Repotting guide

When & how to repot Ceratophyllum demersum (Ceratophyllum demersum)

Also called hornwort, coontail.

More about ceratophyllum demersum

About Ceratophyllum demersum

Ceratophyllum demersum · also called hornwort, coontail · tropical

Ceratophyllum demersum, hornwort or coontail, is a rootless free-floating stem plant for freshwater aquariums and ponds. Whorls of stiff, forked, bristly leaves clothe long fast-growing stems that can be left drifting or anchored. Extremely hardy and an aggressive nutrient sponge, it shades fry and outcompetes algae, though it sheds needles when stressed or moved.

Mature size: Stems readily reach 30-100 cm and longer if unchecked; trim and replant tops to control length and density.

Watch for — Won't stay anchored: Being rootless, it floats up repeatedly. Bunch the bases under hardscape or in substrate, or simply embrace it as a floating plant.

How to tell ceratophyllum demersum needs repotting

Repotting on a calendar is less reliable than reading the plant. For ceratophyllum demersum, watch for these signs:

Roots poking out of the drainage holes or coiling visibly around the inside of the pot.
You are watering far more often than you used to because the rootball dries out within a day or two.
Water runs straight through and out the bottom without soaking in.
Top growth has slowed or new ceratophyllum demersum leaves are noticeably smaller than older ones despite good light.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot ceratophyllum demersum

Every 12–18 months — sooner if roots show fast. Ceratophyllum demersum's growth habit — fast-growing, rootless free-floating stem plant with densely whorled, forked needle-like leaves; can gain several centimetres of stem per week. — sets the pace. Ceratophyllum demersum, hornwort or coontail, is a rootless free-floating stem plant for freshwater aquariums and ponds. Whorls of stiff, forked, bristly leaves clothe long fast-growing stems that can be left drifting or anchored. Extremely hardy and an aggressive nutrient sponge, it shades fry and outcompetes algae, though it sheds needles when stressed or moved.

What size pot to step ceratophyllum demersum up to

Step up one pot size — about 2–3 cm (an inch) wider. Ceratophyllum demersum grows fast, so it will fill that space within a season, but jumping several sizes at once still backfires: the unused soil stays soggy and rots even a vigorous root system. One size at a time, every year or so, is the rhythm.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot ceratophyllum demersum

Early spring, just as new growth restarts, is the ideal window for ceratophyllum demersum. The plant is moving into its strongest growth phase and re-roots into fresh soil quickly. Avoid repotting in winter dormancy or, for flowering plants, while it is in bud or bloom — recovery is slowest then and you risk dropping the flowers.

Step-by-step: repotting ceratophyllum demersum

Time it for spring. Repot ceratophyllum demersum in early spring as growth restarts so it re-roots quickly into the fresh soil.
Choose one size up. Pick a pot about 2–3 cm wider with drainage holes. One step only — a much bigger pot stays soggy and rots roots.
Ease the plant out. Water lightly the day before, then tip ceratophyllum demersum out and gently loosen any roots circling the bottom of the rootball.
Repot at the same depth. Put a layer of fresh no substrate — rootless and free-floating in the new pot, set the plant so its soil line is unchanged, and backfill, firming lightly.
Water and pause feeding. Water once to settle the soil. Hold off fertiliser for about a month — fresh mix already has nutrients and feeding now burns new roots.

Aftercare

Water ceratophyllum demersum once to settle the soil, then let the surface dry before watering again — fresh mix around the roots stays wetter than the old compacted ball, so the commonest post-repot mistake is overwatering. Keep it out of direct sun for a week or two while roots re-establish. Do not fertilise for about 4 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for ceratophyllum demersum

Ceratophyllum demersum wants no substrate — rootless and free-floating. Has no true roots and needs no soil. Let it float, or wedge stem bases into substrate or behind hardscape to hold it down; it feeds entirely through the water column. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting ceratophyllum demersum — frequently asked questions

How often should you repot ceratophyllum demersum?

Every 12–18 months — sooner if roots show fast for ceratophyllum demersum. Repot ceratophyllum demersum roughly every 12–18 months, in early spring as growth restarts. It grows fast and circles its pot quickly, so step up one size (about 2–3 cm wider) into fresh no substrate — rootless and free-floating. Don't jump several sizes — that soggy excess soil is what rots vigorous roots.

What size pot does ceratophyllum demersum need?

When is the best time of year to repot ceratophyllum demersum?

Can you put ceratophyllum demersum straight into a much bigger pot?

No. Even a fast-growing ceratophyllum demersum should only go up one pot size at a time. A vastly oversized pot holds a reservoir of wet soil the roots cannot reach, which stays cold and soggy and rots the roots — the opposite of what you wanted.

Should you fertilise ceratophyllum demersum after repotting?

Not immediately. Wait about 4 weeks after repotting ceratophyllum demersum. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.