Cold hardiness & minimum temperature

Is Perforate St John's-wort (Hypericum perforatum)cold hardy? Hardiness zone & min temp

Also called Perforate St John's-wort, Common St John's Wort, St John's Wort, Klamath Weed.

More about perforate st john's-wort

About Perforate St John's-wort

Hypericum perforatum · also called Perforate St John's-wort, Common St John's Wort · herb

Hypericum perforatum is the medicinal St John's Wort — a upright, freely branching perennial native to grasslands, roadsides, and scrubby habitats across Europe and the UK, bearing bright golden-yellow flowers with distinctive black-dotted margins from June to September. The leaves have translucent oil glands visible when held to the light, giving it the species name 'perforatum'. It thrives in full sun and poor to moderately fertile, well-drained soils and self-seeds prolifically once established. It is confirmed toxic to cats, dogs, and horses via hypericin-driven photosensitization.

Cold limit: USDA 5-9 · RHS H6 (-25 to 30 °C)

What perforate st john's-wort's hardiness rating actually means

Yes — perforate st john's-wort is genuinely cold hardy. Rated RHS H6 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H6 means: Hardy throughout the UK and northern Europe. 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 −20 to −15 °C. Perforate St John's-wort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for perforate st john's-wort as it gets too cold:

It tolerates winter lows to about −20 to −15 °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 perforate 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 perforate st john's-wort can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Perforate St John's-wort hardiness — frequently asked questions

Is perforate st john's-wort cold hardy?

Yes — perforate st john's-wort is genuinely cold hardy. Rated RHS H6 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Perforate 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 perforate st john's-wort can survive?

Minimum survivable temperature is roughly about −20 to −15 °C. Perforate St John's-wort is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is perforate st john's-wort?

Perforate St John's-wort is rated USDA 5-9 and RHS H6 — Hardy throughout the UK and northern Europe.

Can perforate 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 perforate st john's-wort below its minimum temperature?

It tolerates winter lows to about −20 to −15 °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.