Cold hardiness & minimum temperature

Is English Walnut 'Hartley' (Juglans regia 'Hartley')cold hardy? Hardiness zone & min temp

Also called Hartley walnut.

More about english walnut 'hartley'

About English Walnut 'Hartley'

Juglans regia 'Hartley' · also called Hartley walnut · edible

'Hartley' is a leading English (Persian) walnut cultivar, long a benchmark for in-shell quality with large, light, well-sealed nuts. It leafs out late, dodging spring frosts, and is partly self-fertile but yields best with a pollinizer. Grow it in deep, well-drained soil and full sun. All parts produce juglone, and moldy nuts are toxic to dogs.

Cold limit: USDA 5-9 · RHS H6 (-20 to 38°C)

Watch for — Spring frost on young growth: Although 'Hartley' leafs out relatively late to avoid frost, an unusually late freeze can still kill emerging shoots and flowers; choose a frost-free site.

What english walnut 'hartley''s hardiness rating actually means

Yes — english walnut 'hartley' 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. English Walnut 'Hartley' is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for english walnut 'hartley' 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 english walnut 'hartley' 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 english walnut 'hartley' can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

English Walnut 'Hartley' hardiness — frequently asked questions

Is english walnut 'hartley' cold hardy?

Yes — english walnut 'hartley' 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. English Walnut 'Hartley' 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 english walnut 'hartley' can survive?

Minimum survivable temperature is roughly about −20 to −15 °C. English Walnut 'Hartley' is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is english walnut 'hartley'?

English Walnut 'Hartley' is rated USDA 5-9 and RHS H6 — Hardy throughout the UK and northern Europe.

Can english walnut 'hartley' 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 english walnut 'hartley' 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.