Aim to sow
0.25 inches (0.64 cm) deep and try to ensure a gap of at least
3.96 feet (1.22 metres).
By our calculations, you should look at sowing Tea Rose 'Sombreuil' about 35 days before your last frost date.
Tea Rose 'Sombreuil' is hardy, so ensure you wait until all danger of frost has passed in your area before considering planting outside.