Hornwort Facts, Care & Planting Guide (Ceratophyllum demersum)

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Hornwort Facts, Care, Benefits, & Hardiness (Ceratophyllum demersum)

Hornwort helping to filter the water in a tank
Hornwort, also known as coontail, is popular due to it’s oxygenating benefits and ability to deter nuisance species, such as algae. Photo by Patrick Hacker, CC BY 4.0, via Wikimedia Commons

Hornwort, also called rigid hornwort, coontail, and coon’s tail, is an underwater, free-floating aquatic plant that can be found in bodies of water on every continent except Antarctica. Hornwort grows in bunches of 6 to 12 stalks connected at a single root. These long stems are covered in a feathery whorl of thin, green leaves that more closely resemble needles. The bushy growth of these leaves resemble the tail of a raccoon, hence its nickname. These plants range in color from light to dark green depending on environmental conditions.

Does Hornwort Benefit Ponds & Wildlife?

Hornwort is a favorite of many pond and aquarium owners for its oxygenating and anti-algae properties. Hornwort releases a specific combination of chemicals that damage competitive species in a process known as allelopathy. So far, scientists have found that hornwort can halt the growth harmful phytoplankton like blue-green algae and cyanobacteria. These organisms cause toxic blooms that can decrease dissolved oxygen content of the water, release noxious chemicals, and choke out natural plant and animal life.

Hornwort is technically native to and originates from North America, but now has a worldwide distribution (with the exception of Arctic regions) thanks to aquaculture. It’s considered naturalized on most continents, but be wary – its fast growth enable it to easily choke out other vegetation if one is not careful, and so it should absolutely not be planted in natural waterways or in ponds connected to natural waterways. 

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Hornwort Fact Sheet:
Aquatic Perennial Herb
USDA 5-11
Full Sun to Full Shade
Green and Brown
June – August (Summer)
Up to 3m (10ft)
Up to 2.7m (9ft)
pH 6.0-7.5

Hornwort Growth, Hardiness & Wintering

Oxygen bubbles that have formed on hornwort and will oxygenate the pond water
Hornwort is quite hardy, and helps oxygenate water. Public domain.

Hornwort is known across the gardening community as a particularly hardy plant good for beginners and those with less-than-green thumbs. It has a robust growth rate around 13cm (5in) per week, and has been known to reach heights of 2 to 3 meters (6.5 to 10 ft). Hornwort grows well with most degrees of sun exposure, from full sun to full shade. However, it may do best if first introduced to a shady area. Hornwort will let you know if it is in too strong of sunlight by yellowing. If the conditions are too shady, it may thin out and grow more slowly.

Hornwort prefers water temperature between 15 and 30° C (59 and 86° F) and can survive outdoors in winter extremes of -2°C (28° F). You may notice the shedding of needles if conditions near the extremes of the plant’s tolerance.  Hornwort is fairly cold hardy and has even been found growing in parts of Norway. Interestingly, it is usually heat extremes that bother these plants the most. Strong, direct sun exposure should be limited to around 8 hours a day, and water temperatures should be kept below 29° C (85° F) for best growth.

How to Plant Hornwort in Ponds:

Hornwort that was planted in its preferred soil type in a pond, sandy loam
Photo by Bernd Haynold, CC BY-SA 3.0, via Wikimedia Commons

Your new hornwort plant will recover best after the stress of transport if placed in a bucket of room temperature or pond water in a shady area. The stems should be submerged to promote hair-like root formation, and the plant can be kept like this for up to three days. Once your hornwort is happy—as evidenced by its vibrant green color and the beginnings of new hair-like roots, it can be planted in a container or directly into the bottom of your pond.

Hornwort can survive in depths up to 2.7m (9ft) if the water is relatively clear. If planting in a container, hornwort prefers to be anchored in around 2.5 cm (1in) of clay or sandy loam or 3mm (1/8 in) of fine aquatic gravel. Hornwort can also be anchored using fishing line and a light fishing weight or rock. When anchoring the stems of hornwort, it is important to not be too aggressive. Densely covering large parts of the stem can cause it to rot. Alternatively, hornwort can also be floated on the top of your pond if you have fish like hatchetfish that dwell at the water’s surface.

