Cold hardiness & minimum temperature

Is Rock Thyme (Thymus comptus)cold hardy? Hardiness zone & min temp

Also called Rock thyme, Compact thyme.

More about rock thyme

About Rock Thyme

Thymus comptus · also called Rock thyme, Compact thyme · herb

Thymus comptus is a low, mat-forming alpine thyme native to limestone rocks and screes of the western Balkans and southern Alps. It thrives in full sun with sharp drainage and extremely lean, alkaline soil — excess moisture in winter is the single most common killer. Trim lightly after flowering to keep growth compact and prevent woody die-back from the centre. Thyme is listed by the ASPCA as non-toxic to cats, dogs, and horses.

Cold limit: USDA 5-9 · RHS H5 (-20 to 30°C)

Watch for — Root rot in wet soil: The most common cause of failure; roots sitting in waterlogged or clay-heavy soil quickly rot, especially over winter. Ensure perfect drainage and consider raising the bed or adding a deep gravel layer.

What rock thyme's hardiness rating actually means

Yes — rock thyme 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. Rock Thyme is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for rock thyme 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 rock thyme 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 rock thyme can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Rock Thyme hardiness — frequently asked questions

Is rock thyme cold hardy?

Yes — rock thyme 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. Rock Thyme 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 rock thyme can survive?

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

What hardiness zone is rock thyme?

Rock Thyme is rated USDA 5-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can rock thyme 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 rock thyme 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.