Tender annual
How to grow basil
Ocimum basilicum
Basil is a tender annual that needs warmth above all. In the UK sow it indoors at 18 to 25°C in March or April and grow it on a sunny windowsill, in a greenhouse, or outside in a sheltered pot only once nights stay above 10°C, usually from early June. Water in the morning so the compost is not cold and wet overnight, and pinch out the shoot tips every week or two from 15 cm tall to keep it bushy and stop it flowering.
- Hardiness
- Tender, suffers below about 10°C and is killed by frost
- Position
- Full sun, warm, sheltered or indoors
- Soil
- Free-draining, fertile, warm
- Ideal pH
- 6.0 to 7.0
- Water
- Moderate, water in the morning; never wet and cold overnight
- Feeding
- Moderate feeder
Sowing and planting basil
WhenSow Mar to Apr indoors (late May to Jun for outdoors); harvest Jun to Sep
Starting off
Sow indoors Mar to Apr, two or three seeds per module, at 18 to 25°C; germination takes 7 to 14 days. Pot on singly and plant out only when nights stay above 10°C, usually late May or June. In a kitchen garden, give it the warmest, most sheltered bed you have, or a pot near the door. Direct sowing outdoors works late May to Jun under a cloche. Shoot tips root in a jar of water in two weeks.
Spacing and planting
Plant 20 to 30 cm apart each way, sowing 0.5 cm deep (barely covered). Fine in a 15 cm pot per plant, or three plants to a 30 cm pot on a sunny windowsill or patio.
Varieties worth growing
- Sweet Genovese
- The large-leaved Italian basil for pesto and tomatoes, and the most productive type for a greenhouse or windowsill.
- Greek basil
- A neat dome of tiny leaves about 20 cm tall. Tougher in cool weather than Genovese and the best for a pot outdoors.
- Siam Queen
- Thai basil with an aniseed flavour, purple stems and flowers. Holds its flavour in cooking where sweet basil fades.
- Dark Opal
- Deep purple leaves for salads and vinegar. Slower and less vigorous, so give it the warmest spot.
- Mrs Burns' Lemon
- The strongest lemon-scented basil, good with fish and in tea. Runs to flower quickly, so pinch it often.
Basil month by month
Timings are for central England. Allow two weeks later in the north and on cold, heavy soil.
- January
Nothing to do. There is too little light to grow basil well.
- February
Nothing to do. Buy seed.
- March
Sow indoors in modules at 18 to 25°C from the middle of the month. Give seedlings the brightest windowsill.
- April
Sow again. Pot seedlings on singly into 9 cm pots when they have two pairs of true leaves.
- May
Pot on into final pots. Plant in a greenhouse or polytunnel. Begin hardening off for outdoors late in the month.
- June
Plant or stand outside once nights are above 10°C. Last sowing. Start pinching out and picking.
- July
Pick weekly, always cutting just above a pair of leaves. Remove flower spikes. Water in the morning.
- August
Peak harvest. Make pesto to freeze. Root shoot tips in water for windowsill plants.
- September
Bring pots indoors before nights fall below 10°C. Cut and freeze the rest of the crop.
- October
Windowsill plants only; pick lightly as growth slows.
- November
Nothing to do. Discard tired plants.
- December
Nothing to do.
Feeding basil
A moderate feeder grown for leaf. In a pot it needs a balanced feed in the compost and one top-up; in good garden or greenhouse border soil it needs nothing.
A moderate balanced feed for leafy growth; keep it warm and don't overwater.
Feeding plan
Potting into the final pot
5 g per litre of compost, the low end of the label range, mixed through: about 8 g for a 15 cm pot holding 1.5 litres, or 60 g for a 30 cm pot holding 12 litres with three plants.
Six weeks later, pots only
2 g per litre of compost scattered on the surface and watered in: 3 g for the 15 cm pot, about 25 g for the 30 cm pot. Repeat once after another six weeks if leaves turn pale.
Before planting in a bed or border, poor soil only
100 g/m² raked in a week before planting. On soil that had compost this year, none.
