Cold hardiness & minimum temperature

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

Also called Common Water Starwort, Pond Water Starwort.

More about common water starwort

About Common Water Starwort

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

Common Water Starwort is a delicate native European aquatic plant forming rosettes of bright green leaves at the water surface and submerged linear foliage below. An excellent oxygenator for wildlife ponds, it supports aquatic invertebrates and amphibian spawn. Thrives in still to slow-moving water in full sun to partial shade; fully cold-hardy.

Cold limit: USDA 4-9 · RHS H7 (2–22°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.