Seeds react to the soil and water that they are in contact with, not the air above the soil. Just like people, some seeds like it cold, some like it hot. Some like to hide in the dark, some prefer to bask in the light. All seeds have a preference. To be successful starting your vegetable or flowers from seed, knowing what temperature each variety likes best will help. As an example, beets will germinate in soil as cool as 45°, but it can take up to 30 days for germination. At a 70° soil temperature, beets will take an average of 4 days to germinate. Below is a guide to help you determine when is the best time to sow your seeds, both directly in the garden or inside to later transplant into the garden.
Soil temperature generally lags behind air temperature in spring, which may require you to wait longer to sow, consequently eliminating direct-sowing of some longer-season varieties into your garden. For example, some pumpkins take 120 days to mature, but also require soil temperatures of 70°–90°F for germination. If your soil doesn’t typically warm in time for a variety to mature, you can start those seeds indoors, essentially extending your growing season by giving the plants a head start. Even inside, some plants will need additional heating to maintain that ideal soil temperature to germinate. For those plants, use a waterproof seedling heat mat. Using a soil thermometer will also give you an advantage towards successful germination, as you can monitor and adjust the indoor environment to reach the optimal temperature. Germination temperature requirements are often higher than what the plant needs to grow, so once germinated, the seedlings should be removed from a heat mat or transplanted into cooler soils in the garden (after hardening off).
For those seeds that prefer cool soil but the garden soil is too warm (ex: a fall planting of lettuce), you can fool the seeds to germinate by placing them in the refrigerator for a few days prior to sowing.
Bean, Fava 40–75
Bok Choy 75–85
Brussel Sprouts 60–85
Swiss Chard 75–90