Cold hardiness & minimum temperature

Is St. John's Wort (Hypericum perforatum)cold hardy? Hardiness zone & min temp

Also called St. John's wort, common St. John's wort, perforate St. John's wort.

More about st. john's wort

About St. John's Wort

Hypericum perforatum · also called St. John's wort, common St. John's wort · herb

Common St. John's wort is a hardy, sun-loving perennial herb bearing bright yellow five-petalled flowers in midsummer over wiry stems and small, gland-dotted leaves. Vigorous and drought-tolerant, it self-seeds and spreads freely. Long used medicinally, it is a regulated weed in some regions and is toxic to pets and grazing animals.

Cold limit: USDA 5-9 · RHS H5 (-20 to 28°C)

What st. john's wort's hardiness rating actually means

Yes — st. john's wort 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. St. John's Wort is built for winter — once established it takes hard frost and snow in its stride.

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

St. John's Wort hardiness — frequently asked questions

Is st. john's wort cold hardy?

Yes — st. john's wort 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. St. John's Wort 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 st. john's wort can survive?

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

What hardiness zone is st. john's wort?

St. John's Wort is rated USDA 5-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can st. john's wort 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 st. john's wort 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.