Rigid hornwort

Ceratophyllum demersum

In other languages: Grof hoornblad (Nederlands) · Cornifle immergé (Français)

Rigid hornwort is a native, submerged water plant found in still and slow-flowing water over almost the whole world. It has no roots: the long, branched stems with whorls of stiff, forked, dark green leaves float freely in the water or hook loosely into the mud. Because it takes its nutrients straight from the water, it competes with algae and helps keep the water clear. Gardeners use it as an easy oxygenating plant for any pond: you simply drop a few stems into the water. In autumn the stems sink to the bottom, where they overwinter until the water warms up again in spring.

Caution: Grows fast, and every piece of stem takes root again. Thin regularly and put the surplus on the compost heap. Never put plants or pond water into ditches, streams or other natural waters: you would spread unwanted species too.

Updated on 9 October 2026

Light
Sun or partial shade
preferably some sun
Moisture
Moist to wet
must not dry out
Legal basis
Loam, clay or peat
neutral to chalky
Hardiness
Fully hardy
down to -25 °C
Size
0.05 m
tall, 1 m wide

Key facts
Height	0.05 m
Native to Belgium	Yes
Poisonous to people and pets	Not known to be poisonous

Identify

Rigid hornwort: how do you recognise it?

Habit: Long, branched stems floating freely in the water, often more than a metre long, without true roots.
Leaves: In whorls around the stem, stiff and forked once or twice, with fine teeth; dark green and rough to the touch.
Stems: Brittle: they break easily, and every piece grows on into a new plant.
Flowering: Flowers rarely, with tiny flowers under water; you will hardly ever notice them.
Winter: In autumn the stems sink to the bottom, where they overwinter in a dormant state.

Plants: April–August

In your garden

Why grow it?

One of the easiest oxygenators: no basket, no soil, just put it in the water.
Takes nutrients straight from the water and so competes with algae.
Releases oxygen into the water during the day.
A dense underwater forest where tadpoles, young fish, dragonfly larvae and other small aquatic creatures hide and hunt.
Native, fully hardy and still grows with some shade.

Rigid hornwort: where does it grow best?

Sun to partial shade; it still grows in a spot with some shade.
In the open water of the pond, from about 30 cm deep; it is very happy in zones of 50 cm and deeper.
Neutral to chalky, preferably fairly nutrient-rich water; in very soft, acid water it grows less well.
Start with a few bunches per square metre of water surface; it will fill in the rest itself.

Light: Sun and partial shade
Soil type: Loam, clay or peat
Acidity: Neutral or chalky
Moisture: Wet
Size: 0.05 m tall, 1 m wide
Hardiness: Hardy (to -25 °C)

Goes well with

Water lily: Floating leaves shade the water, giving algae less of a chance.
Frogbit: A small native floating plant with white flowers.
Water violet: A submerged plant with lilac flowers held above the water in May–June.
Water soldier: Floats at the surface in summer and is an important plant for dragonflies.

Maintenance

Rigid hornwort: when and how to plant it?

Plant in April–August.

Plant from April to August, when the water is warm enough for growth.
First rinse the stems in a bucket of rainwater and check them for snail eggs, duckweed and other hitchhiking plants.
Simply drop the stems into the water: they need no basket or soil.
To keep them in one place, tie a small stone to a bunch and let it sink.
Spread a few bunches per square metre of water surface around the pond.

Care

Thinning: In summer, remove the surplus with a rake or by hand so it does not fill the whole pond. Always leave a good amount in place.
Sparing wildlife: Leave removed stems on the bank for a day, so tadpoles, larvae and snails can make their way back to the water.
Feeding: Never: it lives on the nutrients in the water.
Topping up: Top up with rainwater; tap water often contains more nutrients, which encourages algae.
Composting: Removed stems are perfect for the compost heap.

Tip: Simply drop a few stems into the pond: they need no basket or soil. In winter they sink to the bottom and in spring they come back by themselves.

Rigid hornwort: when to prune it?

When: Hornwort is not pruned. It is, however, thinned in summer when it takes up too much room.

How:

From June to September, pull or rake part of the stems out of the water.
Work in sections, never the whole pond at once.
Leave the removed material on the edge for a day so aquatic creatures can return.

Caution: Every broken piece grows on: put surplus hornwort on the compost heap, never into a ditch, stream or other natural water.

Pruning calendar and basic rules

Propagation

Cuttings: Every piece of stem 10–15 cm long grows into a new plant: break off a stem and drop it in the water.
By itself: In autumn the stems sink to the bottom; in spring they grow again on their own.

Rigid hornwort: how does it get through the winter?

Hardy to −25 °C.

Fully hardy, to about −25 °C.
In autumn the stems sink to the bottom, where the water does not freeze.
Make sure the pond is at least 60–80 cm deep in one place, so the bottom does not freeze.
In autumn, net fallen leaves out of the pond so they do not rot on top of the overwintering stems.

Diseases and pests

Too much growth: The whole pond is full of stems. Thin regularly in summer.
Blanket weed: Green threads among the stems. Twist them round a stick and lift them out; fewer nutrients in the water helps.
Gone in autumn: The stems sink to the bottom: that is normal, they come back in spring.
Poor growth: Brown, crumbling stems, often in very soft, acid water or just after introduction. Give it time, or choose a different oxygenator for acid water.

Through the year

March: The stems are still resting on the bottom.
April: Plant. The stems start growing again.
May: Plant. Remove blanket weed.
June: Thin out if it becomes too dense.
July: Thin out. Leave removed stems on the edge for a day.
August: Thin out. Plant.
September: Last thinning.
October: Net fallen leaves out of the pond. The stems sink to the bottom.
December: Resting on the bottom.

You and this species

Did you know?

Hornwort has no true roots at all: it takes up water and nutrients through its leaves.
The name Ceratophyllum comes from the Greek words for ‘horn’ and ‘leaf’, after its stiff, forked leaves.
Flowering and pollination take place entirely under water: the pollen drifts through the water to the female flowers.

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