Cold hardiness & minimum temperature

Is Azolla filiculoides (Azolla filiculoides)cold hardy? Hardiness zone & min temp

Also called Water Fern, Fairy Moss, Red Azolla.

More about azolla filiculoides

About Azolla filiculoides

Azolla filiculoides · also called Water Fern, Fairy Moss · houseplant

Azolla is a tiny free-floating water fern whose overlapping fronds turn from green to a vivid red in bright light or cold, carpeting still water. It famously hosts the nitrogen-fixing cyanobacterium Nostoc azollae, so it fertilises its own water. Decorative and used as green manure, it is also invasive in many regions — keep it strictly contained and never release it.

Cold limit: USDA 7-11 (survives mild winters; heavy frost can kill surface mats) · RHS H4 (5 to 30°C)

Watch for — Winter die-off: Hard frost kills surface mats; in cold climates overwinter a portion indoors in a bright frost-free tub if you want to keep a stock.

What azolla filiculoides's hardiness rating actually means

Yes — azolla filiculoides is genuinely cold hardy. Rated RHS H4 and USDA 7-11 (survives mild winters; heavy frost can kill surface mats), 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 7-11 (survives mild winters; heavy frost can kill surface mats) — 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. Azolla filiculoides is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for azolla filiculoides 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 azolla filiculoides go outside or overwinter — and where?

Plant it out within USDA 7-11 (survives mild winters; heavy frost can kill surface mats) 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 azolla filiculoides 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 azolla filiculoides

Azolla filiculoides 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.

Azolla filiculoides hardiness — frequently asked questions

Is azolla filiculoides cold hardy?

Yes — azolla filiculoides is genuinely cold hardy. Rated RHS H4 and USDA 7-11 (survives mild winters; heavy frost can kill surface mats), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Azolla filiculoides is hardy across USDA 7-11 (survives mild winters; heavy frost can kill surface mats); 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 azolla filiculoides can survive?

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

What hardiness zone is azolla filiculoides?

Azolla filiculoides is rated USDA 7-11 (survives mild winters; heavy frost can kill surface mats) and RHS H4 — Hardy in an average winter across much of the temperate world.

Can azolla filiculoides survive winter outside?

Plant it out within USDA 7-11 (survives mild winters; heavy frost can kill surface mats) 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 azolla filiculoides 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.