Cold hardiness & minimum temperature

Is Common Water Starwort (Callitriche stagnalis)cold hardy? Hardiness zone & min temp

Also called Common Water Starwort, Pond Water Starwort, Common Starwort.

More about common water starwort

About Common Water Starwort

Callitriche stagnalis · also called Common Water Starwort, Pond Water Starwort · flowering

Callitriche stagnalis is a submerged and surface-floating aquatic plant native to ponds, ditches, streams, and wet mud across Europe (including the British Isles) and parts of North America. It forms dense, star-shaped rosettes of pale green leaves at the water surface alongside submerged linear leaves, functioning as a valuable oxygenating plant while providing cover for aquatic invertebrates and fish fry. It tolerates a wide range of water conditions and remains active through autumn and winter in mild climates, which makes it a more useful oxygenator than many summer-only alternatives. No confirmed toxicity to cats or dogs is reported; treated as mildly toxic as a precaution.

Cold limit: USDA 4-9 · RHS H7 (0°C to 22°C)

What common water starwort's hardiness rating actually means

Yes — common water starwort is genuinely cold hardy. Rated RHS H7 and USDA 4-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H7 means: Hardy in the severest European continental winters. On the US scale that maps to USDA 4-9 — 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 below about −20 °C. Common Water Starwort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for common water starwort as it gets too cold:

It tolerates winter lows to about −20 °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 common water starwort go outside or overwinter — and where?

Plant it out within USDA 4-9 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 common water starwort can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Common Water Starwort hardiness — frequently asked questions

Is common water starwort cold hardy?

Yes — common water starwort is genuinely cold hardy. Rated RHS H7 and USDA 4-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Common Water Starwort is hardy across USDA 4-9; 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 common water starwort can survive?

Minimum survivable temperature is roughly below about −20 °C. Common Water Starwort is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is common water starwort?

Common Water Starwort is rated USDA 4-9 and RHS H7 — Hardy in the severest European continental winters.

Can common water starwort survive winter outside?

Plant it out within USDA 4-9 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 common water starwort below its minimum temperature?

It tolerates winter lows to about −20 °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.