Repotting guide

When & how to repot Azolla pinnata (Azolla pinnata)

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.

Mature size: Individual fronds 1-2.5 cm wide; colonies spread to cover the entire available water surface within weeks.

How to tell azolla pinnata needs repotting

Repotting on a calendar is less reliable than reading the plant. For azolla pinnata, watch for these signs:

Roots creeping out of the drainage holes or matting tightly across the soil surface.
The rootball dries out within a day or two no matter how much you water.
Water channels straight down the gap between rootball and pot without wetting the centre.
Steady decline — thin growth, persistent crispy edges — that good humidity and watering have not fixed. Only then is the disturbance of a repot worth the risk for azolla pinnata.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot azolla pinnata

Every 1–2 years, disturbing the roots as little as possible. Azolla pinnata's growth habit — free-floating mat-forming fern; individual plants are 1-2.5 cm, branching and overlapping into a continuous carpet that reddens in high light or cold. — sets the pace. 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.

What size pot to step azolla pinnata up to

Go up only one size and handle the rootball as little as possible. Azolla pinnata resents root disturbance, so the goal is to slide the intact rootball into slightly more soil — not to tease, wash or prune the roots. A modest step up means less shock and a faster recovery.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot azolla pinnata

Early spring, just as new growth restarts, is the ideal window for azolla pinnata. The plant is moving into its strongest growth phase and re-roots into fresh soil quickly. Avoid repotting in winter dormancy or, for flowering plants, while it is in bud or bloom — recovery is slowest then and you risk dropping the flowers.

Step-by-step: repotting azolla pinnata

Keep disturbance to a minimum. Azolla pinnata resents root disturbance, so the plan is to move the intact rootball — not to wash, tease or prune the roots.
Choose just one size up. Pick a pot only one size larger with drainage, and have moisture-retentive none — free-floating, no substrate ready.
Slide the rootball out whole. Water the day before, then ease azolla pinnata out keeping the rootball intact. Gently free only the roots that are circling the very bottom.
Nestle it into fresh soil. Add a base layer of fresh mix, set the rootball in at the same depth, and backfill gently around the sides without packing hard.
Water and protect. Water in, then keep it warm, humid and out of direct sun for a few weeks while it re-roots. Expect a short sulk — that is normal.

Aftercare

Expect azolla pinnata to sulk for a couple of weeks — that is normal after any root disturbance for this group. Keep it warm, humid and out of direct sun, water just enough to keep the mix lightly moist, and do not panic and overwater while it re-roots. Do not fertilise for about 4 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for azolla pinnata

Azolla pinnata wants none — free-floating, no substrate. Roots dangle in the water column rather than anchoring in soil. It draws nutrients (especially phosphorus) directly from the water; in paddies it floats above flooded mud. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting azolla pinnata — frequently asked questions

How often should you repot azolla pinnata?

Every 1–2 years, disturbing the roots as little as possible for azolla pinnata. Repot azolla pinnata every 1–2 years, disturbing the roots as little as possible — it sulks for weeks if the rootball is teased apart. Slide it into one size up in spring with fresh none — free-floating, no substrate, keep it warm and humid afterwards, and never bare-root or hard-prune the roots.

What size pot does azolla pinnata need?

When is the best time of year to repot azolla pinnata?

Why does azolla pinnata sulk after repotting?

Azolla pinnata resents root disturbance, so a wilt or stall for a week or two after repotting is normal, not a failure. Minimise it by keeping the rootball intact, stepping up just one size, and keeping the plant warm, humid and out of direct sun while it re-roots.

Should you fertilise azolla pinnata after repotting?

Not immediately. Wait about 4 weeks after repotting azolla pinnata. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.