Cold hardiness & minimum temperature

Is Sea Mouse-ear (Cerastium diffusum)cold hardy? Hardiness zone & min temp

Also called Sea Mouse-ear, Four-stamened Chickweed.

More about sea mouse-ear

About Sea Mouse-ear

Cerastium diffusum · also called Sea Mouse-ear, Four-stamened Chickweed · flowering

Cerastium diffusum is a slender, sticky-hairy annual native to coastal dunes, sandy cliffs, and dry grasslands along the Atlantic coasts of western Europe, including the British Isles. It prefers open, free-draining sandy or gravelly soils in full sun, flowering from March to June with tiny white, deeply notched petals. Its most important care trait is excellent drainage and low soil fertility — enriched or compacted soils cause it to decline rapidly. This species is not listed by the ASPCA; it is an obscure wild annual with no reported toxicity, but classify as mildly-toxic due to absence of confirmation.

Cold limit: USDA 5-9 · RHS H5 (0–20°C)

Watch for — Root rot in wet winters: Although cold-hardy, prolonged waterlogging in clay soils during winter kills the shallow root system; grow in raised beds or rock-garden scree where drainage is rapid.

What sea mouse-ear's hardiness rating actually means

Yes — sea mouse-ear is genuinely cold hardy. Rated RHS H5 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H5 means: Hardy in most of the UK and in cold winters. On the US scale that maps to USDA 5-9 — 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 about −15 to −10 °C. Sea Mouse-ear is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for sea mouse-ear as it gets too cold:

It tolerates winter lows to about −15 to −10 °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 sea mouse-ear go outside or overwinter — and where?

Plant it out within USDA 5-9 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 sea mouse-ear can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Sea Mouse-ear hardiness — frequently asked questions

Is sea mouse-ear cold hardy?

Yes — sea mouse-ear is genuinely cold hardy. Rated RHS H5 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Sea Mouse-ear is hardy across USDA 5-9; 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 sea mouse-ear can survive?

Minimum survivable temperature is roughly about −15 to −10 °C. Sea Mouse-ear is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is sea mouse-ear?

Sea Mouse-ear is rated USDA 5-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can sea mouse-ear survive winter outside?

Plant it out within USDA 5-9 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 sea mouse-ear below its minimum temperature?

It tolerates winter lows to about −15 to −10 °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.