Basil is the herb most people try first and the one that most often disappoints: seedlings that sit still for weeks, plants that blacken on a cool night, a supermarket pot that collapses within a fortnight. Almost every one of those failures comes down to the same thing. Ocimum basilicum is a tropical plant that wants warmth and strong light at the same time, and in a British spring it rarely gets both until May. Get the timing and the temperature right and basil is one of the quickest, most productive herbs you can grow from seed. This guide covers when and how to sow basil, how to pinch it into a bushy plant, the main types worth growing, and what to do as the season ends.
- Sow Mid-March–May indoors at 20–25 °C · outdoors only from June
- Germination 5–10 days when warm · slow and patchy below 18 °C
- Position Full sun, shelter, free-draining compost · never below 10 °C at night
- Harvest From about 6 weeks after sowing · July–September outdoors
Why basil is different from other herbs
Most of the herbs in our beginner's guide to herbs from seed — parsley, chives, coriander, dill — are cool-season plants that shrug off a cold night. Basil is not. It comes from the tropics, it is killed outright by frost, and it is damaged well before that: below about 10 °C the leaves develop dark patches and growth simply stops. A plant chilled in May can sulk for weeks.
That is the single most useful thing to understand about growing basil from seed: warmth matters more than an early start. February sowings only work with a heated propagator and a grow light; mid-March to May is the reliable window, with June and early July sowings for a late-summer crop.
How to sow basil seeds
Basil seed is small, black and has one curious property: the moment it gets wet, the seed coat swells into a clear gel. That gel is how the seed holds on to moisture, and it is also why basil sown too thickly turns into a sticky, crowded mat that invites damping off. Sow thinly, cover lightly and water from below.
Sow 3–5 seeds per module of fine seed compost, cover with no more than 2–3 mm of compost or vermiculite and keep at 20–25 °C — a propagator or warm room, not a cold sill. Seedlings appear in five to ten days; move them straight to the brightest spot you have; basil in poor light grows the same pale, stretched stems described in our guide to leggy seedlings.
Growing on: potting up, planting out and pinching
Basil resents cold, wet roots far more than a small pot, so water in the morning, at the base. When the seedlings have their first pair of true leaves, pot each cluster on into a 9 cm pot without splitting it — a small clump of three or four plants grows into one bushy plant.
Basil only goes outside when nights stay above 10 °C, which in most of Britain means early to mid-June, after a week of hardening off. Choose the warmest, most sheltered spot you have, space plants 20–30 cm apart (15–20 cm for the small-leaved bush types), or keep them under glass or in pots on a sunny patio.
The step that makes the real difference is pinching. When a plant has three or four pairs of leaves and stands about 15 cm tall, snip off the top just above the second pair. Two new shoots grow from the leaf joints below the cut; pinch those in turn, and within a few weeks you have a branched, leafy bush instead of a single stem.
- Sow warm, sow thin Mid-March to May, 3–5 seeds per cell, 2–3 mm deep, at 20–25 °C. Water from below.
- Give maximum light Move seedlings to the brightest spot as soon as they emerge, or under a grow light for early sowings.
- Pot on as a clump At the first true leaves, move each cluster unsplit into a 9 cm pot. Water in the morning, at the base.
- Pinch at 15 cm Cut just above the second pair of leaves. Repeat on each new shoot to build a bushy plant.
- Plant out in June Only when nights stay above 10 °C, after hardening off. Full sun, shelter, 20–30 cm apart.
- Harvest from the top Always cut a stem tip above a leaf pair, never strip the big lower leaves. Pinch out flower buds as they appear.
Harvesting basil — and stopping it flowering
You can start picking about six weeks after sowing. Harvest the same way you pinch: take the top of a stem just above a pair of leaves, so every harvest makes the plant bushier.
Once basil flowers, the leaves turn smaller and tougher, so pinch out flower spikes as they appear (Thai basil and 'Blue Spice' flowers are edible and loved by bees — leave a few if you like). Fresh basil hates the fridge: below about 10 °C the leaves blacken within a day, so keep cut stems in a glass of water on the kitchen counter.
