Tomatoes split when there is a drastic change in the amount of water the plant receives. The surplus of water makes the inside of the tomato grow more quickly than the outside, causing the skin to split. This often occurs after a heavy rainstorm that follows a dry spell.
Tomatoes are more likely to split in the middle of summer, when droughts are common. Additionally, tomatoes at the peak of ripening are more likely to split than green tomatoes. The skin of a green tomato is thicker and less fragile than a ripening tomato, making it more resistant to splitting.
Although there are no ways to completely avoid splitting, precautions can be undertaken to minimize the chance that it occurs. First, the tomato's soil should remain moist at all times. If the plant has constant access to water, a large rainstorm is less likely to cause a split in the skin. Tomatoes should be watered thoroughly every two to three days during dry spells, sometimes more. Mulching also helps tomato plants and soil retain water. Specific hybrids designed to have tougher skins that are resistant to splitting are also available.
A ripe tomato that has split is still safe to eat, however, it does spoil faster than a healthy tomato, and must be eaten within a few days of being taken off the vine.