Repotting guide
When & how to repot Costmary (Tanacetum balsamita)
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.
Watch for — Invasive spreading: Rhizomes spread vigorously and the clump can colonise beds. Plant where it can roam, or contain it in a buried pot or root barrier and divide regularly.
How to tell costmary needs repotting
Repotting on a calendar is less reliable than reading the plant. For costmary, watch for these signs:
- Roots circling the bottom of the module or pot, or poking out of the drainage holes.
- The seedling dries out within a day and growth has visibly stalled.
- Roots are white and matted in a tight spiral when you tip the plant out.
- It has outgrown its current container for the stage of the season — pot costmary on before it becomes hard root-bound.
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 costmary
Pot on seedlings as they grow; not a perennial repot. Costmaryis grown for one season, so the question is really “how often to pot on” — keep moving it up before the roots circle. Vigorous, rhizomatous clump-forming perennial with long, oval, finely toothed silver-green basal leaves and loose clusters of small yellow button or daisy-like flowers in late summer..
What size pot to step costmary up to
Pot costmary on gradually — a seedling jumped straight into a huge pot sits in cold, wet, airless soil and stalls. Step up one or two sizes at a time as the roots fill each container, finishing in a large final pot or the ground. The aim is roots that never circle and never check.
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 costmary
Pot costmary on through the active growing season, whenever roots fill the current container — there is no single date, just "before it becomes root-bound". Avoid potting on during a cold snap.
Step-by-step: repotting costmary
- Pot on before it is root-bound. Check costmary regularly; move it up as soon as roots reach the edge of the cell or pot, not after they have circled.
- Step up one or two sizes. Choose the next container up — not a giant one. Cold, wet, unused soil around a small root system stalls seedlings.
- Knock it out gently. Support the stem, tip the pot, and ease the rootball out without breaking it. A little teasing of circled roots at the base is fine.
- Pot into rich mix. Set it into fresh average, free-draining loam at the same depth (tomatoes are the exception — they can go deeper to root along the stem).
- Water in and grow on. Water well, keep it in good light, and resume feeding once it is established and growing again.
Aftercare
Water costmary in well and keep it in bright light; a freshly potted-on seedling can wilt for a day while roots settle, so do not overcompensate by drowning it. Do not fertilise for about 1 week — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.
The right soil mix for costmary
Costmary wants average, free-draining loam. Adaptable but needs good drainage; pH 6.0-7.5. Tolerates poor soils. Rich, wet ground gives soft growth and rhizome rot, so avoid heavy waterlogged sites. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.
Repotting costmary — frequently asked questions
How often should you repot costmary?
Pot on seedlings as they grow; not a perennial repot for costmary. Costmary is a seasonal crop, so you pot it on as a growing plant rather than repotting a perennial. Step seedlings up gradually into average, free-draining loam so the roots never circle the cell, ending in a large final container. A root-bound transplant stalls and never fully recovers.
What size pot does costmary need?
Pot costmary on gradually — a seedling jumped straight into a huge pot sits in cold, wet, airless soil and stalls. Step up one or two sizes at a time as the roots fill each container, finishing in a large final pot or the ground. The aim is roots that never circle and never check. Use our pot size calculator to size it from the plant's current pot and root spread.
When is the best time of year to repot costmary?
Pot costmary on through the active growing season, whenever roots fill the current container — there is no single date, just "before it becomes root-bound". Avoid potting on during a cold snap.
Can you put costmary straight into a much bigger pot?
No. Even a fast-growing costmary 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 costmary after repotting?
Not immediately. Wait about 1 week after repotting costmary. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.
Related guides
- Costmary care — light, water, soil and common problems
- How often to water costmary — the watering brief
- How to repot a plant — the complete step-by-step method
- Root-bound plant — how to spot and fix it
- Pot size calculator — size the next pot correctly
- When & how to repot basil
- When & how to repot herb garden
- When & how to repot mint
- All 5561 repotting guides in the Growli library