What effect does rain have on tomatoes? That depends on how much rain you get. Tomatoes are 95 percent water, with most varieties soaking up summer rains to become pump and juicy on the vine. Water, along with sunshine and nutrient-rich soil, is needed at every stage of the growing process. Otherwise, tomatoes won't grow, blossom and produce fruit. The manner in which water is delivered, either via rain or via a controlled system, can affect how tomatoes grow in a positive way, or can also have devastating results.
Inconsistent rain and moisture levels can cause cracking, splitting and blossom end rot. Too much rain can cause blight.
Cracking and Splitting
A tomato's fragile skin can easily start splitting and cracking due to inconsistencies in soil moisture. Once a tomato starts to ripen, it forms a protective skin that helps it during harvest, but if rain is heavy during ripening time and the tomato receives too much water, that skin will crack and split.
Laying mulch and setting up a regular watering schedule can help prevent splitting and cracking by keeping the soil moisture consistent despite irregular rain. You can also pick the tomatoes before they fully ripen to prevent cracking. They'll continue to ripen after being picked.
You can also prevent cracking by choosing varieties that are crack resistant. Plum tomatoes and varieties that are small and globular tend to be more crack resistant. Read descriptions of tomato varieties when choosing your plants to find options that say they're crack-resistant.
Blossom End Rot
A tomato plant can start to rot at its blossoms, causing the blossom end of the fruit to turn brown. If rain has been inconsistent, your tomato plants aren't getting enough nutrition, especially calcium, because the soil is dry and not able to deliver the proper nutrients to the plant.
To remedy this, be sure to lay mulch on your garden to retain moisture and don't rely only on rain to water your garden. Water your garden when you don't get enough rain to keep the soil consistently moist. If splitting or cracking has occurred, remove the bad tomatoes and apply a calcium chloride spray to the new growth.
Rain on Tomato Plants: Insects
A strong, heavy rain can be beneficial when it comes to controlling damaging insects on your tomato plants. The rain can help keep your tomato plants free from spider mites, which are barely visible to the human eye, and aphids, which are tiny pear-shaped insects that like to hang out on leaves.
The pressure from a heavy rain can keep these insects at bay by washing them away. If there hasn't been a hard rain in a long time, soapy water or a chemical spray may be needed to control the insects.
Fungal Blight Damage
Fungal blight can devastate a tomato plant, with the dark spots first appearing on lower leaves before they turn yellow and fall off the plant. Blight works its way up the plant if it's not controlled. Fungal blight rarely occurs during dry times, but once the rainy, humid season hits, fungal blight can quickly destroy a tomato plant. While it doesn't affect the fruit, blight often damages the plant to the point that it affects how much it produces. To control blight, fungicide sprays should be applied prior to a rain event.
Rachael Anne Ryals
Born and raised in Florida, Rachael Anne Ryals has been writing since 2007. Her specialties include environmental and health-related issues. Ryals' work has appeared in "The North Florida Herald" and in various other online publications. She graduated from the University of Florida with a Bachelors of Science in journalism. Ryals also studied nutrition at the University of Florida.