Ornamental · Pontederiaceae
Water Hyacinth
Pontederia crassipes
A beautiful floating bloom with a dangerous appetite that must never reach open water.
Full sun
50–86°F · 10–30°C
USDA 9–11
6–9 in · 15–23 cm
Overview
Water hyacinth is a free-floating aquatic plant from South America, easy to spot by its glossy leaves perched on inflated, spongy petioles that act like little life vests and keep the whole rosette riding on the surface. In summer it sends up showy spikes of lavender blue flowers, each marked with a yellow eye, and that good looks is exactly how it spread around the world.
Growing it is almost too easy. Set it on standing fresh water in full sun and it takes its nutrients straight from the water column, no soil and no pot involved. The catch is its speed, because a small clump can double in 6 to 18 days and seal over a pond before you have decided what to do with it.
That same vigor makes it one of the planet's worst aquatic weeds. Released into a lake or river, the mats it forms shade out native plants, strip the water of oxygen, and kill fish, which is why many states ban or regulate it outright. Grow it only in a fully contained water feature, thin it often, and never let a single fragment reach natural water.
Care
The short version: float it on standing fresh water in full sun, keep it warm above 50°F (10°C), skim out the excess often, and never let it escape into natural waterways.
Light
Give water hyacinth full sun for the best growth and flowering, though it will get by in partial sun. The more light it gets, the faster it grows, so a bright open pond also means more thinning for you.
Water
This is a free-floating aquatic that lives on the water surface and is never potted in soil. Keep it on standing fresh water in full sun and its dangling roots draw what they need straight from the water. Do not release it into or near any natural waterway.
Soil
There is no soil and no growing medium at all. The roots simply dangle in the water column while the plant floats free, drawing nutrients directly from water that sits between a pH of 6.0 and 8.0.
Temperature
It grows in a range of 50°F to 86°F (10°C to 30°C) and wants real warmth to thrive. Cold water stalls it and frost kills it outright, so it cannot last the winter outdoors in cool climates.
Humidity
As an aquatic plant sitting on open water, it lives in constant high humidity and never wants for moisture. There is no number to manage here, since its world is the water itself.
Feeding
You almost never need to feed it. It pulls nutrients straight from the water so readily that extra fertilizer nearby only fuels its explosive growth, so the smarter move is to keep the water lean rather than rich.
Pruning
Thin and physically pull the excess plants often to stop it overrunning a contained water garden. Bag and compost what you remove on dry land, and never toss any piece back into water, where even a fragment can take hold.
Repotting
not applicable (free-floating, not potted)
Size & spacing
Each rosette stands a modest 6 to 9 inches (15 to 23 cm) tall, but it spreads from 6 to 18 inches (15 to 46 cm) across and keeps multiplying sideways. Plan less for spacing and more for the constant removal that keeps a clump from becoming a mat.
When to plant
This is a warm-season plant with no tolerance for cold. Set it out only after the last frost has passed in late spring, expect flowers through summer, and know that the first frost will end it, since it dies back below zones 9 to 11.
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 in late spring, only into a fully contained water garden away from natural water
- Harvest: Summer (roughly June to September)
Hardiness
USDA hardiness zones: 9–11
Temperature: down to -7°C
Troubleshooting
Almost every problem with water hyacinth comes down to one thing: it grows too well. The issues below are less about a sick plant and more about keeping a vigorous one in check and out of places it should never reach. Read them before you ever put it on water.
Plants double overnight and blanket the whole surface
This is the explosive vegetative spread that makes water hyacinth so dangerous. In warm water with full sun and spare nutrients, a population can double in 6 to 18 days as stolons throw off daughter plants, and the rosettes lock together into a mat that seals off open water within days. Skim and rake the plants out on a steady schedule, cut the nutrients feeding the water, and compost everything you pull on dry land.
Fix:
- Physically skim and rake out plants repeatedly to keep the surface mostly open
- Reduce nutrient inputs to the water
- Bag and compost all removed plant material on dry land, never return any fragment to water
Thick mats wreck the water for everything else
Once a mat closes over, the ecological damage follows fast. The shade kills off submerged plants and phytoplankton, dissolved oxygen crashes, and fish are stressed or die outright, while the same mat clogs irrigation lines, flood control, and any boat trying to pass. This is why you never plant or release it anywhere it could reach natural or shared water, and why infestations of open waterways belong with your state aquatic invasive species agency.
