Azolla filiculoides (Water Fern) care

Light

Azolla filiculoides needs sun on the leaves, not just bright ambient room light. Bright light to full sun gives the densest growth and the strongest red colouration; in shade it stays green and grows more slowly, spreading less aggressively. A south or west-facing windowsill in the northern hemisphere is the default; anywhere else, expect the plant to stretch and pale out within a season.

Watering

Water azolla filiculoides floats permanently on still water; no separate watering. The actual day count varies with pot size, light, and season — the finger test (or lifting the pot to feel its weight) is more reliable than a fixed calendar. Empty any drainage saucer afterwards so the pot isn't sitting in water. Lives on the surface of calm, nutrient-poor to moderate fresh water. Prefers still conditions; surface turbulence breaks up the mats. It can survive on wet mud if water recedes.

Soil and pot

Azolla filiculoides grows best in 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. A pot with a working drainage hole is non-negotiable for this species — even free-draining mix will turn soggy in a closed planter. If you love the look of a decorative pot without a hole, use it as a cachepot around an inner nursery pot you can lift out to water.

Humidity and temperature

Azolla filiculoides sits happiest at around Not applicable humidity and 5 to 30°C (41 to 86°F). A floating aquatic fern — humidity is irrelevant; it needs an open water surface or saturated mud rather than ambient moisture. If you keep the room above 5 to 30°C year-round and avoid placing the plant near a cold draught, a hot radiator, or an air-conditioning vent, you have already handled the two biggest indoor stressors.

Fertilising

Feed azolla filiculoides sparingly. Never needs feeding — its symbiotic cyanobacterium Nostoc azollae fixes atmospheric nitrogen, making the plant self-fertilising (and itself a nitrogen-rich green manure). Adding nutrients only accelerates already-explosive growth. Skip fertiliser entirely on a stressed, recently-repotted, or actively wilting plant — fertiliser salts make damage worse, not better. Wait for a round of healthy new growth before resuming a feeding rhythm.

Common problems

Below are the issues we see most often on azolla filiculoides in the Growli community. Each is annotated with the most common cause so you know where to start.

• Invasive smothering — It doubles in days and forms thick mats that deoxygenate water and smother other aquatics; it is a listed invasive in many countries — keep it contained and dispose of trimmings away from waterways.
• Light and oxygen blocking — A complete cover shades submerged plants and cuts gas exchange, stressing fish; skim off excess regularly to keep part of the surface clear.
• 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.
• Loss of red colour — Mats stay plain green in shade or rich water; move to brighter, cooler, leaner conditions to bring out the characteristic red flush.

Propagation

Propagates itself by fragmentation — branches naturally break off into new plants. To establish it elsewhere, transfer a handful of floating fronds to another still freshwater body (where legal and contained), and it spreads on its own. Propagation is the cheapest, most satisfying way to expand a collection — and it doubles as insurance against losing a mature plant to an accident. Take a backup cutting once the parent is established and healthy.

Toxicity to pets

Azolla filiculoides is mildly toxic to pets. Azolla filiculoides is not individually listed on the ASPCA Toxic and Non-Toxic Plants database, so a pet-safety claim cannot be confirmed; treat with caution and verify with a vet. Its symbiotic cyanobacterium has largely lost the genes for classic cyanotoxins, but as a cyanobacteria-hosting floating mat it can degrade water quality, so a pet drinking from an Azolla-covered pond is the practical hazard to monitor. If you keep cats, dogs, or curious children in the house, weigh placement carefully — a high shelf or a hanging planter is enough for casual safety. For severe ingestion incidents, call your local vet and the ASPCA Animal Poison Control Center (in the US, 888-426-4435).

Pet-safety status is sourced from the ASPCA Toxic and Non-Toxic Plant List, which catalogues the most-asked-about plants for cats, dogs, and horses.

Azolla filiculoides deep-dive guides

Every aspect of azolla filiculoides care, each with its own calibrated guide:

• Azolla filiculoides watering schedule
• Azolla filiculoides light requirements
• Best soil mix for azolla filiculoides
• Azolla filiculoides fertilizing guide
• When to repot azolla filiculoides
• How to propagate azolla filiculoides
• Azolla filiculoides growth rate & size
• Azolla filiculoides cold hardiness
• Azolla filiculoides temperature & humidity
• Is azolla filiculoides toxic to cats & dogs?
• Is azolla filiculoides toxic to cats?
• Is azolla filiculoides toxic to dogs?

Featured in these plant shortlists

Azolla filiculoides qualifies for 4 curated Growli shortlists — each one filtered objectively from our structured plant-care library, so the selection is consistent and checkable:

• Best small & tabletop houseplants — Compact houseplants that stay under about 40 cm — desk, shelf and windowsill plants that never outgrow a small space.
• Best houseplants for full sun — Houseplants that want direct sun — the species for a hot south or west-facing windowsill where shade-lovers scorch.
• Best houseplants for a cool room — Houseplants that tolerate cool conditions down to about 10°C — for an unheated spare room, hallway, porch or a home kept cool.
• Best fast-growing houseplants — Houseplants documented as fast or vigorous growers — quick to fill a pot, cover a pole or trail down a shelf.
• Browse all 29 plant shortlists — pet-safe, low-light, drought-tolerant and more

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Azolla filiculoides is also known as Water Fern, Fairy Moss, and Red Azolla.

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