Cold hardiness & minimum temperature

Is White mountain heather (Cassiope mertensiana)cold hardy? Hardiness zone & min temp

Also called White mountain heather, Western moss heather, Mertens' cassiope.

More about white mountain heather

About White mountain heather

Cassiope mertensiana · also called White mountain heather, Western moss heather · flowering

White mountain heather is a low-growing alpine subshrub native to western North America, from Alaska to California, found at high elevations near snowfields. Its four-ranked scale-like leaves clothe wiry stems, and it produces delicate white bell flowers on red stalks in early summer. An ideal plant for cool, acidic rock gardens.

Cold limit: USDA 3-7 · RHS H7 (−30 to 18°C)

Watch for — Heat intolerance and summer dieback: Cassiope mertensiana is extremely cold-adapted and suffers badly in warm summers. Temperatures consistently above 20–22°C (68–72°F) cause rapid decline. Position in north-facing aspects, maintain cool root zone with grit mulch, or grow in an alpine house.

What white mountain heather's hardiness rating actually means

Yes — white mountain heather is genuinely cold hardy. Rated RHS H7 and USDA 3-7, 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 3-7 — 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 below about −20 °C. White mountain heather is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for white mountain heather 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 white mountain heather go outside or overwinter — and where?

Plant it out within USDA 3-7 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 white mountain heather can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

White mountain heather hardiness — frequently asked questions

Is white mountain heather cold hardy?

Yes — white mountain heather is genuinely cold hardy. Rated RHS H7 and USDA 3-7, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. White mountain heather is hardy across USDA 3-7; 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 white mountain heather can survive?

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

What hardiness zone is white mountain heather?

White mountain heather is rated USDA 3-7 and RHS H7 — Hardy in the severest European continental winters.

Can white mountain heather survive winter outside?

Plant it out within USDA 3-7 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 white mountain heather 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.