Cold hardiness & minimum temperature

Is Yellow-wort (Blackstonia perfoliata)cold hardy? Hardiness zone & min temp

Also called Yellow-wort, Yellowwort.

More about yellow-wort

About Yellow-wort

Blackstonia perfoliata · also called Yellow-wort, Yellowwort · flowering

Blackstonia perfoliata is a slender annual or biennial wildflower in the gentian family (Gentianaceae), native to calcareous grasslands, chalk downland, limestone screes, and dune slacks across Europe, including England and Wales. Its distinctive grey-green, waxy, perfoliate leaves — appearing to have the stem growing through them — and bright yellow eight-petalled flowers, which open only in sunshine, make it unmistakable. It thrives in thin, alkaline, nutrient-poor soils in full sun and sets seed readily on bare or disturbed chalk. Toxicity data specific to this species is absent from the ASPCA database; treat with caution.

Cold limit: USDA 5-9 · RHS H5 (-10 to 28°C)

What yellow-wort's hardiness rating actually means

Yes — yellow-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. Yellow-wort is built for winter — once established it takes hard frost and snow in its stride.

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

Yellow-wort hardiness — frequently asked questions

Is yellow-wort cold hardy?

Yes — yellow-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. Yellow-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 yellow-wort can survive?

Minimum survivable temperature is roughly about −15 to −10 °C. Yellow-wort is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is yellow-wort?

Yellow-wort is rated USDA 5-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can yellow-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 yellow-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.