Types of basil to grow
All of these are forms of Ocimum basilicum, and they grow the same way; what changes is leaf size, flavour and how compact the plant stays. Our basil collection covers each group:
- Sweet or Genovese basil ('Italiano Classico', the large-leaved sweet basils) — the classic pesto basil, 40–60 cm tall.
- Lettuce-leaved basil ('Napolitano', 'Bolloso Napoletano', 'A Foglie di Lattuga') — huge, crinkled, milder leaves.
- Bush or Greek basil ('Bascuro', 'Bush Ball', 'Fine Green Bush', small-leaf types) — compact domes with tiny leaves, best for pots.
- Purple basil ('Dark Opal', 'Red Rubin', 'Purple Ruffles', 'Red Lizzy', 'Rosie') — slower and more cold-sensitive; give them the warmest spot.
- Flavoured basils — Thai basil and 'Siam Queen' (anise, holds up in stir-fries), lemon basil, cinnamon basil and 'Blue Spice'.
Large-leaved basil
Sweet, Genovese and lettuce-leaved types.
- Biggest harvests for pesto and cooking
- 40–60 cm, needs pinching to stay bushy
- Best in a greenhouse, large pot or warm bed
Small-leaved bush basil
Greek, 'Bush Ball' and fine-leaf types.
- Compact 20–30 cm domes, little pinching needed
- Slower to flower, very aromatic
- Ideal for windowsills and small pots
Basil in autumn and winter
By October, outdoor basil is finished: the first cold night blackens it and the first frost kills it. Pick everything before that happens. Basil dries poorly — it loses most of its flavour — so freeze it instead, chopped in oil in ice-cube trays or as pesto, as described in our guide to harvesting and drying herbs for winter.
Some of our basils are tagged for sowing all year round, but be honest about the light. From November to February a windowsill alone does not give basil enough, and it will sit still or rot. With a grow light on a 12–14 hour timer and a warm room, a pot of small-leaved bush basil will keep you in leaves through winter — see growing herbs indoors in winter. Without a lamp, the better winter option is basil as microgreens, cut after two or three weeks.
Common mistakes
- Sowing too early in the cold — February seed on a cool sill is overtaken by an April sowing in a warm propagator.
- Planting out in May — one night below 10 °C sets basil back for weeks. Wait for June, or keep it under glass.
- Evening watering — cold, wet roots overnight cause damping off in seedlings and root rot in older plants.
- Picking the lower leaves — always harvest from the tip, above a leaf pair, so the plant branches.
- Keeping a supermarket pot as it is — those pots hold dozens of crowded seedlings. Split one into four or five clumps and pot them up separately, and they will grow on for months.
Frequently asked questions
How long does basil take to germinate?
Five to ten days at 20–25 °C. In a cool room it can take three weeks or fail altogether. Our guide to why seeds don't germinate covers the other causes.
Can I sow basil directly outdoors?
Only from June, once the soil is properly warm. Starting indoors from mid-March gives a far longer harvest.
Why are my basil leaves turning black or yellow?
Black patches almost always mean cold — a night below 10 °C, a cold draught or the fridge. Yellowing lower leaves are usually overwatering. Yellowing with a grey-purple fuzz on the underside of the leaves is basil downy mildew: remove affected plants, water at the base and give more air.
Is basil a good companion for tomatoes?
Yes — they want the same warmth, sun and watering, so they share a greenhouse or sunny bed happily. See companion planting and our guide to growing tomatoes from seed. Nasturtiums make a good third partner as a trap crop for aphids, and in autumn you can save nasturtium seeds for next year's bed.
A few warm weeks in spring are all basil asks for, and the reward is a summer of fresh leaves for a fraction of the price of supermarket pots. Browse our basil and organic basil seeds, the wider culinary herbs and herb collection, or everything we tagged for sowing in March. You'll find more in our seed collections and growing guides.
