The second sign for knowing when do you plant tomatoes is the temperature of the soil that you plant to grow your tomatoes in. Ideally, the soil temperature for the best time for planting tomatoes is 60 F. (16 C.).A quick and easy way to tell if the soil is warm enough for planting tomato plants is to thrust a finger in the soil.deally, the soil temperature for the best time for planting tomatoes is 60 F. (16 C.). A quick and easy way to tell if the soil is warm enough for planting tomato plants is to thrust a finger in the soil.