Cold hardiness & minimum temperature

Is Chalk Milkwort (Polygala calcarea)cold hardy? Hardiness zone & min temp

Also called Chalk Milkwort.

More about chalk milkwort

About Chalk Milkwort

Polygala calcarea · also called Chalk Milkwort · flowering

Chalk Milkwort is a compact, mat-forming perennial wildflower endemic to short chalk and limestone grasslands of southern England and parts of northern France, flowering in May and June with vivid blue (occasionally pink or white) blooms. It is a specialist of thin, nutrient-poor, alkaline soils and will not persist in enriched or waterlogged ground. The critical care point is to recreate its native habitat: lean, gritty, limey soil in full sun. It is not recorded as toxic to pets.

Cold limit: USDA 5-7 · RHS H6 (-15 to 25°C)

What chalk milkwort's hardiness rating actually means

Yes — chalk milkwort is genuinely cold hardy. Rated RHS H6 and USDA 5-7, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H6 means: Hardy throughout the UK and northern Europe. On the US scale that maps to USDA 5-7 — 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 −20 to −15 °C. Chalk Milkwort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for chalk milkwort as it gets too cold:

It tolerates winter lows to about −20 to −15 °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 chalk milkwort go outside or overwinter — and where?

Plant it out within USDA 5-7 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 chalk milkwort can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Chalk Milkwort hardiness — frequently asked questions

Is chalk milkwort cold hardy?

Yes — chalk milkwort is genuinely cold hardy. Rated RHS H6 and USDA 5-7, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Chalk Milkwort is hardy across USDA 5-7; 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 chalk milkwort can survive?

Minimum survivable temperature is roughly about −20 to −15 °C. Chalk Milkwort is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is chalk milkwort?

Chalk Milkwort is rated USDA 5-7 and RHS H6 — Hardy throughout the UK and northern Europe.

Can chalk milkwort survive winter outside?

Plant it out within USDA 5-7 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 chalk milkwort below its minimum temperature?

It tolerates winter lows to about −20 to −15 °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.