Cold hardiness & minimum temperature

Is Soft Draba (Draba mollissima)cold hardy? Hardiness zone & min temp

Also called Soft Draba, Caucasus Cushion Draba.

More about soft draba

About Soft Draba

Draba mollissima · also called Soft Draba, Caucasus Cushion Draba · flowering

Draba mollissima is a specialist cushion alpine endemic to the North Caucasus, where it forms flat to hemispherical pads of minute, densely white-hairy rosettes in dry rocky crevices and scree at 2,500–3,500 m elevation. In cultivation it produces fragrant yellow flowers in spring on short scapes above a perfectly symmetrical cushion just 3–10 cm across. It is considered a challenging plant requiring an alpine house or extremely well-drained trough with complete protection from winter wet; experienced alpine growers describe it as one of the most exacting Drabas. Toxicity data are absent from the ASPCA database; classified as mildly-toxic as a precautionary measure.

Cold limit: USDA 4-7 · RHS H5 (-25 to 18°C)

Watch for — Crown rot from winter wet: The principal challenge in cultivation; even brief exposure to standing moisture on the cushion in cool conditions causes irreversible rot — winter glass or cloche cover over an outdoor trough is strongly recommended.

What soft draba's hardiness rating actually means

Yes — soft draba is genuinely cold hardy. Rated RHS H5 and USDA 4-7, 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 4-7 — 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 −15 to −10 °C. Soft Draba is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for soft draba 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 soft draba go outside or overwinter — and where?

Plant it out within USDA 4-7 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 soft draba can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Soft Draba hardiness — frequently asked questions

Is soft draba cold hardy?

Yes — soft draba is genuinely cold hardy. Rated RHS H5 and USDA 4-7, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Soft Draba is hardy across USDA 4-7; 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 soft draba can survive?

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

What hardiness zone is soft draba?

Soft Draba is rated USDA 4-7 and RHS H5 — Hardy in most of the UK and in cold winters.

Can soft draba survive winter outside?

Plant it out within USDA 4-7 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 soft draba 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.