Fertilising guide
How to fertilise Costmary (Tanacetum balsamita)— schedule & NPK
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.
Growth habit: 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 fertiliser costmary actually wants — and why
Costmary is a lean, aromatic herb — the essential-oil flavour you grow it for is strongest in poor soil, so feeding it actively makes it worse.
Little or nothing. If anything, a very weak balanced feed or a thin compost top-dress — never a rich nitrogen feed, which dilutes the aromatic oils and produces soft, bland, floppy growth.
For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for costmary: match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.
How often to feed costmary, and which months
Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For costmary:
Very light feeder. A thin compost mulch or single balanced spring feed is ample. Excess feeding makes growth floppy and dilutes the aromatic oils. In practice: a spring compost top-dress at most, and otherwise leave costmary unfed — lean, sharp-draining soil is exactly what concentrates its flavour.
The dormant-season rule matters more than the exact interval: skip feeding entirely when costmary is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.
What strength to mix for costmary
As weak as it gets for costmary, or none at all. The flavour-versus-growth trade-off runs the opposite way to leafy crops: restraint is the technique.
Feeding always goes onto already-damp soil, never dry roots — water costmary first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the costmary watering schedule.
Signs you are over-feeding costmary
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for costmary:
- Lush, soft, fast growth with noticeably weaker scent and flavour.
- Floppy stems, sparse essential oils, and poor cold/wet hardiness.
- Salt crust in containers and scorched leaf tips from over-feeding.
Signs you are under-feeding costmary
- Rare — these herbs thrive on lean soil.
- Only on truly exhausted soil: pale, thin, very slow growth.
- A short-lived, weak plant in a long-spent container.
If the symptoms point at watering, light or roots rather than nutrition, the full costmary care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Over-feeding is so unlikely with costmary that flushing is rarely needed; if a container has had feed, a single plain-water flush and a switch to a leaner, grittier mix resets it.
Organic vs synthetic feeds for costmary
Organic options
A thin spring mulch of garden compost or leaf-mould is the most these want. UK: a little garden compost; US: a light Espoma Garden-tone top-dress at most. Lean and gritty beats fed and rich every time.
Synthetic / liquid feeds
Generally none for costmary. At absolute most, a very dilute balanced feed once or twice in a container; in the ground, nothing — synthetic feeds work directly against the flavour.
Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.
Fertilising costmary — frequently asked questions
What fertiliser does costmary need?
Little or nothing. If anything, a very weak balanced feed or a thin compost top-dress — never a rich nitrogen feed, which dilutes the aromatic oils and produces soft, bland, floppy growth. Costmary is a lean, aromatic herb — the essential-oil flavour you grow it for is strongest in poor soil, so feeding it actively makes it worse.
How often should I feed costmary?
Very light feeder. A thin compost mulch or single balanced spring feed is ample. Excess feeding makes growth floppy and dilutes the aromatic oils. Very light feeder. A thin compost mulch or single balanced spring feed is ample. Excess feeding makes growth floppy and dilutes the aromatic oils. In practice: a spring compost top-dress at most, and otherwise leave costmary unfed — lean, sharp-draining soil is exactly what concentrates its flavour.
What strength of feed for costmary?
As weak as it gets for costmary, or none at all. The flavour-versus-growth trade-off runs the opposite way to leafy crops: restraint is the technique.
What does over-feeding costmary look like?
Lush, soft, fast growth with noticeably weaker scent and flavour. Floppy stems, sparse essential oils, and poor cold/wet hardiness. Salt crust in containers and scorched leaf tips from over-feeding. Feeding costmary like a leafy vegetable is the defining mistake — rich nitrogen gives you a big, soft, fast plant whose leaves are watery and bland, with weak winter-rot resistance.
Should I flush the soil of costmary?
Over-feeding is so unlikely with costmary that flushing is rarely needed; if a container has had feed, a single plain-water flush and a switch to a leaner, grittier mix resets it.
Keep reading
- Costmary care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water costmary — the watering schedule
- The houseplant fertiliser schedule — feeding through the year
- NPK ratio explained — what the three numbers on the bottle mean
- How to fertilise basil
- How to fertilise herb garden
- How to fertilise mint
- All 5561 fertilising guides in the Growli library