I just read somewhere it should be 60 degrees - I have never measured mine when I plant.

Our soil temp doesn't reach 60 degrees until around August so it is not an essential rule. I plant mine out once the frosts are done. When it is time to plant tomatoes it is time to put your dahlias in the ground, what ever date that is in your area. I pre-start in pots to get them up to a good size first. Doing this they will start to bloom, many in July., nearly all in August and give me 3-4 months of glorious color.