Sow at a depth of approx.
0.23 inches (0.6 cm) and aim for a distance of at least
0.98 inches (2.5 cm) between Cornflower plants.
By our calculations, you should look at sowing Black Gem about 7 days after your last frost date.
As Black Gem is half hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.