Cold hardiness & minimum temperature

Is Iron Butterfly Foamflower (Tiarella 'Iron Butterfly')cold hardy? Hardiness zone & min temp

Also called Iron Butterfly Foamflower, Iron Butterfly Foam Flower.

More about iron butterfly foamflower

About Iron Butterfly Foamflower

Tiarella 'Iron Butterfly' · also called Iron Butterfly Foamflower, Iron Butterfly Foam Flower · flowering

Tiarella 'Iron Butterfly' is a rhizomatous, clump-forming hybrid foamflower bred for its dramatic, deeply cut foliage with strong dark purple maroon vein markings — a standout in the shade garden. It is semi-evergreen and performs best in cool, moist, humus-rich soil in partial to full shade. The most important care fact is protecting the plant from drought and excessive winter wet; established clumps are low-maintenance but need good soil preparation at planting. This cultivar is not listed by the ASPCA; it carries the same precautionary mildly-toxic classification as the genus.

Cold limit: USDA 4-9 · RHS H6 (-20 to 25°C)

What iron butterfly foamflower's hardiness rating actually means

Yes — iron butterfly foamflower is genuinely cold hardy. Rated RHS H6 and USDA 4-9, 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 4-9 — 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. Iron Butterfly Foamflower is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for iron butterfly foamflower 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 iron butterfly foamflower go outside or overwinter — and where?

Plant it out within USDA 4-9 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 iron butterfly foamflower can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Iron Butterfly Foamflower hardiness — frequently asked questions

Is iron butterfly foamflower cold hardy?

Yes — iron butterfly foamflower is genuinely cold hardy. Rated RHS H6 and USDA 4-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Iron Butterfly Foamflower is hardy across USDA 4-9; 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 iron butterfly foamflower can survive?

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

What hardiness zone is iron butterfly foamflower?

Iron Butterfly Foamflower is rated USDA 4-9 and RHS H6 — Hardy throughout the UK and northern Europe.

Can iron butterfly foamflower survive winter outside?

Plant it out within USDA 4-9 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 iron butterfly foamflower 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.