Cold hardiness & minimum temperature

Is Rock Hyssop (Hyssopus officinalis subsp. aristatus)cold hardy? Hardiness zone & min temp

Also called Rock Hyssop, Dwarf Hyssop.

More about rock hyssop

About Rock Hyssop

Hyssopus officinalis subsp. aristatus · also called Rock Hyssop, Dwarf Hyssop · herb

Rock hyssop is a compact, dwarf form of culinary hyssop forming a tidy evergreen mound of narrow, aromatic dark-green leaves topped by spikes of deep blue, bee-friendly flowers in summer. Tougher and neater than the species, it suits rockeries, low hedging and herb edging. This Mediterranean sub-shrub thrives in full sun and dry, sharply drained alkaline soil.

Cold limit: USDA 4-9 · RHS H5 (-23 to 30°C)

Watch for — Root rot in wet soil: The main killer; caused by heavy or waterlogged ground. Plant in sharply drained, gritty soil and avoid overwatering, especially over winter.

What rock hyssop's hardiness rating actually means

Yes — rock hyssop is genuinely cold hardy. Rated RHS H5 and USDA 4-9, 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 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 about −15 to −10 °C. Rock Hyssop is built for winter — once established it takes hard frost and snow in its stride.

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

Rock Hyssop hardiness — frequently asked questions

Is rock hyssop cold hardy?

Yes — rock hyssop is genuinely cold hardy. Rated RHS H5 and USDA 4-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Rock Hyssop 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 rock hyssop can survive?

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

What hardiness zone is rock hyssop?

Rock Hyssop is rated USDA 4-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can rock hyssop 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 rock hyssop 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.