When planting seeds in the spring, the major concern is not the air temperature, but rather the soil temperature. To germinate, seeds need not just enough moisture in the soil, but also enough warmth. The soil temperature will determine if a seed can germinate and how long that will take, and this will vary by plant type. Soil type also plays a role in determining planting dates. Well-drained, sandy soils heat up faster. Heavy clay soils heat up slower, especially if they are wet.