Soil Temperature

• Soil temperatures affect a tomato root system's ability to absorb nutrients, especially phosphorus and nitrogen. Tomato plants with either a phosphorus or nitrogen deficiency tend to be thin, dwarfed and weak-looking. Soil temperatures should average 55 to 60 degrees Fahrenheit before you transplant tomato seedlings. If soil temperatures are cooler than that, consider using row covers, dark plastic mulch, or a cold frame to help warm the soil around the roots until temperatures rise.
  
  

Air Temperature

• The minimum air temperature for tomato plants to grow is 60 degrees. Although air temperatures in the 60s do not inhibit tomato growth, tomato flowers do not like to set fruit if night temperatures fall below 55 degrees. As a result, a late spring cold snap can hurt your tomato crop before it has a chance to grow. Covering plants when night temperatures will drop below this level can keep plants slightly warmer.
  
  

Ideal Growth and Fruit Production

• The ideal temperatures for tomato growth are in between 70 and 85 degrees. While tomatoes can grow outside this range, this is the ideal environment to produce good-tasting tomatoes. Daytime temperatures that consistently exceed these temperatures can result in the interior flesh of the fruit developing white patches and losing flavor. Overnight temperatures consistently above 70 degrees prevent fruit from setting.
  
  

Warming Your Garden

• If you need to keep tomato plants warmer than current temperatures in your area, dark-colored plastic mulch can help to warm the soil while protecting transplanted seedlings from fungal infections that can be transmitted from the soil to plants in cooler, wet weather. During the day, propping a white-colored board near tomato plants can reflect both light and heat from the sun, helping to warm the area immediately around your tomato plants.