Look to ensure a distance
11.7 inches (30.0 cm) between Chicago Heirloom seeds when sowing to make sure your seedlings have enough space.
By our calculations, you should look at sowing Chicago Heirloom about 42 days before your last frost date.
As Chicago Heirloom is hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.