Wolffia arrhiza (Rootless Duckweed) care

Light

Bright but filtered. Wolffia arrhiza burns within days in unfiltered south-facing summer sun, and stops growing within months in deep shade. Multiplies fastest in bright light to full sun. Tolerates moderate light but slows; in tanks, ordinary planted-aquarium lighting suffices and the film must be thinned so light reaches plants beneath. If you only have a south window, set the plant back 1.5 m or hang a sheer curtain — both knock the intensity down into the right range.

Watering

Watering wolffia arrhiza: floats permanently on still water; no separate watering. The number that matters isn't the day of the week — it's how dry the top 2-3 cm of the pot feels. A finger in the soil tells you more than a watering app. After every watering, tip the saucer. Lives on the surface of calm, warm fresh water and detests surface movement, which sinks the minuscule fronds. Best in still ponds or filtered tanks with the outflow baffled to leave the surface undisturbed.

Soil and pot

Wolffia arrhiza grows best in none — free-floating and rootless. Truly rootless (arrhiza = 'without root'); it absorbs all nutrients directly from the water through the frond. No substrate or planting medium is involved. 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

Wolffia arrhiza sits happiest at around Not applicable humidity and 10 to 33°C (50 to 91°F). A floating aquatic surface plant — ambient humidity has no bearing on it. It requires open still water rather than moist air. If you keep the room above 10 to 33°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 wolffia arrhiza sparingly. Needs no feeding; it lives on dissolved nutrients in the water and is itself used to absorb surplus nitrogen and phosphate. Supplementary fertiliser only causes explosive, unmanageable 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 wolffia arrhiza in the Growli community. Each is annotated with the most common cause so you know where to start.

• Impossible to eradicate — Its dust-fine fronds escape nets and recolonise from a single plant; once unwanted it is extremely hard to remove, so introduce it deliberately and contained.
• Surface film and light blocking — A thick meal blocks light and gas exchange below; skim regularly and keep part of the surface open to protect submerged plants and fish.
• Sinks in moving water — Any surface current drives the tiny fronds under where they perish; baffle filter outflows to keep the surface dead calm.
• Clogs filters and pumps — The fine grains are drawn into intakes; fit a fine pre-filter sponge over inlets to prevent blockage.

Propagation

Propagates itself by continuous budding. To spread it, transfer a pinch of floating fronds to another body of still warm fresh water, where they divide and multiply unaided. 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

Wolffia arrhiza is mildly toxic to pets. Wolffia arrhiza 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. The plant carries no recognised toxic principle (some Wolffia species are even eaten as a vegetable in parts of Asia), but dense surface films can shelter cyanobacteria, so water quality is the practical pet concern. 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.

Wolffia arrhiza deep-dive guides

Every aspect of wolffia arrhiza care, each with its own calibrated guide:

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

Featured in these plant shortlists

Wolffia arrhiza qualifies for 3 curated Growli shortlists — each one filtered objectively from our structured plant-care library, so the selection is consistent and checkable:

• Best plants for a north-facing window — Houseplants for a north-facing window: bright, even, indirect light and no scorching direct sun. Each pick verified against its documented light needs.
• 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

Related guides

Wolffia arrhiza is also known as Rootless Duckweed, Watermeal, and Spotless Watermeal.

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