Mature size & growth rate

How big does Wolffia arrhiza (Wolffia arrhiza) get?

Also called Rootless Duckweed, Watermeal, Spotless Watermeal.

More about wolffia arrhiza

About Wolffia arrhiza

Wolffia arrhiza · also called Rootless Duckweed, Watermeal · houseplant

Watermeal is the world's smallest flowering plant — rootless green grains under 1 mm across that float like fine dust on still water. Each plant is a tiny ovoid frond with no root at all, multiplying by budding into a powdery surface film. A curiosity for nano-aquariums and ponds, it is even faster and finer than common duckweed and notoriously hard to remove.

Mature size: Individual plants under 1 mm — the smallest flowering plant known; colonies spread indefinitely across still water

Indoor size vs how big it gets in the wild

Wolffia arrhiza stays fairly low but widens over time — it spreads into a bigger clump by offsets, runners or rhizomes rather than shooting upward. Indoors and in a pot, expect individual plants under 1 mm. In the ground with no restriction it is a completely different plant — the smallest flowering plant known; colonies spread indefinitely across still water — which is why the pot, the light and the pruning matter so much for the size you actually end up with.

Size here is about width, not height: the plant builds an ever-wider clump or sends out plantlets and runners while staying relatively short.

Growth rate and years to mature

Wolffia arrhiza is a fast grower. Realistically, expect two to four years from a young plant to a room-filling specimen in good light. Its feeding profile backs this up: 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.

Want this turned into the right next pot at the right moment? The pot size calculator and the wolffia arrhiza repotting guide cover when and how much to size up — pot size is one of the biggest levers on how fast wolffia arrhiza grows.

How to keep wolffia arrhiza smaller

You are not stuck with the maximum size. For wolffia arrhiza specifically, these are the levers, in order of impact:

Divide the clump every year or two — splitting wolffia arrhiza is the main way to control its spread and refresh it.
Remove runners, plantlets or offsets as they appear if you want it to stay a single tight clump.
Keep it slightly pot-bound; a snug pot naturally limits how wide the clump can get.

The keep-it-smaller method, step by step

Lift the whole plant. Slide wolffia arrhiza out of its pot in spring when the clump has filled it.
Split the clump. Tease or cut the rootball into two or more sections, each with healthy roots and growth.
Repot one division. Put a single division back in the original pot to reset it to a smaller size; pot or give away the rest.
Remove offsets as they form. Through the year, detach new runners or pups to stop it spreading again.

How to grow wolffia arrhiza bigger or faster

If you want it to fill the space sooner, push the conditions rather than hoping — for wolffia arrhiza the accelerators are:

Give it a wider pot and let the clump fill it — width is exactly how this plant gets bigger.
Good light plus regular feeding maximises offset and runner production.
Leave plantlets and offsets attached and feed through the growing season for the fastest spread.

Light is almost always the ceiling. The wolffia arrhiza light requirements page covers exactly how bright a spot it needs to grow at its potential instead of stalling.

When wolffia arrhiza outgrows the room (or the pot)

"Too big" usually arrives as one of these signs for wolffia arrhiza:

The clump bulging over the pot rim or splitting the pot — the cue to divide, not to find a bigger room.
A dense centre that goes bare or tired while the edges keep spreading.
Runners or offsets escaping across the shelf or into neighbouring pots.

If it is the pot rather than the room, it is a repotting job, not a goodbye — see the wolffia arrhiza repotting guide. If you want more of this plant instead of a bigger one, the wolffia arrhiza propagation guide turns prunings into new plants.

Wolffia arrhiza size — frequently asked questions

How big does wolffia arrhiza get?

Wolffia arrhiza reaches individual plants under 1 mm when grown indoors, and far larger where it grows unrestricted (the smallest flowering plant known; colonies spread indefinitely across still water). Size here is about width, not height: the plant builds an ever-wider clump or sends out plantlets and runners while staying relatively short.

Is wolffia arrhiza slow or fast growing?

Wolffia arrhiza is a fast grower. Expect two to four years from a young plant to a room-filling specimen in good light. Wolffia arrhiza stays fairly low but widens over time — it spreads into a bigger clump by offsets, runners or rhizomes rather than shooting upward.

How long does wolffia arrhiza take to reach full size?

Roughly two to four years from a young plant to a room-filling specimen in good light. Light, pot size and feeding move that timeline more than anything else.

How do I keep wolffia arrhiza smaller?

Divide the clump every year or two — splitting wolffia arrhiza is the main way to control its spread and refresh it. Remove runners, plantlets or offsets as they appear if you want it to stay a single tight clump. Keep it slightly pot-bound; a snug pot naturally limits how wide the clump can get.

How can I make wolffia arrhiza grow bigger or faster?

Give it a wider pot and let the clump fill it — width is exactly how this plant gets bigger. Good light plus regular feeding maximises offset and runner production. Leave plantlets and offsets attached and feed through the growing season for the fastest spread.