Mature size & growth rate

How big does Fanal astilbe (Astilbe x arendsii 'Fanal') get?

Also called Fanal astilbe, Fanal false spirea.

More about fanal astilbe

About Fanal astilbe

Astilbe x arendsii 'Fanal' · also called Fanal astilbe, Fanal false spirea · flowering

Astilbe x arendsii 'Fanal' is one of the most celebrated astilbe cultivars, bearing intensely deep crimson-red plumes in early to midsummer — among the darkest reds in the genus. Its dark bronze-green foliage provides a striking backdrop. 'Fanal' is compact relative to the hybrid group, making it ideal for border edging, containers, and shady streamside plantings.

Mature size: 50–60 cm tall (including flower plumes), 45–60 cm wide

Indoor size vs how big it gets in the wild

Fanal astilbe 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 50–60 cm tall (including flower plumes), 45–60 cm wide. A pot, your light levels and a little pruning are what set the final size in a home, far more than the plant's theoretical potential.

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

Fanal astilbe 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: feed with a balanced slow-release granular fertiliser in early spring. a liquid feed of low-nitrogen, high-potassium fertiliser (e.g. tomato feed) in early june can deepen flower colour. over-feeding with nitrogen produces lush foliage at the expense of blooms.

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

How to keep fanal astilbe smaller

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

Divide the clump every year or two — splitting fanal astilbe 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 fanal astilbe 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 fanal astilbe bigger or faster

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

Give it a wider pot and let the clump fill it — width is exactly how this plant gets bigger.
Brighter light speeds up clump and offset production noticeably.
Leave plantlets and offsets attached and feed through the growing season for the fastest spread.

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

When fanal astilbe outgrows the room (or the pot)

"Too big" usually arrives as one of these signs for fanal astilbe:

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 fanal astilbe repotting guide. If you want more of this plant instead of a bigger one, the fanal astilbe propagation guide turns prunings into new plants.

Fanal astilbe size — frequently asked questions

How big does fanal astilbe get?

Fanal astilbe reaches 50–60 cm tall (including flower plumes), 45–60 cm wide when grown indoors. 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 fanal astilbe slow or fast growing?

Fanal astilbe is a moderate grower. Expect three to six years to reach mature indoor size, gaining a steady amount each growing season. Fanal astilbe 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 fanal astilbe 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 fanal astilbe smaller?

Divide the clump every year or two — splitting fanal astilbe 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 fanal astilbe grow bigger or faster?

Give it a wider pot and let the clump fill it — width is exactly how this plant gets bigger. Brighter light speeds up clump and offset production noticeably. Leave plantlets and offsets attached and feed through the growing season for the fastest spread.