Sowing basil: warmth, light and a gentle move outdoors

Start basil in a warm, bright place rather than sowing outdoors at the first hint of spring. The French SNHF gives March–April at 18–20°C, while the RHS recommends about 20°C for germination. These are starting conditions, not a universal planting date. Basil is tender: wait until frost has passed and nights are regularly above 10°C before leaving plants outdoors.

Warmth matters, but so does light

Fill a small pot or tray with fine, lightly moist seed compost. For indoor sowing, the RHS advises scattering seed on the surface without covering it, then keeping the container warm and bright. Keep compost moist without waterlogging. If you cover the tray to slow drying, ventilate it and remove the cover as soon as seedlings emerge.

Thin and pot on with care

Once seedlings have several leaves, pot each one into a 7.5cm pot; for a larger container, the RHS gives 7.5cm spacing. Hold each plant by a leaf rather than its stem and keep the rootball intact. Place the pots in bright light and water without saturating the compost. Stretched stems often indicate insufficient light; cold, waterlogged compost can also slow recovery.

Harden plants off before planting out

A plant raised indoors is not ready for a cold night outside. Put pots in a sheltered spot for a few hours by day, then gradually lengthen their time outdoors over several days. Bring them in if temperatures fall, wind is drying the leaves or a cold spell is forecast. This gradual change helps plants adjust to brighter light, moving air and lower temperatures.

The SNHF says basil does not tolerate cold and advises temperatures above 10°C. The RHS likewise recommends moving plants out when nights are regularly above that level. Read the threshold alongside local conditions: an exposed balcony or upland garden may be colder than a nearby weather station. After hardening off, plant into warmed soil in a bright, sheltered position.

Sow outside only when conditions are settled

Direct sowing avoids transplanting, but the RHS limits it to warm locations in early summer: sow seed on the surface without covering it, then thin seedlings to 10–20cm apart. The SNHF says to wait until frost risk has passed. In either case, wait for nights to remain above 10°C; in France the window shifts with region, altitude and cover.

If a first attempt suffers in cold weather, sow again when conditions settle rather than assuming the seed was poor. Record the date and position in your garden plan or journal: that observation adds detail to general guidance.

After planting: keep growth steady

Water at the base when the surface starts to dry, and do not leave pots standing in water. Bright light and steady growth help plants stay compact. Pinch growing tips if you want a bushier plant, and remove flower spikes if your aim is to keep picking leaves. When harvesting, the SNHF advises leaving two pairs of leaves low on the stem so the plant can bush out; the RHS recommends taking shoot tips regularly.

Basil may sit beside tomatoes in a garden, but that does not prove it protects them from disease; see our guide to what to plant next to tomatoes. The vegetable sowing and planting calendar gives seasonal context, not a personalised date for this variety or plot. Marmanda’s plan and journal can help organise crops and keep these observations; check local conditions in the garden before acting.

References: SNHF, Basil (in French); RHS, Basil. Temperature thresholds are growing guides, not guarantees for every microclimate.