Mature size & growth rate

How big does Costmary (Tanacetum balsamita) get?

Also called Costmary, Bible Leaf, Mint Geranium.

More about costmary

About Costmary

Tanacetum balsamita · also called Costmary, Bible Leaf · herb

Costmary is a hardy, spreading perennial with long, balsam-and-mint-scented silver-green leaves once used to flavour ale and as a fragrant bookmark in bibles. It is tough, drought-tolerant, and undemanding, spreading by rhizomes in full sun and free-draining soil. Flowers are small and yellow; many gardeners grow it purely for the aromatic foliage.

Mature size: 60-100 cm tall in flower; spreading clumps 45-60 cm or wider.

Indoor size vs how big it gets in the wild

Costmary 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 60-100 cm tall in flower. In the ground with no restriction it is a completely different plant — spreading clumps 45-60 cm or wider. — 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

Costmary 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: very light feeder. a thin compost mulch or single balanced spring feed is ample. excess feeding makes growth floppy and dilutes the aromatic oils.

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

How to keep costmary smaller

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

Divide the clump every year or two — splitting costmary 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 costmary 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 costmary bigger or faster

If you want it to fill the space sooner, push the conditions rather than hoping — for costmary 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 costmary light requirements page covers exactly how bright a spot it needs to grow at its potential instead of stalling.

When costmary outgrows the room (or the pot)

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

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

Costmary size — frequently asked questions

How big does costmary get?

Costmary reaches 60-100 cm tall in flower when grown indoors, and far larger where it grows unrestricted (spreading clumps 45-60 cm or wider.). 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 costmary slow or fast growing?

Costmary is a fast grower. Expect two to four years from a young plant to a room-filling specimen in good light. Costmary 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 costmary 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 costmary smaller?

Divide the clump every year or two — splitting costmary 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 costmary 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.