Cold hardiness & minimum temperature

Is Ceratophyllum demersum (Ceratophyllum demersum)cold hardy? Hardiness zone & min temp

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.

Cold limit: USDA 5-11 (cold-hardy pond plant outdoors; overwinters as turions) · RHS H4 (15-28°C)

What ceratophyllum demersum's hardiness rating actually means

Yes — ceratophyllum demersum is genuinely cold hardy. Rated RHS H4 and USDA 5-11 (cold-hardy pond plant outdoors; overwinters as turions), it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H4 means: Hardy in an average winter across much of the temperate world. On the US scale that maps to USDA 5-11 (cold-hardy pond plant outdoors; overwinters as turions) — the zones where it can be left outdoors year-round.

New to these scales? The USDA hardiness zone map explained covers how the zone numbers work, and you can find your own zone with the zone finder.

Minimum temperature — and what happens below it

Minimum survivable temperature is roughly about −10 to −5 °C. Ceratophyllum demersum is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for ceratophyllum demersum as it gets too cold:

It tolerates winter lows to about −10 to −5 °C once established.
Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root.
First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.

Can ceratophyllum demersum go outside or overwinter — and where?

Plant it out within USDA 5-11 (cold-hardy pond plant outdoors; overwinters as turions) and it overwinters with little or no help.
It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy.
The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

Work back from your local frost dates with the frost-date calculator: the last spring frost and first autumn frost are what really decide when ceratophyllum demersum can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Ceratophyllum demersum hardiness — frequently asked questions

Is ceratophyllum demersum cold hardy?

Yes — ceratophyllum demersum is genuinely cold hardy. Rated RHS H4 and USDA 5-11 (cold-hardy pond plant outdoors; overwinters as turions), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Ceratophyllum demersum is hardy across USDA 5-11 (cold-hardy pond plant outdoors; overwinters as turions); it belongs in the ground or a frost-proof container, not on a windowsill, and many types actively need a cold winter to perform.

What is the minimum temperature ceratophyllum demersum can survive?

Minimum survivable temperature is roughly about −10 to −5 °C. Ceratophyllum demersum is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is ceratophyllum demersum?

Ceratophyllum demersum is rated USDA 5-11 (cold-hardy pond plant outdoors; overwinters as turions) and RHS H4 — Hardy in an average winter across much of the temperate world.

Can ceratophyllum demersum survive winter outside?

Plant it out within USDA 5-11 (cold-hardy pond plant outdoors; overwinters as turions) and it overwinters with little or no help. It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy. The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

What happens to ceratophyllum demersum below its minimum temperature?

It tolerates winter lows to about −10 to −5 °C once established. Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root. First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.