Cold hardiness & minimum temperature

Is Hemlock Water Dropwort (Oenanthe crocata)cold hardy? Hardiness zone & min temp

Also called Hemlock Water Dropwort, Dead Man's Fingers, Water Hemlock.

More about hemlock water dropwort

About Hemlock Water Dropwort

Oenanthe crocata · also called Hemlock Water Dropwort, Dead Man's Fingers · flowering

Oenanthe crocata is a robust, hairless perennial of the carrot family (Apiaceae), native to western and central Europe including the UK, growing along river banks, drainage ditches, wet meadows, and pond margins. It reaches 1–1.5 m tall and produces flat-topped umbels of white flowers in summer. The single most critical fact about this plant is that it is widely considered the most poisonous plant native to Britain — all parts, especially the fleshy white tuberous roots, contain the potent polyacetylene neurotoxin oenanthotoxin, which can cause fatal seizures in humans and animals within minutes of ingestion. It is extremely toxic to pets and humans.

Cold limit: USDA 5-9 · RHS H7 (-20–28°C)

What hemlock water dropwort's hardiness rating actually means

Yes — hemlock water dropwort is genuinely cold hardy. Rated RHS H7 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H7 means: Hardy in the severest European continental 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 below about −20 °C. Hemlock Water Dropwort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for hemlock water dropwort as it gets too cold:

It tolerates winter lows to about −20 °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 hemlock water dropwort 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 hemlock water dropwort can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Hemlock Water Dropwort hardiness — frequently asked questions

Is hemlock water dropwort cold hardy?

Yes — hemlock water dropwort is genuinely cold hardy. Rated RHS H7 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Hemlock Water Dropwort 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 hemlock water dropwort can survive?

Minimum survivable temperature is roughly below about −20 °C. Hemlock Water Dropwort is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is hemlock water dropwort?

Hemlock Water Dropwort is rated USDA 5-9 and RHS H7 — Hardy in the severest European continental winters.

Can hemlock water dropwort 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 hemlock water dropwort below its minimum temperature?

It tolerates winter lows to about −20 °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.