Hornwort - Ceratophyllum

Hornwort is a very common water plant throughout the world. According to the Royal Horticultural Society's A-Z Encyclopaedia of Garden plants, there are about 30 species, all similar aquatics. However there are two common types. The list below shows the identifying features:

Both are cosmopolitan in their range, so there are lots of alternative common names. So if you are doing, for instance, a Google search, use the Latin name and not the common name!

Since both occur over a wide range, both are very variable in their appearance, depending on temperature, light level and also on water hardness. The two seem to be quite similar, but most of the following refers specifically to Ceratophyllum demersum.

Most books say it is a floating plant and so it is for the most part. However this is not completely true. Depending on water conditions, it may sink, float or be neutral buoyancy. So in winter it forms dense shoots where the leaves are very tightly packed and are not filled with gas bubbles, so they sink to the bottom (a form known as turions). When the light level rises these start to grow and their metabolism forms gas bubbles within the leaves, so the plant floats to the surface and grows very fast. At least, I believe the growth is triggered by light rather than temperature as I have had these 'winter buds' in a tropical aquarium for many months (at around26°C) for many months without triggering growth!

In an aquarium common hornwort can grow into a tropical form, which may easily be taken for a different species from the common cold-water form. This is the one I have photographed, and it is neutral buoyancy, floating in mid-water.

You can of course anchor the plant and aquarium suppliers usually supply is as a lead-weighted, bunched plant. However of anchored at the bottom the stems rapidly extend until the bulk of the plant is on the surface again, so it's not always entirely satisfactory anchored. It grown very fast and can quickly take over a tank, so be prepared for lots of pruning. Of course removing growing plants removes nitrates and other chemicals that may foul the water, so a fast growing plant is useful.

It is said that the spineless variety (Ceratophyllum submersum) is a prettier aquarium plant, being bushier with longer 'leaves'. That may be true, but the common hornwort's tropical form is a beautiful aquarium plant, being much more colourful than the spineless one, as the photograph of Ceratophyllum demersum shows.

Common hornwort also flowers readily in a tropical aquarium - the specimen in the photo is in flower - but the flowers are underwater and very insignificant, being little more than buds on the main (pink) stem.

The fruits are slightly more obvious than the flowers: they are green, slightly oval and about 4mm in diameter. Each fruit has three spines, roughly in the same plane and each about 5mm long. I have had the spined hornwort flowering and fruiting in a tropical aquarium (26°c) - it is these spines on the fruits that give it its common name!


There's a full size photo (415kB) available, click ln the picture to download it.

This photo was taken with ambient aquarium light (two 38w fluorescent tubes) in a 122cm (4') aquarium, using an Olympus C-2020Z digital camera. More light would have given a better depth of focus! The specimen is unusual, being lighter and pinker than normal. The tank water is relatively soft, being mainly rain water. Tank temperature about 26°C

Ceratophyllum demersum can be found in many ponds in England, see the PAWL site and makes a splendid aquarium plant. Be warned however that it grows very fast - and may want to take over your aquarium or pond, but it's easily removed!

Let me know if you use it and what you think.

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Last modified: Mon, 05 Aug 2019 15:34:19 BST
Page first published 22nd November 2002.
Page written and © 2002 - by Richard Torrens