Making Soils Warmer in the Spring

Many people contacted me whether they could plant seed or put transplants into the ground because of this unusually cold weather. My answer is the same to them as it is here. It depends. Soil preparation and soil temperature are very important for successful seed germination, preventing plant diseases and encouraging root growth of transplants.

Fluffing Up the Soil

Soil preparation and increased soil temperature are interrelated, which we tend to forget. “Fluffing up” the soil with amendments like compost allows warm air to enter the soil. These soils also cool off rapidly at night but can be planted earlier than soils not “fluffed up”. Amended soils warm up quickly, particularly if they are in full sun. This means amendments like compost are rototilled into the soil or “double dug” into these future growing areas.

Preparation of the soil before planting, the location of the garden spot in relation to the sun and the type of seed or transplant affects what and how soon we can plant after cold temperatures pass.

Warm Season Plants

 Warm season plants come from the tropics and include tomatoes, peppers, potatoes, squash and melons, annual flowers like impatiens, marigolds, petunias, geraniums, salvia, celosia and zinnias. These require warm soil temperatures, 60-70 F, at the time of planting.

Cool Season Plants

Cool season plants like peas, beans spinach, radishes, beets, pansies, snapdragons, sweet alyssum, California poppy, godetia, larkspur, violas, and bachelor's buttons can handle cooler air temperatures and soils, 40-50 F, and so can be planted earlier than warm season types.

Variability Within the Groups

Within the cool and warm season groups different plants, and even varieties within these plant groups, vary by quite a bit. If you aren’t sure, check seed packets for the best soil temperature for germination or online sources for the best root growth of transplants. Use a soil thermometer, with its tip pushed into the ground about 1 inch deep, to measure soil temperatures and to gauge when to plant seed and transplants.

Soil Thermometer I use