Cold hardiness & minimum temperature

Is Mountain Alyssum (Alyssum montanum)cold hardy? Hardiness zone & min temp

Also called Mountain Alyssum, Mountain Madwort.

More about mountain alyssum

About Mountain Alyssum

Alyssum montanum · also called Mountain Alyssum, Mountain Madwort · flowering

Mountain Alyssum is a low-growing, silver-grey cushion perennial native to rocky slopes and cliffs of central Europe. In spring it is smothered in dense clusters of bright yellow, honey-scented flowers. Exceptionally hardy and drought-tolerant, it suits rock gardens, dry stone walls, and raised beds. Often confused with Aurinia saxatilis but smaller and more refined.

Cold limit: USDA 4–9 · RHS H7 (-20 to 28°C)

Watch for — Root rot from poor drainage: Sitting in wet soil, especially in winter, rapidly causes root and crown rot. Grow in raised beds, scree gardens, or slopes; top-dress with grit around the crown.

What mountain alyssum's hardiness rating actually means

Yes — mountain alyssum 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. Mountain Alyssum is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for mountain alyssum 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 mountain alyssum 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 mountain alyssum can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Mountain Alyssum hardiness — frequently asked questions

Is mountain alyssum cold hardy?

Yes — mountain alyssum 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. Mountain Alyssum 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 mountain alyssum can survive?

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

What hardiness zone is mountain alyssum?

Mountain Alyssum is rated USDA 4–9 and RHS H7 — Hardy in the severest European continental winters.

Can mountain alyssum 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 mountain alyssum 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.