Hornwort (Ceratophyllum demersum): the rootless oxygenator that competes with algae

If we had to recommend just one oxygenating plant for a garden pond, this would often be it. Submerged hornwort, Ceratophyllum demersum, has neither spectacular flowers nor floating foliage: it lives entirely underwater, in dark green clumps with fine, stiff leaves. But it is a formidable competitor for algae because it draws the nutrients they need directly from the water.

Identity card

How it works against algae

The algae that turn the water green and filamentous algae feed on dissolved mineral salts: nitrates, ammonium, and phosphates. Most aquatic plants take these elements from the substrate through their roots. Hornwort has no roots: it absorbs them directly from the water through the entire surface of its leaves. It therefore competes head-on with algae, at the same pantry and in the same place.

Several laboratory studies have also shown that certain submerged plants, including hornwort, release compounds capable of slowing the growth of certain microalgae and cyanobacteria. This is known as allelopathy. In a real pond, this effect is difficult to measure in isolation, but it adds to competition for nutrients and shading.

During the day, like any green plant, it produces oxygen through photosynthesis. At night, it consumes a little: a pond overgrown with submerged plants can therefore experience low oxygen levels toward the end of the night in hot weather, which is why it is important to maintain a reasonable density.

Installation: don't plant it, just place it

No basket, no potting soil. Simply place the bunches in the water. They will stabilize between the surface and the bottom or sink slightly, and some developed stems will anchor themselves to the bottom. If you want it to stay in a specific spot, weigh down the base of the bunch with a small stone attached with a soft tie.

• Choose a calm area, sheltered from the pump's direct current.
• Distribute several bunches rather than one large clump.
• Combine it with other oxygenating plants for variety.
• At first, remove stems that turn yellow: the plant is adapting to your water.

How much should you add?

There is no universal quantity, as everything depends on the pond's fish and nutrient load. In practice, start with a few bunches per square metre of surface area in deep zones, and let it multiply. It grows quickly in summer: in one season, a few bunches can colonize a good part of the volume. The goal is to have plenty of submerged vegetation without the plant occupying the entire water column.

Maintenance: thin out, don't pull up

The only regular maintenance is removing excess growth in summer and autumn. This is also one of the simplest ways to remove nutrients from the pond: each handful of hornwort removed takes away the nitrogen and phosphorus it has stored. Use an aquatic plant net or a rake, and leave the removed stems at the water's edge for a few hours so that small animals can return to the pond. Then put them in the compost.

Hornwort and fish

With goldfish, hornwort cohabits very well and even serves as a spawning ground: the eggs stick to its leaves and the fry hide among them. With koi, it's a different story: they readily nibble and shred it. In a koi pond, keep it in a separate filtration zone, where it can fully perform its water-purifying role.

What it doesn't do

Hornwort helps a great deal, but it does not replace suitable filtration in a pond heavily stocked with fish, nor a UV clarifier when faced with established green water. It acts over the long term by making the environment less favorable to algae. To support this balance, also reduce the sludge at the bottom, which continuously releases nutrients.

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Frequently asked questions

— Does hornwort need soil?

No. It has no true roots and feeds directly from the water. Simply place it in the pond.

— Why does my hornwort turn yellow after purchase?

This is often an adaptation to different water conditions. Remove the yellow parts: the green tips will start growing again within a few weeks.

— What happens to it in winter?

It slows down and sinks to the bottom, where it overwinters, then starts growing again in spring. No protection is necessary.

— Can it take over the pond?

It can become very dense in summer in nutrient-rich water. Regular thinning is enough to contain it, and it is a good indicator: if it grows explosively, your water is rich in nutrients.

— Is it useful in an aquarium?

Yes, it is widely used in cold-water aquariums for the same reasons: rapid growth, nutrient absorption, and shelter for fry.

In summary

• A native oxygenating plant without roots that feeds directly from the water.
• A direct competitor to algae for nitrogen and phosphorus.
• Simply place it in a calm area, without a basket.
• Thin regularly to remove nutrients and maintain a reasonable density.
• Ideal with goldfish; protect it from koi.

Last updated: September 28, 2026.