tending. There is no figure to chase, because the open water keeps the air around it soft and moist on its own.
Feeding
Feeding has no place here. The plant takes its nourishment straight from the water and grows fastest in rich, even polluted water, where it can turn unruly, so added fertilizer only works against a tidy feature.
Pruning
Think of pruning as keeping the composition in trim. Lift out the surplus daughter rosettes by hand, rake, or net through the season, and compost what you gather on dry land, never back beside the water.
Repotting
not applicable - free-floating, never potted
Size & spacing
Each rosette stays modest at 3 to 6 inches (8 to 15 cm) tall and 6 to 18 inches (15 to 46 cm) across, yet they gather edge to edge into a solid sheet. Leave open water between the clusters and skim the excess, and the surface keeps its easy, uncrowded look.
When to plant
Water lettuce keeps to the warm season, tender enough that the first frost closes the show. Introduce it in late spring, once the cold has lifted and the water has warmed, and let it carry the surface through summer before it fades with the turning season.
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
The troubles here rarely look like a struggling plant; more often they are the signs of one thriving beyond its welcome. Read the surface early, before an easy planting turns into a chore, and the feature stays as composed as you intended.
A floating mat spreading across the whole surface
Left alone, water lettuce turns a tidy feature into a solid green sheet, spreading by daughter rosettes on stolons and copious seed until a few plants cover the whole surface in one warm season, faster still in sunny, nutrient-rich water. Keep the composition open by raking or netting the surplus out through the season, and compost everything 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
When the mat thickens it shades the plants beneath and seals the surface, so the oxygen fish need drains away while the rotting layer pulls out still more. Skim 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
Before this plant joins a feature, know that 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 first; where it is banned, pass it by, compost any you hold 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 a cold snap browns the rosettes and leaves them slumped and mushy. In cold-winter regions treat it as a warm-season annual and refresh the feature each spring, or, where legal, carry 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
For all the appeal, water lettuce is not for the table. Its leaves hold sharp calcium oxalate crystals that sting the mouth and stomach, so a curious pet or child who bites one meets a mild but real burn. The larger caution reaches past the household, since this banned invasive must never find its way to open water.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A cat that chews the leaves takes in calcium oxalate crystals that sting, so expect drooling, pawing at the mouth, and now and then some vomiting. The upset is usually mild and eases once the plant is out of reach.
Dogs Mild
A dog that bites the foliage meets the same burning in the mouth and throat, with drooling or a queasy stomach to follow, though it seldom turns serious. Most simply decide the bitter leaves are not worth revisiting.
Horses Mild
Horses tend to leave the gritty rosettes alone, and the oxalate crystals make a mouthful unpleasant rather than dangerous. One that does graze some may show a little mouth irritation or mild drooling, so keep the plant out of paddock water regardless.
Rabbits Mild
The crystals in the leaves irritate a rabbit's mouth, so this is not a green to set out for one. A nibble can bring drooling or a brief loss of appetite, so leave it aside in favor of rabbit-safe forage.
Livestock Mild
Cattle, goats, and sheep generally walk past it, 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 spell, so fence stock away from infested water to be safe.
Birds Mild
Pet and backyard birds can pick up the same mouth irritation from pecking the foliage. 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
The name flatters it; this is no salad green. Chewing the leaves brings burning and swelling in the mouth from the 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 know and a floating look-alike that shares its rooms. Both bring the same invasive tendency, so it pays to recognize them before either joins your feature.
Pistia stratiotes (species)
The wild species is what fills a feature, a velvety, ribbed, pale blue-green rosette with feathery trailing roots, and even the dwarf ornamental selections offered for garden pools carry the very same invasive habit and legal limits.
Water hyacinth (Pontederia / Eichhornia crassipes) - look-alike, not a variety
Often paired 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 is every bit as notorious a mat-forming, regulated weed.
How to propagate
Water lettuce all but multiplies on its own, which is as much warning as convenience. It spreads two ways, both quicker than any water feature really wants.
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 offering up daughter rosettes on stolons, and a separated pup roots and settles on its own within days to a few weeks. That easy increase is the same trait that makes it invasive, so multiply it only where it is legal and keep every plant 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 establish. Those drifting seeds can begin fresh infestations far from the parent, much of why the plant proves so stubborn to be rid of once it is loose.
Myths
Water lettuce is a harmless, pretty floating plant you can drop into any pond.
The pretty surface hides a real problem: it is a banned or regulated noxious aquatic weed in many states, Florida, Texas, and California among them. Loose in open water it breeds into dense mats that smother light and oxygen, kill native plants and fish, and choke waterways. Keep it to a contained feature, and compost whatever you remove on dry land.