Cold hardiness & minimum temperature

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

Also called Feathered Mosquito Fern, Water Velvet.

More about azolla pinnata

About Azolla pinnata

Azolla pinnata · also called Feathered Mosquito Fern, Water Velvet · houseplant

Azolla pinnata is a tiny free-floating aquatic fern that forms a dense green-to-red carpet on still water. It hosts the nitrogen-fixing cyanobacterium Anabaena, making it a living fertiliser in rice paddies. In aquariums and ponds it shades water, curbs algae and provides shelter, but it multiplies explosively and must be thinned regularly.

Cold limit: USDA 9-11 (frost-tender; grown as a warm-season or indoor floater in most US regions) · RHS H2 (18-28°C)

Watch for — Yellowing or browning fronds: Usually too little light, cold shock, or stagnant low-oxygen water. Increase light, raise temperature into range, and remove decaying plants before they foul the water.

What azolla pinnata's hardiness rating actually means

Azolla pinnata is half-hardy (RHS H2). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Its RHS rating of H2 means: Tender — survives a frost-free greenhouse or a very mild, sheltered spot. On the US scale that maps to USDA 9-11 (frost-tender; grown as a warm-season or indoor floater in most US regions) — 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 1 to 5 °C — tolerates cold but no real frost. Azolla pinnata shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for azolla pinnata as it gets too cold:

Down to roughly about 1 to 5 °C it copes, especially if dry and sheltered.
A sustained hard frost collapses the top growth; whether it returns depends on whether the roots, crown or tubers froze.
Wet cold is far more lethal than dry cold for this plant — soggy, frozen soil is the usual killer.

Can azolla pinnata go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 (frost-tender; grown as a warm-season or indoor floater in most US regions) or a frost-free UK microclimate.
In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter.
A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

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 pinnata can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H2 figure above.

Frost protection for borderline azolla pinnata

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

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost.
Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse.
Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones.
Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.

Azolla pinnata hardiness — frequently asked questions

Is azolla pinnata cold hardy?

Azolla pinnata is half-hardy (RHS H2). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Borderline outdoors. In its mild end of USDA 9-11 (frost-tender; grown as a warm-season or indoor floater in most US regions) (and sheltered UK gardens) azolla pinnata can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature azolla pinnata can survive?

Minimum survivable temperature is roughly about 1 to 5 °C — tolerates cold but no real frost. Azolla pinnata shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is azolla pinnata?

Azolla pinnata is rated USDA 9-11 (frost-tender; grown as a warm-season or indoor floater in most US regions) and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can azolla pinnata survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 (frost-tender; grown as a warm-season or indoor floater in most US regions) or a frost-free UK microclimate. In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter. A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

How do I protect azolla pinnata from frost?

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost. Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse. Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones. Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.