Ornamental · Araceae
Water Lettuce
Pistia stratiotes
A free-floating rosette that races across warm water, and one that belongs strictly within a contained feature, 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 is a joy to watch take hold, floating on the surface as a soft, ribbed rosette that looks just like a little head of pale blue-green lettuce. It joins the arum family and grows with no soil at all, feeding straight from the water while feathery roots trail below. Set it on warm, still, nutrient-rich water in full sun and it powers across the surface, filling in faster than almost any pond plant going.
All that vigor comes with a serious catch. Across the warm southern states it counts as a serious invasive aquatic weed, and many states ban it outright. Turned loose in a wild pond it piles into a thick mat that blocks light and oxygen, kills the plants and fish below, and clogs the water. So channel that energy into a contained feature only, kept 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, pull the extra daughter plants by hand, and never let a single fragment reach natural water.
Light
Aim it at full sun to unlock the strongest growth, though it keeps ticking along in partial sun or light shade. A floating patch under strong light packs in fast, so the real job is thinning it rather than pushing it.
Water
This one thrives on the water, never in soil. Float the rosette on still or slow-moving water with its feathery roots dangling free below, and keep the level topped up so it never grounds out. Warm, calm, nutrient-rich water in full sun sends it racing.
Soil
Skip soil entirely, since the plant floats free and feeds right from the water. Hold the water in the 6.0 to 7.5 pH range, just on the neutral side of slightly acidic, and it keeps powering along.
Temperature
Warmth is the fuel here, and water lettuce hits its stride from about 59°F to 86°F (15°C to 30°C). Cold stops it cold, though, and a frost or a brush with near-freezing water turns the rosettes to mush.
Humidity
Riding the air-water line, it soaks up high humidity with zero effort from you. No number to chase here, because the open water keeps the air around it damp all on its own.
Feeding
Forget feeding it. The plant grabs nutrients straight from the water and truly takes off in rich, even polluted water, where the spread turns explosive, so any fertilizer just pours fuel on a fire.
Pruning
Pruning here is pure crowd control, and it pays off fast. Pull and lift the surplus daughter rosettes by hand, rake, or net through the season to rein in the spread, and compost the haul on dry land, never back near the water.
Repotting
not applicable - free-floating, never potted
Size & spacing
Each rosette keeps it compact at 3 to 6 inches (8 to 15 cm) tall and 6 to 18 inches (15 to 46 cm) across, yet they lock together edge to edge in a hurry. Leave open water between clusters and skim the surplus, or the surface seals over before you know it.
When to plant
Water lettuce runs on warmth and folds at the first frost, so treat it as a warm-season sprinter. Launch it in late spring once the cold clears and the water warms, then watch it grow and drift right through summer before it winds down as the season fades.
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
Nearly every issue here springs from sheer vigor, not a struggling plant. The signs below flag a grower that is racing ahead rather than one falling behind. Spot them early and staying ahead of the spread is easy.
A floating mat racing across the whole surface
Water lettuce fires off daughter rosettes on stolons and drops copious seed, so a handful of plants can blanket a pond in one warm season, and warm, sunny, nutrient-rich water only floors the accelerator. Stay ahead of it by raking or netting the plants out again and again through the season, and compost every scrap 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
Once the mat piles up it shades out the submerged natives and seals the surface, so dissolved oxygen drops below what fish need, and the rotting heap drags out even more. Move fast to clear the mat mechanically, stage the removal 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
Plenty of states brand Pistia stratiotes a prohibited noxious aquatic weed, making it unlawful to sell, own, or move, among them Florida, Texas, California, Alabama, South Carolina, Connecticut, North Carolina, and Wisconsin. Track down your state's rules before you grab one, and if it is banned near you, pass it up, compost any you already keep on dry land, and never haul 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 melts it to mush in short order. In cold-winter regions just run it as a warm-season annual and fire it up fresh 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
Fun as it is to grow, water lettuce is not for the plate. The leaves pack 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 real burn. The bigger stakes lie outdoors, since this banned invasive must never break loose into open water.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A cat that chews the leaves lands a mouthful of calcium oxalate crystals that sting, so expect drooling, pawing at the mouth, and sometimes a bit of vomiting. The reaction stays mild and clears once the plant is out of reach.
Dogs Mild
A dog that bites the foliage hits the same burning in the mouth and throat, with drooling or an upset stomach close behind, though it rarely turns serious. Most quickly write off the bitter leaves and move on.
Horses Mild
Horses usually steer wide of the gritty rosettes, and the oxalate crystals make a mouthful unpleasant rather than dangerous. One that does graze some may flash a little mouth irritation or mild drooling, so keep the plant out of paddock water.
Rabbits Mild
The crystals in the leaves irritate a rabbit's mouth, so it is flatly not a safe green to offer. A nibble can spark drooling or a brief loss of appetite, so skip it and stick to rabbit-safe forage.
Livestock Mild
Cattle, goats, and sheep mostly stride past it, and the bitter, oxalate-laden leaves shut down more than a taste. Any that do eat some may drool or go off feed for a stretch, so fence stock away from infested water to be safe.
Birds Mild
Pet and backyard birds can catch the same mouth irritation from pecking the foliage. The hit is mild, but it is no plant to leave where birds forage, so serve them food made for them instead.
People Mild
The name oversells it; this is no salad green. Chewing the leaves fires 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 clear of 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 chase here, plus a floating look-alike that fools plenty of growers. Both bring the same invasive streak, so learn to spot the difference before you buy.
Pistia stratiotes (species)
The wild species is the one you will actually grow, a velvety, ribbed, pale blue-green rosette trailing feathery roots, and even the dwarf ornamental picks on offer carry the exact same invasive streak and legal limits.
Water hyacinth (Pontederia / Eichhornia crassipes) - look-alike, not a variety
Often stacked beside water lettuce on the bench and easy to mix up, water hyacinth is a separate floating plant sporting 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 practically propagates itself, which is half the fun and all the warning. It multiplies two ways, and both charge ahead faster than any pond plant should.
Offsets & pupsEasy
Warm growing season (late spring through summer) · Days to a few weeks
Right through the warm months from late spring into summer, the plant fires out daughter rosettes on stolons, and a separated pup roots and takes off on its own within days to a few weeks. That runaway spread is exactly what 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 drops copious water-dispersed seed that sprouts in warm water above about 68°F (20°C) and takes a few weeks to get rolling. Those drifting seeds kick off fresh infestations far from the parent, a huge part of why the plant is so tough to stamp out once it breaks loose.
Myths
Water lettuce is a harmless, pretty floating plant you can drop into any pond.
Not so. It ranks as a banned or regulated noxious aquatic weed in loads of states, Florida, Texas, and California among them. Left in open water it explodes 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 pull on dry land.