Cold hardiness & minimum temperature

Is Large-Leaved Waterleaf (Hydrophyllum macrophyllum)cold hardy? Hardiness zone & min temp

Also called Large-Leaved Waterleaf, Hairy Waterleaf, Largeleaf Waterleaf.

More about large-leaved waterleaf

About Large-Leaved Waterleaf

Hydrophyllum macrophyllum · also called Large-Leaved Waterleaf, Hairy Waterleaf · herb

Hydrophyllum macrophyllum is a hairy-stemmed woodland perennial native to mesic, rocky, calcareous forests of the Midwest and Upper South of the eastern United States. It produces large (up to 15 cm / 6 in), prominently lobed leaves and clusters of cream-coloured flowers in late spring. It grows up to 70 cm tall and colonises shaded, moist slopes through rhizome spread, making it a useful large-scale groundcover under tall trees. Hydrophyllum is not listed in the ASPCA plant database; classified mildly-toxic as a precaution pending confirmed species-level toxicity data.

Cold limit: USDA 5-7 · RHS H6 (-25 to 25°C)

What large-leaved waterleaf's hardiness rating actually means

Yes — large-leaved waterleaf is genuinely cold hardy. Rated RHS H6 and USDA 5-7, 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 5-7 — 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. Large-Leaved Waterleaf is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for large-leaved waterleaf 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 large-leaved waterleaf go outside or overwinter — and where?

Plant it out within USDA 5-7 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 large-leaved waterleaf can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Large-Leaved Waterleaf hardiness — frequently asked questions

Is large-leaved waterleaf cold hardy?

Yes — large-leaved waterleaf is genuinely cold hardy. Rated RHS H6 and USDA 5-7, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Large-Leaved Waterleaf is hardy across USDA 5-7; 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 large-leaved waterleaf can survive?

Minimum survivable temperature is roughly about −20 to −15 °C. Large-Leaved Waterleaf is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is large-leaved waterleaf?

Large-Leaved Waterleaf is rated USDA 5-7 and RHS H6 — Hardy throughout the UK and northern Europe.

Can large-leaved waterleaf survive winter outside?

Plant it out within USDA 5-7 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 large-leaved waterleaf 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.