Mature size & growth rate

How big does Strap Water Fern (Blechnum patersonii) get?

Also called Strap Fern, Paterson's Water Fern.

More about strap water fern

About Strap Water Fern

Blechnum patersonii · also called Strap Fern, Paterson's Water Fern · tropical

Blechnum patersonii is an unusual Australian fern with simple, undivided strap-like fronds unlike the typical feathery fern silhouette. It grows along stream banks in the wild, preferring consistently moist, shaded conditions. Non-toxic to pets; a distinctive choice for humid terrariums or bathrooms.

Mature size: 20-40 cm tall; fronds to 30 cm long

Watch for — Pale fronds in low light: While shade-tolerant, excessively dark conditions cause weak, etiolated growth. Supplement with a grow light if needed.

Indoor size vs how big it gets in the wild

Strap Water Fern is a naturally small plant — it stays shelf- and desk-sized for its whole life, so it never becomes a space problem. Indoors and in a pot, expect 20-40 cm tall. In the ground with no restriction it is a completely different plant — fronds to 30 cm long — which is why the pot, the light and the pruning matter so much for the size you actually end up with.

It grows mostly by adding leaves, offsets or a slightly wider rosette rather than gaining height — the footprint barely changes year to year.

Growth rate and years to mature

Strap Water Fern is a moderate grower. Realistically, expect three to six years to reach mature indoor size, gaining a steady amount each growing season. Its feeding profile backs this up: apply a very dilute balanced liquid fertiliser (quarter strength) every 4-6 weeks during the growing season. this fern is a light feeder and excess fertiliser causes frond tip burn.

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

How to keep strap water fern smaller

Good news — strap water fern barely needs managing. If you do want to keep it tidy:

Divide or remove offsets when the pot looks crowded to keep strap water fern to a single tidy clump.
Keeping it slightly pot-bound and easing back on feed naturally caps the size.
Pinch or remove the oldest, tiredest leaves so energy goes into a compact, fresh-looking plant.

How to grow strap water fern bigger or faster

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

Move it to brighter (but not scorching) light — that is the single biggest growth lever for a small plant.
A small step up in pot size every couple of years gives the roots a little more room without triggering a size jump.
Feed lightly through the growing season; this plant simply will not race however hard you push it.

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

When strap water fern outgrows the room (or the pot)

"Too big" usually arrives as one of these signs for strap water fern:

Roots circling the bottom or pushing out of the drainage hole — it wants a pot one size up, not a bigger room.
Offsets crowding the surface so the original plant looks squashed.
Honestly, strap water fern rarely outgrows a room — outgrowing its pot is the only realistic limit.

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

Strap Water Fern size — frequently asked questions

How big does strap water fern get?

Strap Water Fern reaches 20-40 cm tall when grown indoors, and far larger where it grows unrestricted (fronds to 30 cm long). It grows mostly by adding leaves, offsets or a slightly wider rosette rather than gaining height — the footprint barely changes year to year.

Is strap water fern slow or fast growing?

Strap Water Fern is a moderate grower. Expect three to six years to reach mature indoor size, gaining a steady amount each growing season. Strap Water Fern is a naturally small plant — it stays shelf- and desk-sized for its whole life, so it never becomes a space problem.

How long does strap water fern take to reach full size?

Roughly three to six years to reach mature indoor size, gaining a steady amount each growing season. Light, pot size and feeding move that timeline more than anything else.

How do I keep strap water fern smaller?

Divide or remove offsets when the pot looks crowded to keep strap water fern to a single tidy clump. Keeping it slightly pot-bound and easing back on feed naturally caps the size. Pinch or remove the oldest, tiredest leaves so energy goes into a compact, fresh-looking plant.

How can I make strap water fern grow bigger or faster?

Move it to brighter (but not scorching) light — that is the single biggest growth lever for a small plant. A small step up in pot size every couple of years gives the roots a little more room without triggering a size jump. Feed lightly through the growing season; this plant simply will not race however hard you push it.