Ornamental · Pontederiaceae
Water Hyacinth
Pontederia crassipes
A lovely floating bloom with a runaway growth habit that should never reach open water.
Full sun
50–86°F · 10–30°C
USDA 9–11
6–9 in · 15–23 cm
Overview
If you have never grown an aquatic plant, water hyacinth is a gentle place to begin. This free-floating native of South America rides on the surface thanks to its inflated, spongy petioles, little built-in floats that hold each glossy-leaved rosette up on the water. Come summer it rewards you with spikes of lavender blue flowers, each carrying a small yellow eye, and that charm is exactly how it traveled the world.
The good news for a nervous beginner is that keeping it alive takes almost no effort. Rest it on standing fresh water in full sun and it feeds itself straight from the water column, with no soil and no pot to fuss over. The one thing to watch is its pace, because a small clump can double in 6 to 18 days and cover a pond faster than you might expect.
That same eagerness is why it needs a careful home. Let loose in a lake or river, its mats shade out native plants, pull oxygen from the water, and harm fish, so many states ban or regulate it. Grow it only in a fully contained water feature, scoop out the extra often, and you will do just fine as long as no fragment ever slips into natural water.
Care
The short version: rest it on standing fresh water in full sun, keep it warm above 50°F (10°C), lift out the extra often, and never let it slip into natural waterways.
Light
Give water hyacinth full sun and it will grow and bloom at its happiest, though a spot with partial sun still suits it. The brighter the water, the faster it fills in, so more light simply means a little more scooping for you.
Water
This is a free-floating aquatic that lives right on the water surface and never sits in soil. Set it on standing fresh water in full sun and its dangling roots sip everything they need straight from the water. Please do not release it into or near any natural waterway.
Soil
There is no soil to worry about at all, which makes this an easy one. The roots simply dangle in the water column while the plant floats free, drawing nutrients from water that sits between a pH of 6.0 and 8.0.
Temperature
It feels best in a range of 50°F to 86°F (10°C to 30°C) and wants genuine warmth to thrive. Cold water slows it down and frost will end it, so it cannot make it through winter outdoors where the air turns cold.
Humidity
Floating on open water, it lives in constant high humidity and never goes thirsty. There is no reading to chase here, since its whole world is the water beneath it.
Feeding
You will almost never need to feed it, which is one less job to remember. It draws nutrients from the water so easily that extra fertilizer nearby only fuels its fast growth, so keeping the water lean is the kinder choice.
Pruning
Thin and lift out the extra plants often so it does not take over a contained water garden. Bag and compost what you remove on dry land, and never let a piece drift back into water, where even a small fragment can settle in and grow.
Repotting
not applicable (free-floating, not potted)
Size & spacing
Each little rosette rises just 6 to 9 inches (15 to 23 cm) tall, yet it spreads from 6 to 18 inches (15 to 46 cm) across and keeps widening. Think less about spacing and more about the regular scooping that keeps a clump from turning into a mat.
When to plant
This is a warm-season friend with no patience for cold. Wait until the last frost has passed in late spring to set it out, enjoy the flowers all summer, and remember the first frost will finish it, since it cannot hold on 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
Nearly every trouble with water hyacinth comes from the same happy problem: it grows a little too well. What follows is less about nursing a sick plant and more about gently keeping an eager one in bounds and away from places it should never go. Give it a read before you float your first one.
Plants seem to double overnight and cover the surface
This is the explosive vegetative spread that makes water hyacinth so risky, and it can catch a new grower off guard. Given warm water, full sun, and spare nutrients, a population can double in 6 to 18 days as stolons send out daughter plants and the rosettes knit into a mat that seals off open water within days. Skim and rake them out on a steady schedule, ease 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 spoil the water for everything else
Once a mat closes over, the ecological harm comes quickly. The shade kills submerged plants and phytoplankton, the dissolved oxygen drops away, and fish grow stressed or die, while the same mat clogs irrigation, flood control, and any boat trying to pass. This is why you keep it far from 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 not be legal to own where you live
Because the plant does so much damage, many states have made it a prohibited or noxious aquatic weed, with penalties that cover owning, 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 bring one home, and where it is banned, choose 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 slump after a frost
Water hyacinth is frost-tender and will not live through winter outside zones 9 to 11. When the cold arrives the leaves blacken, the plant slumps, and it rots down into the water. In cool 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
You can relax on the poison front, because the ASPCA lists water hyacinth as non-toxic. It is still not something to chew, since the leaves hold insoluble calcium oxalate crystals that prick and irritate the mouth and gut, leaving most pets, livestock, and people with mild drooling or an upset stomach if they bite in. The real worry with this plant is not your pet but the environment, where it counts as a serious 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 be sick as the oxalate crystals sting the soft tissue inside. The upset usually stays mild and eases once the chewing stops.
Dogs Mild
A dog that chews the foliage tends to drool, get a sore mouth, and have the odd bout of vomiting or loose stool. Most spit it out fast, since the raphides make it unpleasant to eat.
Horses Mild
The crystals can irritate a horse's mouth and gut, so a mouthful usually brings mild drooling and a bit of stomach upset rather than anything serious.
Rabbits Mild
A nibbling rabbit may show some mouth irritation and go off its food for a short while. Keep this plant off the menu and stick to the greens you know are safe.
Livestock Mild
Cattle, goats, and sheep that browse the leaves can pick up mild mouth and gut irritation from the oxalate. The bigger concern is that plants lifted from polluted water may carry heavy metals and other contaminants.
Birds Mild
Birds seldom bother with it, but a curious one that chews the leaves can get 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 treat no part of it 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 just the wild species to grow, since no one has bred it into garden cultivars. The only thing worth learning is how to tell it apart from the plant people mix it up with.
Pontederia crassipes (species)
This is simply the wild species grown as it comes, with no real ornamental cultivars to pick from, and it remains a banned or regulated noxious invasive across many states.
Water lettuce (Pistia stratiotes) - look-alike, not a variety
People often confuse this separate floating plant with water hyacinth, but it shows velvety, ribbed pale green rosettes and lacks the inflated bulbous petioles and lavender blue flowers of the true plant.
How to propagate
Making more of this plant will never be your challenge; slowing it down will. It multiplies on its own faster than almost anything else in a water garden, which is exactly why spreading it is restricted.
DivisionEasy
Warm season (late spring through summer) · 1 to 2 weeks
You will honestly never need to propagate this one, because it handles that itself. All through the warm season it puts out 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 very speed is why moving or spreading it is discouraged and outright illegal in many states, so never let a division reach natural water.
SeedModerate
Warm season · Variable
It also sets seed across the warm season, and how long the seed takes to sprout varies. Seed is a secondary route beside its runaway vegetative spread, but it still adds to the ways the plant travels.
Myths
Water hyacinth is just a harmless, pretty pond floater.
The lavender flowers really are lovely, but this ranks among the world's worst aquatic invasive weeds and is regulated or banned in many states. Turned loose in natural water, its mat-forming growth smothers whole waterways and kills the native plants and fish below. Never release it into or near open water, and always dispose of plants on dry land.