How to grow thai basil
Overview
Thai basil (Ocimum basilicum var. thyrsiflora) is a compact, bushy herb prized for its strong anise-clove flavour and purple stems. It is grown for repeated summer harvests and requires warm weather to thrive, with regular trimming to prevent flowering.
Varieties
- Siam Queen — Compact, very purple stems, strong anise-clove flavour, and high leaf-to-stem ratio.
- Queenette — Dwarf variety ideal for pots, with tight branching and extra-large leaves.
Soil Preparation
Prefers slightly acidic to neutral pH (6.0–7.0) in rich, well-draining loam. Add compost or aged manure before planting. Improve drainage with coarse sand if soil is heavy clay.
Growing Guide
Sow seeds 5mm deep in well-draining soil after frost risk passes. Space plants 25–30cm apart in full sun. Pinch out the central tip at 15cm to encourage branching. Water at soil level to avoid leaf fungal issues. In cooler zones, use black pots or cloches to warm the root zone.
Sowing
Sow seeds 5mm deep, 30cm apart, in seed-raising mix. Germinates in 7–14 days at 20–25°C. Direct sow after frost or start in punnets for earlier crops. Thin to strongest seedling.
Care
Feed every 2–3 weeks with a balanced liquid fertiliser (e.g., seaweed or fish emulsion) during active growth. Pinch out flower spikes as they appear to keep plants leafy. Remove lower leaves as the plant matures to encourage bushy growth.
Companion Planting
Plant near chilli, eggplant, and tomatoes to share heat and deter pests. Avoid fennel and sage, which may compete or inhibit growth. Marigolds nearby can repel aphids.
Common Pests & Risks
Aphids cluster on new growth; blast off with water or use neem oil. Spider mites cause stippling in dry heat; increase humidity with misting. Slugs and snails may attack young seedlings; use iron-based pellets or beer traps.
Harvesting
Harvest leafy stems from 20cm tall, just before flower buds open, for peak flavour. Cut above a leaf pair to promote regrowth. Regular picking every 1–2 weeks keeps plants productive and delays bolting.
In the Kitchen
Use fresh leaves in stir-fries, curries, and salads; the flavour holds well in cooking. Store stems in a jar of water on the bench for 5–7 days. Freeze leaves whole in oil for later use.
Troubleshooting
Yellow lower leaves: overwatering or poor drainage; reduce water and check soil. Brown leaf edges: wind or sun scorch; move to sheltered spot or shade cloth. Stunted growth: cold soil below 15°C; wait for warmth or use heat mat. Leggy stems: not enough light; move to full sun. White powdery spots: powdery mildew in humid air; improve air circulation and avoid overhead watering.