Try to aim for a seed spacing of at least
0.2 inches (0.5 cm) and sow at a depth of around
0.1 inches (0.25 cm).
By our calculations, you should look at sowing Double Flowered Chater's Tall about 56 days before your last frost date.
As Double Flowered Chater's Tall is very hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.