Aim to sow
0.5 inches (1.27 cm) deep and try to ensure a gap of at least
1.88 feet (58.0 cm). Soil temperature should be kept higher than
21°C / 70°F to ensure good germination.
By our calculations, you should look at sowing Petit Gris de Rennes about 14 days before your last frost date.
As Petit Gris de Rennes is hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.