Cold hardiness & minimum temperature

Is Rough Marshmallow (Althaea hirsuta)cold hardy? Hardiness zone & min temp

Also called Rough Marshmallow, Hairy Marshmallow, Hispid Marshmallow.

More about rough marshmallow

About Rough Marshmallow

Althaea hirsuta · also called Rough Marshmallow, Hairy Marshmallow · herb

Althaea hirsuta is a bristly-hairy annual or biennial wildflower native to southern Europe and southwest Asia, producing small, pale pink to lilac flowers through summer. Found on dry, chalky disturbed ground and field margins, it is occasionally cultivated for wildflower meadows and herbal use. Its mucilaginous properties are similar to other Althaea species, though it is less commonly used medicinally.

Cold limit: USDA 7-10 · RHS H4 (-5°C to 40°C)

What rough marshmallow's hardiness rating actually means

Yes — rough marshmallow is genuinely cold hardy. Rated RHS H4 and USDA 7-10, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H4 means: Hardy in an average winter across much of the temperate world. On the US scale that maps to USDA 7-10 — 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 −10 to −5 °C. Rough Marshmallow is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for rough marshmallow as it gets too cold:

It tolerates winter lows to about −10 to −5 °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 rough marshmallow go outside or overwinter — and where?

Plant it out within USDA 7-10 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 rough marshmallow can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Frost protection for borderline rough marshmallow

Rough Marshmallow is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

At the cold edge of its range, mulch the root zone in late autumn to buffer the deepest freezes.
Protect container specimens — pots freeze through far faster than open ground, costing roughly a zone of hardiness.
Shelter new growth from late spring frosts with fleece if a hard night is forecast.

Rough Marshmallow hardiness — frequently asked questions

Is rough marshmallow cold hardy?

Yes — rough marshmallow is genuinely cold hardy. Rated RHS H4 and USDA 7-10, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Rough Marshmallow is hardy across USDA 7-10; 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 rough marshmallow can survive?

Minimum survivable temperature is roughly about −10 to −5 °C. Rough Marshmallow is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is rough marshmallow?

Rough Marshmallow is rated USDA 7-10 and RHS H4 — Hardy in an average winter across much of the temperate world.

Can rough marshmallow survive winter outside?

Plant it out within USDA 7-10 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.

How do I protect rough marshmallow from frost?

At the cold edge of its range, mulch the root zone in late autumn to buffer the deepest freezes. Protect container specimens — pots freeze through far faster than open ground, costing roughly a zone of hardiness. Shelter new growth from late spring frosts with fleece if a hard night is forecast.