When to Plant Tomatoes
You should plant tomatoes when the soil temperature reaches 60 degrees Fahrenheit.
More Info: The time of year you plant your tomatoes will depend where you are located, but all threat of frost must have passed and the soil temperature should warm to at least 60 degrees Fahrenheit.
Frost Zones Vary Across the United States
The first step to take before you plant is to determine in which plant hardiness zone you reside. For example, there is obviously a huge difference in air and soil temperatures between Maine and Florida at any given time of the year. You can consult the United States National Arboretum website for a national map, or county and state extension services for more detailed information.
Use Common Sense When Planting
Because tomatoes are a warm weather crop, it's important to note how each year's spring-into-summer season progresses. For example, if you've had an unusually cold spring, you may need to wait an extra week or two, regardless of what the plant hardiness zones show. Soil and air temperatures need to be about 55-60 degrees during the day before you consider planting. When in doubt, wait.
Be Creative to Optimize Planting
Though it may seem like a lot of trouble, tomato plants may be put in the ground a bit earlier by using one little trick. If you prepare the gardening beds early, you can cover the beds with black or red plastic to enhance the ground temperature. The heat from a late spring sun can be captured into the soil in this inventive way and allow for earlier planting.