Repotting guide

When & how to repot Azolla filiculoides (Azolla filiculoides)

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.

Mature size: Individual plants 1-2.5 cm; colonies spread indefinitely to blanket still water surfaces

How to tell azolla filiculoides needs repotting

Repotting on a calendar is less reliable than reading the plant. For azolla filiculoides, 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 filiculoides.

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 filiculoides

Every 1–2 years, disturbing the roots as little as possible. Azolla filiculoides's growth habit — free-floating, mat-forming aquatic fern; reproduces vegetatively by fragmentation as side branches break off, doubling rapidly in warm bright water to form dense green-to-red surface mats. — sets the pace. 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.

What size pot to step azolla filiculoides up to

Go up only one size and handle the rootball as little as possible. Azolla filiculoides 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 filiculoides

Early spring, just as new growth restarts, is the ideal window for azolla filiculoides. 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 filiculoides

Keep disturbance to a minimum. Azolla filiculoides 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 ready.
Slide the rootball out whole. Water the day before, then ease azolla filiculoides 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 filiculoides 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 filiculoides

Azolla filiculoides wants none — free-floating. Rooted only by fine dangling rootlets that hang in the water, not in substrate. It draws nutrients from the water and fixes its own nitrogen via its cyanobacterial symbiont, so no soil is needed. 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 filiculoides — frequently asked questions

How often should you repot azolla filiculoides?

Every 1–2 years, disturbing the roots as little as possible for azolla filiculoides. Repot azolla filiculoides 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, keep it warm and humid afterwards, and never bare-root or hard-prune the roots.

What size pot does azolla filiculoides need?

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

Why does azolla filiculoides sulk after repotting?

Azolla filiculoides 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 filiculoides after repotting?

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