Fertilizing
Thyme needs little feeding, and too much fertilizer makes it soft, leggy and less aromatic. Feed lightly from spring to late summer only.
- Use a balanced liquid fertilizer at half strength every 2–4 weeks, or a single dose of slow-release fertilizer in spring.
- Avoid fertilizers for acid-loving plants, as thyme prefers neutral to slightly alkaline soil.
- Do not feed in autumn and winter.
Placement
Keep your thyme bonsai outdoors all year in full sun, with at least 6 hours of direct sun a day and good air circulation. Indoors it quickly becomes weak and leggy and attracts pests, so it is not suited to life as a houseplant.
Thyme tolerates frost when its roots are dry, but a small pot freezes through quickly. Protect it below -5°C (23°F) and from long spells of cold rain in an unheated greenhouse, cold frame or under the eaves; a frost-free, unheated spot is enough, and it should not be brought into a warm room.
Pruning
Pinch or trim new shoots regularly during the growing season to keep the shape compact and encourage branching. Thyme grows quickly, so it needs more frequent trimming than most bonsai.
- Do heavier pruning in spring as growth starts, or right after flowering in early summer.
- Always cut back to a point that still has green leaves: thyme rarely buds back from old, bare wood.
- Stop trimming in early autumn so new growth can harden before winter.
The thin stems are best shaped by pruning; if you wire, use thin wire, apply it gently and check it often.
Repotting
Repot every one to two years in spring, as growth begins. Use a very free-draining, gritty mix, such as bonsai soil with pumice or perlite and coarse sand, and add a little garden lime, as thyme dislikes acidic soil.
Thyme has a fine, shallow root system, so remove only a little of the root mass. Use a pot with good drainage, and water carefully until new growth appears.
Watering
Water thoroughly, then let the soil dry slightly before watering again. The small pots used for thyme dry out quickly in summer heat, so check them daily, but never water on a fixed schedule.
Thyme comes from dry, stony hillsides: constantly wet soil rots its roots, and humid, still air encourages fungal disease. Do not mist it or use a humidity tray; water in the morning and keep the leaves dry. In winter, water only when the soil is dry.
Seasonal care
- Spring: Repot every one to two years into a fresh, gritty mix with a little lime, and do any heavier pruning as growth starts. Begin light feeding and pinch new shoots often to build dense branching.
- Summer: Keep the plant in full sun with good air circulation. It flowers in early summer; trim it back to green leaves once flowering has finished, and water more often in hot weather while letting the soil dry slightly between waterings.
- Autumn: Stop feeding and trimming, and water less as growth slows. Leave it outdoors in full sun; it does not need to come inside.
- Winter: Keep it outdoors in a bright, sheltered spot, and protect the pot below -5°C (23°F) and from long cold rain, for example in an unheated greenhouse or cold frame. Water only when the soil is dry, as cold, wet roots rot quickly.
Common problems
Thyme forgives a lot, but a few mistakes and pests come up regularly.
- Root rot from overwatering: Constantly wet soil, especially in cold weather, quickly rots the roots, causing grey, wilting shoots. Let the soil dry slightly between waterings and use a gritty mix with pumice or perlite and sand.
- Leggy, bare stems: Thyme rarely buds back from old, leafless wood, so cutting too hard leaves bare stubs. Pinch regularly and always cut back to a point that still has green leaves.
- Weak, pale growth: Too little light, too much fertilizer or keeping it indoors produce long, soft shoots and few flowers. Keep it outdoors in full sun and feed only lightly.
- Spider mites and aphids: These appear mostly on weak plants or indoors in warm, dry air, causing speckled or curled leaves. Rinse the plant, keep it outdoors and treat with insecticidal soap.
- Grey mould and mildew: Humid, stagnant air encourages fungal disease on the dense foliage. Improve air circulation, thin out crowded growth and avoid wetting the leaves.