Signs to watch
Pale, small leaves on a pot plant
Nitrogen running out in the compost
A light top-dress of the dry feed in the feeding plan, watered in
Purple-tinged, stalled leaves in a cool spell
Phosphorus locked up by cold, not a shortage
Warmth, not feed
Lush, floppy growth with little scent
Too much nitrogen
Stop feeding
Pests, diseases and problems
Seedlings collapse at soil level soon after germinating
Damping off, a group of soil fungi that thrive in cold, wet, crowded conditions
Sow thinly in fresh compost in clean pots, water from below with tap water, keep at 18°C or more and give good light and air. Resow rather than try to save a tray.
Leaves turn black or develop dark patches overnight
Cold. Basil is damaged below about 10°C, by cold draughts on a windowsill, and in the fridge
Move plants off the windowsill behind the curtain on cold nights, do not plant out before June, and store cut basil in a glass of water at room temperature.
Leaves yellow between the veins, with a grey-purple fuzz on the underside
Basil downy mildew, which spreads fast in humid, still air
There is no cure. Bin affected plants straight away. Prevent it by spacing plants, watering at the base in the morning and ventilating the greenhouse. Thai and lemon basils are less affected than Genovese types.
Fluffy grey mould on stems and leaves, starting where leaves touch
Grey mould, in a dense, overwatered pot
Remove affected parts, thin the plant by harvesting, and let the surface of the compost dry between waterings.
Ragged holes and missing seedlings outdoors, with slime trails
Slugs and snails, which seek basil out
Grow outdoor basil in pots raised off the ground, check under the rims at night, and plant out only sturdy plants 10 cm tall or more.
Sticky leaves, with clusters of small green insects or tiny white flies that rise when the plant is touched
Aphids or glasshouse whitefly on plants under cover
Rinse the plant under the tap, squash small colonies, and hang yellow sticky traps in the greenhouse. Harvest hard and let it regrow clean.
Harvest and rotation
Harvest and storage
Start picking 6 to 8 weeks after sowing once plants are 15 cm tall, pinching tips just above a pair of leaves so two new shoots form. A healthy plant gives 50 to 100 g a month through Jun to Sep. Freeze as pesto or chopped in oil in ice-cube trays; dried basil is weak and fridge storage below 10°C blackens the leaves.
Rotation and companions
An annual in the mint family: no rotation needed. Grow beside tomatoes and peppers, which want the same warmth, and whose canopy gives basil light shade in a heatwave. Keep it away from tall crops that cast shade and from mint, whose runners smother it.
Good to knowPinch out the tips and any flower spikes to keep it bushy; it hates cold and wet feet.
Growing basil in pots
Basil is better in a pot than in open ground in most British summers, because a pot can follow the warmth. Grow one plant in a 15 cm pot or three in a 30 cm pot of peat-free multipurpose compost with a little added grit, in the sunniest sheltered spot you have: a south-facing windowsill, a greenhouse bench or the foot of a warm wall. Water in the morning, at the base, when the top of the compost is dry, and never leave the pot standing in water overnight. A supermarket pot is dozens of seedlings sown together: split it into four or five clumps and pot each separately, and it will last for months.
Basil: common questions
Why does my supermarket basil keep dying?
It is overcrowded and usually overwatered. A supermarket pot holds dozens of seedlings in a small volume of compost, raised fast in warmth, so they exhaust it within a couple of weeks. Split the root ball into four or five clumps, pot each into fresh compost, keep them warm and bright, and water only when the surface is dry.
Should I let basil flower?
No, not if you want leaves. Once basil flowers it stops making new leaf and the existing leaves turn smaller and more bitter. Pinch out every flower spike as it appears, cutting back to the pair of leaves below. Leave a plant or two to flower at the end of summer if you want to feed bees.
Can basil grow outside in the UK?
Yes, from June to early September in a warm, sheltered, sunny spot, and it does best in a pot. It will not survive a frost and stops growing below about 10°C, so wait until nights are reliably mild. Greek basil copes best outdoors; in a cool, wet summer large-leaved types are more reliable in a greenhouse or on a windowsill.
Sources
Growing information on this page was checked against these references in November 2026. Feeding rates are Dr Forest label rates.