Basil

Basil is a warm weather annual from tropical Asia, and it behaves like one. It sulks below about 10 degrees, runs to flower in a heatwave, rots in a cold wet spring, and gives its best leaves to whoever pinches it most often. Everything that goes wrong with it is a version of one of those four things. Grown well it is one of the most productive things you can put on a windowsill, and a single plant kept pinched will crop for four or five months.

Sowing

Sow on the surface of moist seed compost, cover with the barest scatter of vermiculite or compost, and keep it warm and bright. Seedlings appear in about a week at 21 degrees, and much more slowly below that. Below 10 degrees the leaves blacken and the plant stalls, which is why sowing early outdoors gains nothing. Indoors you can sow from early spring; outdoors, wait until nights are reliably above 10 degrees, which means March to June in the northern hemisphere and September to January in the southern.

Sow a small batch every three to four weeks rather than one large one. Basil does not stand still: each plant has a productive life of a few months and then flowers, so a succession keeps leaves coming all season.

Thin the seedlings hard. Three or four plants in a fifteen centimetre pot will each grow into something useful; twenty crammed together, which is what a supermarket pot contains, will exhaust the compost, compete for light and collapse within a fortnight.

Light

Six hours of sun is the minimum, and more is better. Indoors that means the sunniest windowsill in the house, and a plant on a shaded sill grows pale, leggy and thin flavoured. Turn the pot every few days so it does not lean.

In the hottest part of summer, plants in small pots outdoors appreciate shade from the fiercest afternoon sun, not because basil dislikes heat but because a small pot in full sun dries out faster than you can keep up with, and drying out is what triggers bolting.

Water

Evenly moist compost is the whole recipe. Water in the morning, at the base of the plant, so the leaves are dry by evening; wet leaves overnight is how the fungal problems start. In warm weather a pot needs water every other day, and in cooler spells every three or four.

Let the surface dry slightly between waterings but never let the pot go bone dry. A basil plant that wilts hard once will usually recover, but it takes the drought as a signal that its season is ending and pushes out flower buds soon afterwards.

Feeding and repotting

Feed fortnightly with a balanced liquid feed at half strength once the plants are established and you are harvesting regularly. A plant giving you leaves every week is being asked for a lot.

Pot on from a seed tray into individual pots as soon as the seedlings have two pairs of true leaves, and again into their final fifteen to twenty centimetre pot when the roots reach the sides. Basil dislikes root disturbance less than most herbs, but it still sulks for a few days after each move.

Harvesting

Pinch out the growing tips from the top, cutting just above a pair of leaves, rather than stripping the lower leaves off the stem. The plant responds by branching from that pair, so every harvest doubles the number of growing points and you end up with far more basil than if you had picked from the bottom. Start when the plant has two or three pairs of true leaves and keep doing it weekly.

Remove flower buds the day you notice them. Once a basil plant flowers, leaf production slows sharply and the flavour turns bitter. If a plant has got away from you, cut it back by a third and it will usually come again.

Ailments

Leggy pale growth is light. Wilting with damp compost is root rot, and wilting with dry compost is simple thirst. Black patches on the stem near the compost in a cool wet spring are damping off, which cannot be cured, only avoided by sowing later into warmer conditions. Early flowering is the one every grower meets, and the ledger below has the fix.