Hornwort Facts, Care & Planting Guide (Ceratophyllum demersum)

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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.

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Hornwort Fact Sheet:
PLANT TYPE
Aquatic Perennial Herb
HARDINESS ZONES
USDA 5-11
LIGHT REQUIREMENTS
Full Sun to Full Shade
BLOOM COLOR
Green and Brown
BLOOM PERIOD
June – August (Summer)
MAXIMUM GROWTH
Up to 3m (10ft)
PLANTING DEPTH
Up to 2.7m (9ft)
WATER QUALITY
pH 6.0-7.5

Hornwort Growth, Hardiness & Wintering

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 being to 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:

Photo by Bernd Haynold. Available under a Creative Commons Attribution-Share Alike 3.0 Unported license.

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:

Photo by Renjusplace. Available under a Creative Commons Attribution-Share Alike 3.0 Unported license.

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 and toxic 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.


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.

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