Cold hardiness & minimum temperature

Is Coris-Leaved St John's Wort (Hypericum coris)cold hardy? Hardiness zone & min temp

Also called Coris-leaved St John's wort, Heath-leaved St John's wort.

More about coris-leaved st john's wort

About Coris-Leaved St John's Wort

Hypericum coris · also called Coris-leaved St John's wort, Heath-leaved St John's wort · flowering

Hypericum coris is a compact, mound-forming, semi-evergreen subshrub native to the southwestern and central Alps and northern Italy, where it colonises sunny limestone rocks and scree at elevations up to 2,000 m. It produces whorls of narrow, heath-like leaves on wiry stems and bears clusters of small golden-yellow, cup-shaped flowers in summer, making it an elegant choice for rock gardens and gravel beds. The single most important care point is sharp drainage — permanently wet soil will kill it, particularly in winter. Per the ASPCA, Hypericum (St John's wort) is toxic to cats, dogs, and horses, with hypericin as the toxic principle.

Cold limit: USDA 6-9 · RHS H5 (-15 to 25°C)

Watch for — Root and crown rot: The most common cause of failure; results from waterlogged soil especially in winter — ensure very sharp drainage and avoid overhead watering.

What coris-leaved st john's wort's hardiness rating actually means

Yes — coris-leaved st john's wort is genuinely cold hardy. Rated RHS H5 and USDA 6-9, 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 6-9 — 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. Coris-Leaved St John's Wort is built for winter — once established it takes hard frost and snow in its stride.

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

Plant it out within USDA 6-9 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 coris-leaved 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.

Coris-Leaved St John's Wort hardiness — frequently asked questions

Is coris-leaved st john's wort cold hardy?

Yes — coris-leaved st john's wort is genuinely cold hardy. Rated RHS H5 and USDA 6-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Coris-Leaved St John's Wort is hardy across USDA 6-9; 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 coris-leaved st john's wort can survive?

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

What hardiness zone is coris-leaved st john's wort?

Coris-Leaved St John's Wort is rated USDA 6-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can coris-leaved st john's wort survive winter outside?

Plant it out within USDA 6-9 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 coris-leaved 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.