Lemna minor (Common Duckweed) care

Light

Lemna minor is what florists mean by "bright spot, no direct sun" — close enough to a south or east window to feel the brightness, with a sheer curtain or a few feet of distance keeping the sun off the leaves. Grows fastest in bright light to full sun; tolerates moderate light but multiplies slowly. In aquariums, standard planted-tank lighting is ample and may need thinning to keep light reaching plants below. A phone lux-meter at the leaf surface should read 1,500-3,000 lux at noon.

Watering

Water lemna minor floats permanently on still or slow 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 standing fresh water. Prefers calm, nutrient-rich water (still ponds, tanks without strong surface flow); reduce filter outflow or surface agitation, which sinks and shreds the fronds.

Soil and pot

Lemna minor grows best in none — free-floating. Rootless of substrate; it takes all nutrients directly from the water through its single trailing root and frond underside. No soil or planting medium is used. 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

Lemna minor sits happiest at around Not applicable humidity and 6 to 33°C (43 to 91°F). An aquatic surface plant — humidity is irrelevant. It needs an open water surface rather than ambient moisture. If you keep the room above 6 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 lemna minor sparingly. Generally needs no feeding — it thrives on dissolved nitrogen and phosphate already in pond and aquarium water, which is precisely why it is used to strip excess nutrients. Adding fertiliser only triggers runaway 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 lemna minor in the Growli community. Each is annotated with the most common cause so you know where to start.

• Overgrowth and oxygen crash — A complete surface blanket blocks light and gas exchange, deoxygenating the water below and stressing fish; skim off excess weekly to keep part of the surface clear.
• Sinking and dying off — Strong filter flow or surface turbulence pushes fronds underwater where they rot; reduce surface agitation to keep them floating.
• Winter die-back — Fronds form turions and sink in cold water, vanishing each autumn; this is normal — they resurface and regrow when water warms in spring.
• Hitchhiking pests and weeds — Wild-collected duckweed can introduce snails, algae or invasive species; quarantine and rinse new batches before adding to a clean system.

Propagation

Effortless — it propagates itself by vegetative budding. To establish it elsewhere, simply transfer a few floating fronds to another body of still fresh water, where they divide and spread on their 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

Lemna minor is mildly toxic to pets. Lemna minor 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. Duckweed itself contains no known toxic principle, but floating mats can harbour cyanobacteria (blue-green algae) and accumulate waterborne toxins, so a pet drinking from a duckweed-choked pond is the real hazard to watch. 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.

Lemna minor deep-dive guides

Every aspect of lemna minor care, each with its own calibrated guide:

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

Featured in these plant shortlists

Lemna minor qualifies for 2 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.
• Browse all 29 plant shortlists — pet-safe, low-light, drought-tolerant and more

Related guides

Lemna minor is also known as Common Duckweed, Lesser Duckweed, and Bayroot.

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