Cold hardiness & minimum temperature

Is Square-stalked St John's-wort (Hypericum tetrapterum)cold hardy? Hardiness zone & min temp

Also called Square-stalked St John's-wort, Square-stalked St John's Wort, Peterwort.

More about square-stalked st john's-wort

About Square-stalked St John's-wort

Hypericum tetrapterum · also called Square-stalked St John's-wort, Square-stalked St John's Wort · flowering

Hypericum tetrapterum is a rhizomatous perennial native to damp meadows, fens, stream margins, and wet woodland rides across the UK and Europe, instantly recognisable by its square, four-winged stems. It bears clusters of small pale yellow flowers from June to September and is well suited to pond margins and rain gardens. Unlike most Hypericum species it tolerates and even thrives in waterlogged soils — the most important care distinction from its relatives. It is toxic to cats, dogs, and horses due to hypericin, consistent with all Hypericum species.

Cold limit: USDA 5-9 · RHS H6 (-20 to 25 °C)

What square-stalked st john's-wort's hardiness rating actually means

Yes — square-stalked 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. Square-stalked St John's-wort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for square-stalked 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 square-stalked 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 square-stalked 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.

Square-stalked St John's-wort hardiness — frequently asked questions

Is square-stalked st john's-wort cold hardy?

Yes — square-stalked 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. Square-stalked 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 square-stalked st john's-wort can survive?

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

What hardiness zone is square-stalked st john's-wort?

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

Can square-stalked 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 square-stalked 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.