Pre-warm cold soil before sowing or transplanting warm-weather crops into the garden. Black or clear plastic sheeting is a simple and inexpensive way to warm the soil and get a jump on the growing season. You can also dig, loosen, and turn over the soil in planting beds so that solar heat reaches it.
Warm-weather seeds and seedlings require the soil to be 65° to 75°F for best germination and early growth. Peppers, melons, cucumbers, summer squashes, winter squashes, and pumpkins grow best in warm soil. Soil temperature more than air temperature will determine how early you can set out these crops. Air temperature for warm-season crops should not fall below 55°F at night, 60°F or slightly warmer is optimal.
Using Plastic Sheeting to Warm the Soil
If you use clear or black plastic to warm planting beds, pull it tight over the soil to eliminate as much air as possible. Air between solar heated plastic and chilly soil can insulate the soil and prevent it from warming. Plastic sheeting heated by solar energy can raise soil temperature by as much as 16°F on a cloudless day.
Soil stores solar heat; two to six weeks of solar heating planting beds can raise the temperature of late winter and early spring soil to the warmth required by seeds and transplants. Each day a few additional degrees of residual solar heat will be stored in the soil.
Measuring Soil Temperature
There are two ways to determine if the soil is sufficiently warm for planting; first, is to simply to take a handful of planting bed soil in your hand—if it feels chilly, it is; second, you can use a basic thermometer to take the soil’s temperature. Place the thermometer in a hole about 4 inches deep, mark the spot with a small stake or irrigation flag, cover the thermometer and then come back in about 15 minutes to take the reading. Soil temperature should be about 65°F at 4 inches below the soil surface for seeds and seedlings to get started.
Planting Through Plastic Sheeting
Once the soil has warmed, you can cut a slit in the plastic and sow seed in the soil or set transplants. Plastic can stay in place for the whole season in most regions—only in extremely hot regions might the soil grow too hot for plant growth. Plastic sheeting across planting beds through the season will protect vining crops such as melons that creep across the garden from soil-borne pests and diseases.
Other Ways to Pre-Warm the Soil
• Floating row covers, plastic tunnels, and cold frames can also be set in place over planting beds to pre-warm the soil. Warming the soil under these season extenders may take several days more than plastic sheeting stretched tight over the soil.
• Mounded or raised wide rows running east and west can be planted; solar heat will more quickly warm raised mounds and beds than planting beds flat in the ground. East and west beds will be evenly warmed from early until late in the day.
• V-shaped trenches for sowing seed or setting transplants can be dug in advance of planting. Mound the soil to the north side of the trench then allow the soil beneath to warm over the course of a couple of weeks before backfilling the trench once the soil has warmed.
• Hills or mounds can be made for crops planted in clusters. Make small mounds for corn and beans. Make larger, wider mounds for melons, sweet potatoes, cucumbers, and squashes. These crops will do best on mounds 6 to 8 inches high and 3 feet wide.
Also of interest see several articles at the Season Extension Category.