Try to aim for a seed spacing of at least
11.9 inches (30.5 cm) and sow at a depth of around
0.25 inches (0.635 cm). Soil temperature should be kept higher than
18°C / 64°F to ensure good germination.
By our calculations, you should look at sowing Summertime about 49 days before your last frost date.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Summertime is a hardy plant.