How to Grow Basil: Sowing, Care & Harvest

Basil is the herb everyone buys and almost everyone kills — usually within a fortnight of bringing a supermarket pot home. Yet grown with a little understanding of what it actually wants, basil is one of the most generous plants you can keep: a couple of plants on a warm windowsill will supply pesto, salads, and garnishes for months. Here is how to grow it properly, from seed or rescued supermarket pot, indoors or out.

What basil wants

Basil is a tropical plant that thinks it lives in the Mediterranean. That means three non-negotiables: warmth, light, and drainage. It sulks below 10°C, stops growing in deep shade, and rots in soggy compost. Get those three right and everything else is fine-tuning.

Sowing basil from seed

Sow indoors from early spring to midsummer in small pots of peat-free seed compost. Scatter a few seeds on the surface, cover with a thin layer of compost, water gently, and keep at around 20°C — a bright windowsill with a clear plastic bag over the pot works as well as a propagator. Seedlings usually appear within one to two weeks.

Once seedlings have a couple of pairs of true leaves, move each into its own 7.5 cm pot. Keep them warm and bright, and water sparingly — wet, cold compost is exactly what the fungal disease damping off loves, and it can flatten a whole tray of seedlings overnight.

Planting out — timing is everything

• Wait until night temperatures stay reliably above 10°C before moving basil outdoors.
• Harden plants off over a week or so: outside by day, inside at night.
• Space plants at least 10 cm apart; in containers, use pots at least 20 cm wide and deep.
• Pick the sunniest, most sheltered spot you have — against a warm wall is ideal.

Watering: mornings only

Water basil in the morning so the compost is evenly moist through the warm hours but the roots aren't sitting cold and wet overnight — basil hates wet feet after dark. Aim water at the base of the plant rather than over the leaves, and in hot spells check containers daily. Underwatered basil wilts theatrically but recovers fast; overwatered basil yellows from the bottom and doesn't.

Pinching: the secret to a bushy plant

Left alone, basil grows a single lanky stem, flowers, and calls it a career. To get the dense, leafy plant you actually want, pinch out the growing tip just above a pair of leaves once the plant has three or four pairs. Each pinch forces two new shoots from below the cut. Repeat every week or two — and yes, the pinchings go straight in the kitchen.

Tip: that leggy supermarket basil pot is really 20+ seedlings crammed together. Split the root ball gently into three or four clumps, pot each into its own container, and water well. They will look miserable for a few days, then take off — one €2 pot becomes a summer's supply.

Bolting: when basil tries to flower

Heat, drought stress, and overcrowding push basil to bolt — send up flower spikes — after which the leaves turn sparse and bitter. Pinch flower stems out the moment you see them. If a plant has already bolted, cut it back hard to a low pair of leaves; it will usually push out a fresh flush of tender growth.

Harvesting and keeping the flavour

Harvest little and often, always taking shoot tips rather than stripping single leaves from the stem. For a big pesto batch, cut whole stems back by half — a well-pinched plant recovers within two weeks. Basil loses most of its aroma when cooked, so add it at the end, off the heat. To preserve a glut, blitz leaves with olive oil and freeze in ice-cube trays; dried basil is a shadow of its fresh self.

Common problems

• Damping off: seedlings collapse at soil level — sow thinner, water less, improve airflow.
• Slugs and snails: they adore young basil outdoors; protect new plants from day one.
• Aphids: check shoot tips and leaf undersides; wash off with water or use insecticidal soap.
• Yellow lower leaves: almost always overwatering or cold roots, not hunger.
• Grey mould: fuzzy patches in humid, crowded conditions — space plants out and ventilate.

Warning: basil is extremely frost-tender. A single night near 0°C blackens the leaves beyond recovery. At the end of the season, either bring a pot onto a warm windowsill for winter or harvest everything the day before the first frost is forecast.

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