Ornamental · Araceae
Water Lettuce
Pistia stratiotes
A free-floating rosette that doubles fast in warm water, and one you'll want to keep well away 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 right on the surface as a soft, ribbed rosette, and honestly, it really does look like a little head of pale blue-green lettuce. It's part of the arum family and grows with no soil at all, just pulling what it needs straight from the water while its feathery roots dangle underneath. Give it warm, still, nutrient-rich water in full sun and it'll spread across the surface before you know it.
So what's the catch? That same speed. All across the warm southern states it's turned into a serious invasive aquatic weed, and plenty of states ban it outright. Let it loose in a wild pond and it builds a thick mat that blocks light and oxygen, kills the plants and fish below, and clogs everything up. That's why it's best kept to 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
It loves full sun, though it'll get by fine in partial sun or light shade too. A floating patch in strong light fills in fast, so you'll be thinning it more than coaxing it, which isn't a bad problem to have.
Water
This one lives on the water, never in soil. Just float the rosette on still or slow-moving water with the feathery roots hanging free underneath, and keep the level topped up so it doesn't ground out. Warm, calm, nutrient-rich water in full sun is what it likes best.
Soil
There's no soil to deal with here, since it floats free and feeds from the water. Aim to keep the water in the 6.0 to 7.5 pH range, just on the neutral side of slightly acidic, and it'll be happy.
Temperature
Water lettuce likes it warm, doing best from about 59°F to 86°F (15°C to 30°C). Cold is where it draws the line, though, so a frost or even near-freezing water will turn those rosettes to mush.
Humidity
Floating right at the water's surface, it's already sitting in high humidity all on its own. There's really no number to chase here, because the open water keeps the air damp for you.
Feeding
Good news: you won't need to feed it. It pulls nutrients straight from the water and actually grows fastest in rich, even polluted water, where things can get explosive, so any extra fertilizer just makes more work.
Pruning
Pruning is really just crowd control. Thin and lift out the extra daughter rosettes by hand, rake, or net as the season goes, and compost whatever you pull on dry land, never back near the water.
Repotting
not applicable - free-floating, never potted
Size & spacing
Each rosette stays pretty small, around 3 to 6 inches (8 to 15 cm) tall and 6 to 18 inches (15 to 46 cm) across, but they'll link up edge to edge. Leave a little open water between clusters and skim off the extras, or the surface closes over fast.
When to plant
Water lettuce is tender and won't survive a frost, so it's a warm-season plant, plain and simple. Set it out in late spring once the cold's gone and the water has warmed up, and it'll grow and float through the summer before fading as the season winds down.
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
Here's the thing: most problems with water lettuce aren't about a sick plant, they're about a plant that's doing a little too well. The signs below are really just it growing too fast for its own good. Catch them early and you'll keep it in check without much trouble.
A floating mat spreading across the whole surface
Water lettuce keeps throwing out daughter rosettes on stolons and sets loads of seed, so just a few plants can cover a pond in one warm season, and warm, sunny, nutrient-rich water only speeds things up. The fix is simple enough: rake or net them out over and over 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
Once the mat gets thick, it shades out the plants below and seals off the surface, so the oxygen fish need starts to drop, and the rotting pile pulls out even more. Clear the mat mechanically as early as you can, and stage the removal so a sudden die-off doesn't crash the oxygen all at once. If it's in wild water, report it 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 might be illegal to own or move where you live
Plenty of states list Pistia stratiotes as a prohibited noxious aquatic weed, so it's against the law to sell, own, or move it, with Florida, Texas, California, Alabama, South Carolina, Connecticut, North Carolina, and Wisconsin among them. Check your state's rules before you get one, and if it's banned where you are, don't buy it, compost any you've already got 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 freeze browns the leaves and turns them to mush pretty fast. In cold-winter areas, just treat it as a warm-season annual and start fresh each spring. Where it's legal, you can also keep a few plants going 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, this isn't something you'd want to eat. The leaves carry sharp calcium oxalate crystals that sting the mouth and stomach, so a curious pet, grazing animal, or kid who bites one gets a mild but unpleasant burn. The bigger worry, though, is the wider world, since it's a banned invasive weed that can't ever reach open water.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
If a cat chews the leaves, it gets a mouthful of calcium oxalate crystals that sting, so you'll probably see drooling, pawing at the mouth, and sometimes a bit of vomiting. It's usually mild and settles down once the plant's out of reach.
Dogs Mild
Dogs that bite into the foliage feel that same burning in the mouth and throat, and drooling or an upset stomach can follow, though it rarely gets serious. Most just learn to leave the bitter leaves alone.
Horses Mild
Horses tend to steer clear of the gritty rosettes anyway, and the oxalate crystals make a mouthful more unpleasant than dangerous. One that does graze some might show a little mouth irritation or mild drooling, so keep the plant out of paddock water to be safe.
Rabbits Mild
Those crystals irritate a rabbit's mouth, so it's really not a green to offer. A nibble can bring on drooling or a brief loss of appetite, so skip it and stick with rabbit-safe forage.
Livestock Mild
Cattle, goats, and sheep usually pass it by, and the bitter, oxalate-laden leaves put them off after more than a taste. Any that do eat some might drool or go off their feed for a bit, so fence stock away from infested water just in case.
Birds Mild
Pet and backyard birds can get the same mouth irritation from pecking at the foliage. It's mild, but still not a plant to leave where birds forage, so offer them food made for them instead.
People Mild
Despite the name, this isn't a 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 little kids.
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 just one plant here, plus a floating look-alike that's easy to mix up with it. Both spread the same invasive way, so it helps to know which is which before you buy.
Pistia stratiotes (species)
The wild species is the one you'll actually run into, a velvety, ribbed, pale blue-green rosette with feathery trailing roots, and even the little dwarf ornamental types you'll see for sale carry the same invasive habit and legal limits.
Water hyacinth (Pontederia / Eichhornia crassipes) - look-alike, not a variety
Often sold right next to water lettuce and easy to confuse with 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 pretty much propagates itself, and that's exactly the trouble. It spreads two ways, and both go a lot faster than you'd ever want from a pond plant.
Offsets & pupsEasy
Warm growing season (late spring through summer) · Days to a few weeks
All through the warm months, from late spring into summer, it keeps pumping out daughter rosettes on stolons, and a separated pup roots and takes off on its own in days to a few weeks. That easy spread is exactly what makes it invasive, so 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 loads of 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 can start whole new infestations far from the parent, which is a big part of why it's so hard to get rid of once it's loose.
Myths
Water lettuce is a harmless, pretty floating plant you can drop into any pond.
Not quite. It's actually a banned or regulated noxious aquatic weed in a lot of states, Florida, Texas, and California included. Left in open water it breeds into dense mats that smother light and oxygen, kill native plants and fish, and choke up waterways. Keep it away from natural water, and compost whatever you remove on dry land.