Sow
0.0 inches (0.0 cm) deep with a guideline distance of
5.85 inches (15.0 cm). Soil temperature should be kept higher than
16°C / 61°F to ensure good germination.
By our calculations, you should look at sowing stewartii about 70 days before your last frost date.
As stewartii is hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.