When to Plant Tomatoes

Q. When can I put tomato plants outside in the garden?

We all want these, right?Putting tomato plants into the garden at the right time is critical in our hot, desert climate. They don't set fruit when temperatures get above 95° F. They grow poorly at temperatures in the 60s. That "sweet spot" for tomato fruit set can disappear quickly in the hot desert climates, particularly after prolonged cool wet springs.

A. This has been a crazy Spring! Tomato plants grow best when soils are warm and air temperatures are above 60° F and below 90°. Traditionally, our last freeze is after mid-March. Most gardeners like to get their tomato plants out earlier than this, any time after mid-February, if its warm enough.

Tomato stem rot occurs on tomato plants frequently when soil temperatures are cold and tomatoes are put outside too early.

            Start watching weather projections, up to a few weeks ahead, around mid-February. As soon as weather projections predict warm weather for a 2 to 3 week period after mid-February, put transplants outside and help them to adjust from the protected greenhouse to the harsher garden environment.

             Put transplants in a location protected from strong wind and intense afternoon sunlight. They should get eight hours of sun every day to remain vigorous. Never plant them in the same spot in the garden year after year. Plant them in a different part of the garden each year to reduce disease problems.

This tomato plant stem was buried in the garden soil. New roots were supposed to grow from the buried stem. Instead, the tomatoes them died along with the plant. The transplant was put in the garden when soils were still cold and the soil did not drain water easily. Instead, the tomato transplant developed stem rot.

            Prepare the garden soil for planting while transplants acclimate. Remember, tomatoes like warm soils so garden preparation should focus on “fluffing” the soil so that it warms more quickly. Add compost to it and double dig, spade or till the soil to open it and let warm air and sunlight raise its temperature.

Hot caps and wall-o-waters are sometimes used to protect tender new transplants when air temperatures are still cold and potentially freezing. Transplants are protected from freezing temperatures but unfortunately the soil temperatures are still cold. These cold soil temperatures don't permit transplants to grow as fast as they might.

            Heavy, wet soils warm slowly when temperatures are rising. Alternatively, cover these spots with clear plastic, if you have to, to trap heat and warm it more quickly. Pin the edges of the clear plastic to the soil to keep the heat trapped under it.

A potentially better technique is to warm the soil first with clear plastic and plant tomato seed directly into trenches in the warmed soil. This can be done in the middle of winter. This was demonstrated at the University Orchard in North Las Vegas.

            When weather projections are positive, plant tomatoes transplants into warm soil along with a pre-plant fertilizer to improve rooting. If a generous application of compost was used during garden preparation, then add only a phosphorus fertilizer to the soil surrounding the transplant. This phosphorus fertilizer could be a mineral type such as triple super phosphate or a natural phosphorus source like bone meal.