Tomatoes have one of the most delicate flowers imaginable. Tiny and frail, it must withstand the elements for long enough to pollinate. (Tomatoes do not require bees for pollination, although bumble bees do frequent the flowers) Wind, either cold or hot, can shrivel the flowers. Cold nights can stop them from pollinating. And heavy rain can rot them or knock them off. Most do not survive this process, which is why healthy tomato plants set many dozens of flowers.
The flowers are attached to the plant with stems just millimeters thick, yet somehow capable of delivering enough water and nutrients to produce a fruit that can swell in just a few weeks to a half a pound or more. Extremely hot weather can interfere with this process by overwhelming the plant’s ability to deliver enough water from the roots to the foliage. When stressed in this way, tomato plants abandon their flowers first, and then their fruit. When the flow of water is interrupted to the fruit, a calcium deficiency can occur that causes the bottom to rot and turn black.
Hybridized tomato varieties have been bred over the years to better withstand the effects of hot and cold weather. They set lots of flowers over a long period, which improves the chances that a large number of them will set fruit.
Heirloom tomatoes, on the other hand, are much less tolerant of extreme weather. Some varieties do well in the heat but can’t stand cool nights. Others are the opposite. Almost none do well in both.
For over ten years now in the Central Valley of California, high temperatures in the summer have been trending up. A large number of new record daily highs are being set every year. This year, most of the record-breaking days were in the spring, but the high temperature has been “above average” almost every day this summer. 2014 might end up setting a record for the number of “above average” temperature days.
Luckily for us, tomatoes don’t seem to mind “above average” heat. And “record high” temperatures of 85 or 90 degrees in April actually benefit tomato plants. The record highs of 114 in July like we had last summer, on the other hand, essentially wiped out most of our heirloom varieties for the rest of the summer.
It doesn’t take a lot of extreme weather to damage a crop. All over the world, short-term events like heatwaves, tornadoes and hard freezes have always caused local catastrophes. The question is whether climate change will make these so common that food production worldwide will suffer, and the answer increasingly appears to be “Yes”.
For the last several years that has certainly been the case on our farm. So we are happy — and keeping our fingers crossed — to have a normal summer, even if it still “above average”. Now if we could just have “above average” rain next winter, without it breaking any records, we’d be thrilled.