Eager to plant those tomatoes? Take these steps to keep them warm from root to leaf

Are you in a race to have the first vine-ripened tomato on the block? Many of us can hardly wait to pick the first one of the season. We all know the flavor is superior to any grocery store purchase. The question is, will planting early translate to an earlier harvest?

Tomatoes are a warm-season crop. They grow best under the milder temperatures of summer. They are not frost tolerant, nor do the roots like to reside in cold, moist soils. The recommended time to plant tomatoes is mid-May or Mother’s Day weekend. By then, we are usually past the chance of a late-season frost, and the soils have warmed to support root growth.

Tomato transplants planted in cold soils will not grow. They rest there and wait for warmer days. Planting too early stresses a healthy plant and causes them to rebound slowly once the weather warms. The bottom line is that early planting without warming the soil and air usually will not result in an earlier harvest.

Warming the air to protect the transplant from frost is easy. The plants can be covered with a hot cap, milk jug, or another device to warm the air on a sunny day, which will retain heat overnight. These coverings should remain on the plants until mid-May, and the temperatures are consistently in the upper 70s.

Gardeners face the challenge of warming the soil mass to promote root development. Tomato roots will only grow once the soil temperatures reach around 65 to 70 degrees, usually occurring naturally in mid-May. Remember, it is the root development that drives the top growth. Not the other way around.

The easiest way to warm the soil quickly in the spring is to put down a layer of black plastic. The plastic traps the sun’s rays and raises the temperature to the optimum range for root growth. The ends of the plastic are then covered with soil to reduce the chance of it blowing away. Plastic sheeting can be purchased, or a trash bag can be cut.

Plastic should be put down a couple of weeks before the estimated planting date as it takes time to raise the soil temperature to 4 to 6 inches deep where the roots develop. Punch a hole in the plastic sheet when it is time to plant.

Most commercial tomato growers use plastic sheeting as it provides many advantages. The tomato growth shades the plastic and helps moderate the summer soil temperatures. The plastic can be covered with mulch during the summer if you are concerned about overheating. Plastic mulch has other advantages. It keeps weed growth in check and reduces foliar diseases.

A soaker hose or drip tubing can be placed under the plastic to water the tomatoes during the growing season. Cages to support the plants can be stuck through plastic.

If you want to have a vine-ripe tomato well before the Fourth of July, remember that it requires both warm soil and air temperatures to support the growth.

Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Need help? Contact the Johnson County Extension gardening hotline at 913-715-7050 or email garden.help@jocogov.org. The Kansas Garden Guide, your one-stop vegetable resource, has just been updated. Find your copy by going to ksre.k-state.edu and searching Kansas Garden Guide.

This story was originally published April 21, 2023 at 6:00 AM.