Cold hardiness & minimum temperature

Is Costmary (Tanacetum balsamita)cold hardy? Hardiness zone & min temp

Also called Costmary, Bible Leaf, Mint Geranium.

More about costmary

About Costmary

Tanacetum balsamita · also called Costmary, Bible Leaf · herb

Costmary is a hardy, spreading perennial with long, balsam-and-mint-scented silver-green leaves once used to flavour ale and as a fragrant bookmark in bibles. It is tough, drought-tolerant, and undemanding, spreading by rhizomes in full sun and free-draining soil. Flowers are small and yellow; many gardeners grow it purely for the aromatic foliage.

Cold limit: USDA 4-9 · RHS H5 (10-26°C)

What costmary's hardiness rating actually means

Yes — costmary 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. Costmary is built for winter — once established it takes hard frost and snow in its stride.

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

Costmary hardiness — frequently asked questions

Is costmary cold hardy?

Yes — costmary 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. Costmary 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 costmary can survive?

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

What hardiness zone is costmary?

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

Can costmary 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 costmary 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.