Costmary (Bible Leaf) care

Light

Most houseplants will scorch where costmary thrives. Give it the windowsill you'd otherwise leave empty because everything else burned there. Full sun for the most aromatic foliage and best flowering, though it tolerates light shade. In too much shade plants grow lax and leaves are less fragrant and rarely bloom. A plant moved abruptly from low light to direct sun bleaches in 48 hours — always acclimatise over a week.

Watering

Aim for when the top 3-4 cm of soil is dry, about every 7-10 days; less once established for costmary, but treat that as a starting point rather than a rule. A south-facing summer windowsill will dry the pot twice as fast as a north-facing winter room. Lift the pot; if it feels noticeably lighter than it did wet, water it. Water to establish, then only in prolonged drought. Costmary is notably drought-tolerant and dislikes constantly wet soil, which rots the rhizomes.

Soil and pot

Costmary grows best in 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. A pot with a working drainage hole is non-negotiable for this species — even free-draining mix will turn soggy in a closed planter. If you love the look of a decorative pot without a hole, use it as a cachepot around an inner nursery pot you can lift out to water.

Humidity and temperature

Costmary sits happiest at around 30-50% humidity and 10-26°C (50-79°F). Prefers dry to moderate air and good airflow. Crowded, humid conditions can encourage rust and powdery mildew on the foliage. If you keep the room above 10 year-round and avoid placing the plant near a cold draught, a hot radiator, or an air-conditioning vent, you have already handled the two biggest indoor stressors.

Fertilising

Feed costmary sparingly. Very light feeder. A thin compost mulch or single balanced spring feed is ample. Excess feeding makes growth floppy and dilutes the aromatic oils. Skip fertiliser entirely on a stressed, recently-repotted, or actively wilting plant — fertiliser salts make damage worse, not better. Wait for a round of healthy new growth before resuming a feeding rhythm.

Common problems

Below are the issues we see most often on costmary in the Growli community. Each is annotated with the most common cause so you know where to start.

• 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.
• Rust and powdery mildew — Crowded, humid plantings develop orange rust pustules or white mildew. Thin and divide clumps, improve airflow, and remove affected leaves.
• Sparse or no flowering — In shade or cool conditions costmary often stays leafy and rarely blooms. Give it full sun for flowers, though foliage is the main crop anyway.
• Rhizome rot in wet ground — Heavy, waterlogged soil rots the spreading roots. Plant in free-draining soil and avoid overwatering established clumps.

Propagation

Almost always by division of the rhizomatous clump in spring or autumn, as plants rarely set viable seed in cool climates. Rooted offsets transplant easily. Propagation is the cheapest, most satisfying way to expand a collection — and it doubles as insurance against losing a mature plant to an accident. Take a backup cutting once the parent is established and healthy.

Toxicity to pets

Costmary is mildly toxic to pets. Tanacetum balsamita is not individually listed by the ASPCA, but the Tanacetum genus contains thujone (its relative common tansy, T. vulgare, is recognised as toxic to cats and dogs). Treat costmary as potentially harmful, keep it away from pets, and verify with a vet before any exposure. If you keep cats, dogs, or curious children in the house, weigh placement carefully — a high shelf or a hanging planter is enough for casual safety. For severe ingestion incidents, call your local vet and the ASPCA Animal Poison Control Center (in the US, 888-426-4435).

Pet-safety status is sourced from the ASPCA Toxic and Non-Toxic Plant List, which catalogues the most-asked-about plants for cats, dogs, and horses.

Costmary deep-dive guides

Every aspect of costmary care, each with its own calibrated guide:

• Costmary watering schedule
• Costmary light requirements
• Best soil mix for costmary
• Costmary fertilizing guide
• When to repot costmary
• How to propagate costmary
• Costmary growth rate & size
• Costmary cold hardiness
• Costmary temperature & humidity
• Is costmary toxic to cats & dogs?
• Is costmary toxic to cats?
• Is costmary toxic to dogs?

Featured in these plant shortlists

Costmary qualifies for 1 curated Growli shortlist — each one filtered objectively from our structured plant-care library, so the selection is consistent and checkable:

• Best fragrant houseplants — Indoor plants with scented flowers or aromatic foliage — greenery you can smell, selected from our care library.
• Browse all 29 plant shortlists — pet-safe, low-light, drought-tolerant and more

Related guides

Costmary is also known as Costmary, Bible Leaf, and Mint Geranium.

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• Herb garden care — light, water and common problems
• Mint care — light, water and common problems
• All 5561 plant care guides in the Growli library