When to plant basil?
Sow basil indoors at about 20 °C: from March in the north and centre, and earlier in the south only with warmth and plenty of light. It goes outside once nights stay above 10 °C and frost has passed, in a pot at least 20 cm wide and deep.
When, by region
| North | Centre | South | |
|---|---|---|---|
| Sow indoors | From March to July, at about 20 °C, seed on the surface, with plenty of light. Without heat, a windowsill is cold at night. | As in the north. | Before March only if you can hold about 20 °C and plenty of light. Without heat, from March. |
| Outdoors | Once frost has passed, nights stay above 10 °C and the soil for heat-loving crops is about 18 °C. That is often later than in the centre. | The same thresholds. Plants go out once nights clear them, usually sooner than in the north. | The same thresholds, met sooner. In a pot, in the hot months, the compost can dry out in a day. |
Thresholds from the RHS and University of Minnesota Extension. The month matters less than the nights and the soil.
The 20 °C, sowing from March to July, seed sown on the surface, and planting out in early summer once nights stay above 10 °C, are from the RHS. University of Minnesota Extension groups basil with the heat-loving crops, alongside beans and tomatoes: plant once the soil reaches about 18 °C (65 °F) and frost has passed. The seed-tray method, lights included, is in starting seeds indoors and the homemade seedbed. Tomato months, which use a different threshold, are in when to plant tomatoes.
How to sow
Sow thinly, on the surface of damp seed compost, and cover with a clear lid or a plastic bag. The RHS gives about 20 °C (68 °F) for germination. A few sowings, a few weeks apart, stretch the harvest from March to July. Once the seedlings have several leaves, move them into a 7.5 cm (3 in) pot, or set several in a wider pot, 7.5 cm apart.
Too much water brings damping off. Wet the compost, and don't leave the saucer full. A windowsill is cold at night and can overheat by day: University of Minnesota Extension says so for indoor sowing in general, and prefers lights on for 12–16 hours. The detail is in the guide to starting seeds indoors.
Outdoors, the RHS waits until early summer and nights that stay above 10 °C (50 °F). Results are less reliable than indoors, and only warm spots are worth it. Thin to 10–20 cm (4–8 in). Crowded plants flower sooner.
Pots and planting out
Harden them off for a couple of weeks, then plant in early summer, at the same depth as before. A pot at least 20 cm (8 in) wide and deep, multi-purpose compost, plants at least 10 cm (4 in) apart. Full sun, out of the wind. On a balcony the compost dries out fast: see the container garden.
Where nights stay cold, the RHS keeps basil in a greenhouse, in pots or a growing bag. That is the better path in a cool climate, so it matters more in the north than in the south. Frost stops it even after a mild day: hardening off does not make it hardy.
Flowers, water, tomatoes
Pinch the tips to keep it bushy, and take off flower stems as soon as they show: the RHS says so, to keep the leaves coming. A flower left on ends the harvest from that stem. That is why the sowings repeat.
Once established, water in dry spells so the soil stays even, without drowning the pot. Water at the base, in the morning, is in the watering guide.
Next to tomatoes it is the pairing people mention most. What is documented and what is tradition is in companion planting: French marigold has a glasshouse trial, basil does not. Tomato months stay those in when to plant them.
In the app
Every plant in the catalogue has its own sowing calendar, and weather alerts for your area help you judge whether to sow now or hold off.
Frequently asked questions
Can I sow basil in January?
Only if you can hold about 20 °C and plenty of light, and it makes most sense in the south. Without heat the seedlings grow thin or stall. In the north and centre, start in March.
Is a windowsill enough?
Barely. It is cold at night and can overheat by day, and winter daylight is weak. University of Minnesota Extension prefers lights on for 12–16 hours, a few centimetres above the seedlings.
How big should the pot be?
At least 20 cm wide and deep, with plants at least 10 cm apart. Outdoors, thinned, they sit 10–20 cm apart.
Should I remove the flowers?
Yes, as soon as they show, and pinch the tips. The plant makes more leaves. Repeat sowings, from March to July, fill the gaps.
Does basil protect tomatoes?
It is a traditional pairing. The glasshouse trial cited for French marigold does not cover basil. It does no harm, but it is not a defence to count on.