Cold hardiness & minimum temperature

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

Also called Hairy St John's-wort, Hairy St John's Wort.

More about hairy st john's-wort

About Hairy St John's-wort

Hypericum hirsutum · also called Hairy St John's-wort, Hairy St John's Wort · flowering

Hypericum hirsutum is a softly hairy, erect perennial native to calcareous woodland edges, scrub, and hedgebanks across the UK and Europe, reaching 40–80 cm with terminal clusters of pale yellow five-petalled flowers from July to August. It grows in partial to full shade, tolerating conditions too shady for most Hypericum species, and prefers moist, well-drained soils. The most important care fact is that, like all members of the genus, it contains hypericin, which is toxic to cats, dogs, and horses. Keep pets away from all parts of the plant.

Cold limit: USDA 4-8 · RHS H6 (-20 to 25 °C)

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

Yes — hairy st john's-wort is genuinely cold hardy. Rated RHS H6 and USDA 4-8, 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 4-8 — 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. Hairy St John's-wort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for hairy 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 hairy st john's-wort go outside or overwinter — and where?

Plant it out within USDA 4-8 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 hairy 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.

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

Is hairy st john's-wort cold hardy?

Yes — hairy st john's-wort is genuinely cold hardy. Rated RHS H6 and USDA 4-8, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Hairy St John's-wort is hardy across USDA 4-8; 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 hairy st john's-wort can survive?

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

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

Hairy St John's-wort is rated USDA 4-8 and RHS H6 — Hardy throughout the UK and northern Europe.

Can hairy st john's-wort survive winter outside?

Plant it out within USDA 4-8 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 hairy 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.