How to Care for Hornwort in Ponds:

Hornwort that has been clipped to maintain the desired size
Hornwort can be regularly clipped to help maintain the desired size. Photo by Renjusplace, CC BY-SA 3.0, via Wikimedia Commons

Hornwort should be trimmed weekly or bi-monthly to keep at the desired size. Clippings should be bagged or burned for disposal so you do not unintentionally introduce the fast-growing plant into native waterways. Hornwort is incredibly sensitive to copper, so certain medications for fish can rapidly kill off your hornwort plants, potentially releasing toxins and damaging the health of your pond.

Your new hornwort plant may shed needles up to a month after its first planting as it gets accustomed to its new surroundings. It is a good idea, especially if you do not have fish or shrimp that like to eat this debris, to remove the needles from the bottom of your pond to prevent decomposition.

Hornwort can survive a light frost, especially if anchored to the bottom of your pond. Since this plant has such a high growth rate, many gardeners simply plant new cuttings come spring. Alternatively, you can easily remove your hornwort before first frost and grow them inside in either a tub or aquarium and access to sun or artificial light.

Is Hornwort Toxic, Poisonous or Invasive?

Hornwort is considered an invasive weed in the Australian state of Tasmania and in New Zealand. Hornwort’s fast-growing nature has allowed it to outcompete native freshwater flora, and has been a particular nuisance for the area’s hydroelectric water plants. For some unknown reason, hornwort grows particularly well in the waterways of New Zealand, and plants in certain areas can reach the height of a 3-story building. 

It is not known to be toxic, and in fact is eaten by many fish and animals alike. 

Will Koi & Pond Fish Eat Hornwort?

Fish and animals alike love hornwort. In particular, various species of ducks and teal, including the long-tailed duck (Clangula hyemalis), which is classified as vulnerable by the International Union for the Conservation of Nature, love feasting on the feathery plant. Hornwort is also a known favorite of turtles, with slider (Trachemys scripta), snapping (Chelydra serpentine), musk (Sternotherus odoratus), and Blanding’s (Emys blandingii) turtles all known to eat it.

This plant is eaten by a wide variety of fish including carp, and is one of the few aquatic plants able to survive the merciless nibbling of goldfish. Hornwort is also a favorite of individuals who breed fish. The plant provides great protection for small fry, and some fish may even lay their eggs in its bushy branches.

8 thoughts on “Hornwort Facts, Care & Planting Guide (Ceratophyllum demersum)”

    • Hi Laura,

      Hornwort isn’t known to put out any toxic compounds and is safe to plant with really any other plants! It can grow fast, so there’s risk of it choking out other plants over time but this can be mitigated by cutting the hornwort either yearly or as-needed to help control its growth.

  1. Hi, One clump of ceratophyllum demersum in my pond at about 50cms depth has become very pale green and seems to be disintegrating. (wildlife Pond, no fish)
    We’re battling blanket weed which I thought might be killing it but other clumps have remained dark green. Any idea what has caused this and should we remove it? We have other oxygenators besides hornwort. Many thanks for your time.

    • Hi Felicity,

      Massive apologies for the delayed response! Is this particular hornwort in a more sunny location than the others? It could be severe sunburn causing the leaves to pale and fall apart. Another possibility is that this particular hornwort just wasn’t able to acclimate to your pond, while the others did just fine – it happens!

    • Hi Jacqueline,

      While many species belonging to the Chara/Ceratophyllum genus often have a distinctive garlicy or slightly skunky smell when their foliage is disturbed, it’s not typical for Ceratophyllum demersum specifically to possess this particular feature. It’s possible that you were sold the wrong species, as there are hundreds of hornwort species and they can often be mislabeled/mistaken for one another.

  2. Hi i have been growing hornwart in my pond for the last 8 months it grows really rapidly and i have been selling it in bunches, will this continue to grow over the winter or should i transplant some indoors ie into my garage in tanks so i can continue to sell it does the tank water in gagage require to be heated at all regards cliff


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