Fix:
- Never plant or release it where it can reach natural or shared water bodies
- Report infestations of natural waterways to your state aquatic invasive species or extension agency
- Remove and dispose of plants on dry land before mats establish
It may be illegal to own where you live
Because the plant is so destructive, many states have made it a prohibited or noxious aquatic weed, and the penalties cover possessing, selling, and moving it. Florida lists it as a Noxious Weed and Class 1 Prohibited Aquatic Plant, and Texas makes it illegal to possess or transport at all. Check your state department of agriculture or natural resources before you ever acquire it, and where it is banned, pick a legal aquatic ornamental instead.
Fix:
- Confirm legality in your state before obtaining the plant
- Do not buy, sell, move, or transport it where it is regulated
- Dispose of any existing plants by composting on dry land
Leaves blacken and collapse after a frost
Water hyacinth is frost-tender and simply does not survive winter outside zones 9 to 11. Once cold hits, the leaves blacken, the plant collapses, and it rots down into the water. In cold climates treat it as an annual and lift the dead material out to compost on dry land before it fouls the water.
Fix:
- Treat it as an annual in cold climates
- Remove and compost dead material on dry land before it fouls the water
Toxicity
The ASPCA lists water hyacinth as non-toxic, so it sits low on the poison scale, but it is not harmless to chew. The leaves carry insoluble calcium oxalate crystals that prick and irritate the mouth and gut, leaving most pets, livestock, and people with mild drooling or stomach upset if they bite in. The real danger of this plant is not to your pet at all but to the environment, where it ranks as a notorious invasive weed that must never reach open water.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A cat that bites the leaves may drool, paw at its mouth, and vomit as the oxalate crystals sting the soft tissue inside. The reaction usually stays mild and settles once it stops chewing.
Dogs Mild
Expect drooling, an irritated mouth, and the odd bout of vomiting or loose stool if a dog chews the foliage. Most dogs spit it out quickly because the raphides make it unpleasant to eat.
Horses Mild
The crystals can irritate a horse's mouth and gut, so grazing on it tends to bring mild drooling and stomach upset rather than serious harm.
Rabbits Mild
A nibbling rabbit may show mouth irritation and a brief loss of appetite. Keep this plant off the menu and stick to greens you know are safe.
Livestock Mild
Cattle, goats, and sheep that browse the foliage can get mild mouth and gut irritation from the oxalate. A bigger worry is that plants pulled from polluted water may carry heavy metals and other contaminants.
Birds Mild
Birds rarely make a meal of it, but a curious one that chews the leaves can suffer the same mild mouth irritation. There is no safe edible part to offer.
People Caution
This is not a food plant, and biting into the raw leaves or stems leaves a burning, prickling feeling in the mouth from the calcium oxalate. Keep children from chewing it, and never treat any part as edible.
Chewing the oxalate-containing leaves can irritate the mouth and stomach; the plant can also accumulate heavy metals and other contaminants from polluted water. Keep pets and children from chewing it.
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 really only the wild species to grow, since breeders have not turned it into garden cultivars. The one thing worth sorting out is the plant it gets confused with.
Pontederia crassipes (species)
This is the wild species grown as is, with no meaningful ornamental cultivars to choose from, and it stays a banned or regulated noxious invasive across many states.
Water lettuce (Pistia stratiotes) - look-alike, not a variety
Often mistaken for water hyacinth, this separate floating plant gives itself away with velvety, ribbed pale green rosettes and none of the inflated bulbous petioles or lavender blue flowers of the real thing.
How to propagate
You will never struggle to make more of this plant, only to stop it. It multiplies on its own faster than almost anything in the water garden, which is exactly why spreading it is restricted.
DivisionEasy
Warm season (late spring through summer) · 1 to 2 weeks
You never really need to propagate this plant, because it does the work itself. Through the warm season it throws off stolons and daughter plants and can double its population in 6 to 18 days, with each division settling in within a week or two. That same speed is why moving or spreading it is discouraged and outright illegal in many states, so never let divisions reach natural water.
SeedModerate
Warm season · Variable
It also sets seed through the warm season, and the timing to sprout varies. Seed is a secondary route next to its runaway vegetative spread, but it still adds to how the plant gets around.
Myths
Water hyacinth is just a harmless, pretty floating pond plant.
The lavender flowers are lovely, but this is one of the world's worst aquatic invasive weeds and a regulated or banned plant in many states. Let loose in natural water, its mat-forming growth chokes whole waterways and kills the native plants and fish underneath. Never release it into or near open water, and dispose of any plants on dry land.