Repotting guide

When & how to repot Lemna minor (Lemna minor)

Also called Common Duckweed, Lesser Duckweed, Bayroot.

More about lemna minor

About Lemna minor

Lemna minor · also called Common Duckweed, Lesser Duckweed · houseplant

Common duckweed is a tiny free-floating aquatic plant, each frond just 2-4 mm across with a single dangling root. It multiplies explosively to carpet still water in green, shading out algae and oxygenating fish ponds and aquariums. Useful for nutrient uptake and shade, but it spreads so fast it must be skimmed back regularly to avoid blanketing the surface.

Mature size: Individual fronds 2-4 mm; colonies spread indefinitely to cover the entire water surface

How to tell lemna minor needs repotting

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

Roots poking out of the drainage holes or coiling visibly around the inside of the pot.
You are watering far more often than you used to because the rootball dries out within a day or two.
Water runs straight through and out the bottom without soaking in.
Top growth has slowed or new lemna minor leaves are noticeably smaller than older ones despite good light.

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 lemna minor

Every 12–18 months — sooner if roots show fast. Lemna minor's growth habit — free-floating, mat-forming aquatic; reproduces almost entirely by budding daughter fronds, doubling its population in days under warm bright conditions and forming a continuous green carpet. — sets the pace. Common duckweed is a tiny free-floating aquatic plant, each frond just 2-4 mm across with a single dangling root. It multiplies explosively to carpet still water in green, shading out algae and oxygenating fish ponds and aquariums. Useful for nutrient uptake and shade, but it spreads so fast it must be skimmed back regularly to avoid blanketing the surface.

What size pot to step lemna minor up to

Step up one pot size — about 2–3 cm (an inch) wider. Lemna minor grows fast, so it will fill that space within a season, but jumping several sizes at once still backfires: the unused soil stays soggy and rots even a vigorous root system. One size at a time, every year or so, is the rhythm.

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 lemna minor

Early spring, just as new growth restarts, is the ideal window for lemna minor. 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 lemna minor

Time it for spring. Repot lemna minor in early spring as growth restarts so it re-roots quickly into the fresh soil.
Choose one size up. Pick a pot about 2–3 cm wider with drainage holes. One step only — a much bigger pot stays soggy and rots roots.
Ease the plant out. Water lightly the day before, then tip lemna minor out and gently loosen any roots circling the bottom of the rootball.
Repot at the same depth. Put a layer of fresh none — free-floating in the new pot, set the plant so its soil line is unchanged, and backfill, firming lightly.
Water and pause feeding. Water once to settle the soil. Hold off fertiliser for about a month — fresh mix already has nutrients and feeding now burns new roots.

Aftercare

Water lemna minor once to settle the soil, then let the surface dry before watering again — fresh mix around the roots stays wetter than the old compacted ball, so the commonest post-repot mistake is overwatering. Keep it out of direct sun for a week or two while roots re-establish. Do not fertilise for about 4 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for lemna minor

Lemna minor wants 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. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting lemna minor — frequently asked questions

How often should you repot lemna minor?

Every 12–18 months — sooner if roots show fast for lemna minor. Repot lemna minor roughly every 12–18 months, in early spring as growth restarts. It grows fast and circles its pot quickly, so step up one size (about 2–3 cm wider) into fresh none — free-floating. Don't jump several sizes — that soggy excess soil is what rots vigorous roots.

What size pot does lemna minor need?

When is the best time of year to repot lemna minor?

Can you put lemna minor straight into a much bigger pot?

No. Even a fast-growing lemna minor should only go up one pot size at a time. A vastly oversized pot holds a reservoir of wet soil the roots cannot reach, which stays cold and soggy and rots the roots — the opposite of what you wanted.

Should you fertilise lemna minor after repotting?

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