Thai Basil
Basil in the US

Basil is generally an easy plant to get going from seed, and you won't often find plug plants for it. For most folks, you'll be sowing in April or May, but this really varies by your USDA zone. Some areas can sow as early as January, others wait 'til July. Just check your local last frost date. Sow seeds about 1/4 inch deep in warm soil, around 65-70°F (18-21°C). Many varieties do best indoors first, or undercover in a greenhouse or cold frame, then get moved out once the weather warms up. You can sow a small batch every few weeks for a continuous supply right through to fall.

Once your seedlings have their first true leaves, it's time to prick them out carefully and pot them up into individual small pots. Keep them warm and watered. Before planting them outside, you'll need to harden them off for about a week, gradually exposing them to outdoor conditions. Basil is quite tender, so wait until all danger of the last frost has well and truly passed, and the soil is nice and warm. Plant them about 6-12 inches (15-30 cm) apart in fertile, well-draining soil, or they do brilliantly in containers if space is tight.