How to grow basil from seed

Basil is a warmth-lover from the subtropics, and cold is what holds it back in a Central European or British summer. Growth stalls below about 50°F, and cold, damp spells invite fungal problems. Everything about timing follows from that.

Sow indoors from late March to May. There is little to gain from sowing earlier: the seedlings have to wait on the windowsill until the nights are warm, and leggy plants in March light are weaker than compact ones sown in April.

Plant out once the nights stay above 50°F. In Germany, Austria and Switzerland that is usually after the Ice Saints in mid-May, often closer to the end of the month. In the UK, aim for early summer. Direct sowing outdoors works from mid-May to the end of June, but it is less reliable than raising plants inside.

Fill a tray or small pots with seed compost, water it, and scatter the seeds thinly on the surface. Basil needs light to germinate, so press the seeds in gently and don't cover them, or at most with a dusting of compost. A spray bottle keeps the surface moist without washing the seeds away.

Keep the tray bright and warm, ideally at 68–77°F. Seedlings then appear in about a week; in cooler rooms it can take two to three weeks. Once they have two pairs of leaves, prick them out into their own pots, setting them a little deeper than before.

Harden the plants off for a week or so before planting out: a few hours outside on mild days at first, then longer. Choose the warmest, sunniest and most sheltered spot you have, with humus-rich, free-draining soil. About 3 litres of compost per square metre is plenty. Skip fresh manure.

Space plants 8 in apart in rows 10 in apart. A 120 × 80 cm raised bed then takes three rows of six, 18 plants, which is far more than most households need. Three to six plants usually keep a kitchen in pesto and salads, and the rest of the bed can go to tomatoes and peppers, which like the same warm spot.

Keep the soil evenly moist but never soggy. Water in the morning and at the base rather than over the leaves: basil dislikes sitting wet overnight, and leaves that stay damp into the night are where downy mildew gets going.

Give plants room and keep them weed-free so the leaves dry off quickly after rain. In a cold, wet spell basil simply stops growing; it picks up again when the warmth returns. In a poor summer, a pot on a sunny balcony or a spot in the greenhouse often does better than the open bed, and a pot also keeps the slugs at bay.

A supermarket pot is not one plant but dozens of seedlings crammed together, which is why it collapses so quickly at home. Buy one before it grows too big, split the root ball into clumps of two or three plants and pot each clump on its own. They then grow on like home-raised plants.

Don't pluck single leaves from the bottom up; that weakens the plant. Cut whole shoot tips instead, always just above a pair of leaves. The plant then branches from the leaf joints below the cut and grows bushier with every harvest. Leave a good set of leaves on each stem.

Pinch out flower stems when you see them if you want leaves: once basil flowers, the leaves turn tougher and a little harsher. The flowers themselves are edible and loved by bees, so letting one or two plants bloom does no harm.

Basil tastes best fresh and raw, and loses much of its aroma when dried or cooked, so add it to hot dishes at the very end. To keep some for winter, make pesto with oil and salt, or freeze the leaves and crumble them straight from the freezer.

Large-leaved Genovese is the classic for pesto and tomatoes and copes well in the open bed. Small-leaved Greek basil forms a dense ball and is often more aromatic. Thai, lemon, cinnamon and anise basils, and purple types such as 'Opal', bring different scents and colours.

If downy mildew has hit your basil before, look for varieties bred for resistance, such as the 'Prospera' series or the Rutgers varieties marked DMR. Classic Genovese is the most susceptible; red, Thai and lemon basils tend to suffer less.

Downy mildew is the most serious basil disease. The first sign is yellowing leaves, starting low on the plant, that look like a nutrient deficiency; turn a leaf over and you'll find a grey-purple fuzz on the underside. It spreads in warm, humid weather and overnight leaf wetness, and can arrive on seed. Pull out affected plants promptly. The leaves are safe to eat, just unappealing.

Fusarium wilt makes plants wilt and the stems brown from below, often on one side first. The fungus survives in soil for many years, so don't grow basil in the same spot or the same potting compost after an outbreak, and start with clean seed.

Slugs and snails love young basil, and aphids and grey mould turn up too, especially in crowded, damp conditions.

Basil belongs to the mint family, Lamiaceae, with thyme, sage and oregano. As a light, short-lived crop it slots easily into a vegetable rotation; the main rule is to move it after any wilt or mildew problem.

Basil and tomatoes are the classic pairing, and it makes practical sense: both want warmth, sun and a rich but not over-manured soil. Claims that basil makes tomatoes taste better are tradition.

There is some evidence of a real effect on pests. In a glasshouse study, basil planted among tomatoes reduced whitefly about as well as French marigolds did. That was under glass, against one pest, so don't expect basil to protect an outdoor bed.