Aim to sow
0.23 inches (0.6 cm) deep and try to ensure a gap of at least
1.95 inches (5.0 cm). Soil temperature should be kept higher than
15°C / 59°F to ensure good germination.
By our calculations, you should look at sowing Snowdrift about 49 days before your last frost date.
As Snowdrift is half hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.