Cold hardiness & minimum temperature

Is Thalictrum delavayi 'Hewitt's Double' (Thalictrum delavayi 'Hewitt's Double')cold hardy? Hardiness zone & min temp

Also called Hewitt's Double meadow rue.

More about thalictrum delavayi 'hewitt's double'

About Thalictrum delavayi 'Hewitt's Double'

Thalictrum delavayi 'Hewitt's Double' · also called Hewitt's Double meadow rue · flowering

'Hewitt's Double' is a tall, airy meadow rue prized for clouds of tiny, fully double lilac-mauve pompon flowers held on wiry stems above delicate, fern-like blue-green foliage in mid to late summer. A graceful herbaceous perennial reaching 1.2-1.8 m, it brings see-through height to the middle or back of a moist, partly shaded border.

Cold limit: USDA 4-8 (cool-temperate perennial) · RHS H6 (-20 to 24°C)

What thalictrum delavayi 'hewitt's double''s hardiness rating actually means

Yes — thalictrum delavayi 'hewitt's double' is genuinely cold hardy. Rated RHS H6 and USDA 4-8 (cool-temperate perennial), 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 (cool-temperate perennial) — 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. Thalictrum delavayi 'Hewitt's Double' is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for thalictrum delavayi 'hewitt's double' 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 thalictrum delavayi 'hewitt's double' go outside or overwinter — and where?

Plant it out within USDA 4-8 (cool-temperate perennial) 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 thalictrum delavayi 'hewitt's double' can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Thalictrum delavayi 'Hewitt's Double' hardiness — frequently asked questions

Is thalictrum delavayi 'hewitt's double' cold hardy?

Yes — thalictrum delavayi 'hewitt's double' is genuinely cold hardy. Rated RHS H6 and USDA 4-8 (cool-temperate perennial), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Thalictrum delavayi 'Hewitt's Double' is hardy across USDA 4-8 (cool-temperate perennial); 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 thalictrum delavayi 'hewitt's double' can survive?

Minimum survivable temperature is roughly about −20 to −15 °C. Thalictrum delavayi 'Hewitt's Double' is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is thalictrum delavayi 'hewitt's double'?

Thalictrum delavayi 'Hewitt's Double' is rated USDA 4-8 (cool-temperate perennial) and RHS H6 — Hardy throughout the UK and northern Europe.

Can thalictrum delavayi 'hewitt's double' survive winter outside?

Plant it out within USDA 4-8 (cool-temperate perennial) 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 thalictrum delavayi 'hewitt's double' 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.