Why Tomatoes Won’t Ripen During a Heat Wave

It’s a hot summer across North America, and many of our gardens are feeling the effects of it. Tomatoes in particular have a complex relationship with temperature, and a great many related gardening myths. This summer we’ve gotten a few questions about tomatoes and hot weather, but the most common has been “why won’t my tomatoes ripen during a heat wave?” The answers interesting and provide a lot of insight into how tomatoes grown. So if your tomatoes are stuck green during this hot summer, we may have some answers. Let’s dig in and learn a bit about how heat waves affect tomato ripening!

Tomatoes and Temperature

Let’s start with some basics of how temperature affects tomato growth, and in particular how tomatoes ripen. Tomatoes, like all plants, have optimal temperature ranges for growth and setting fruit. Like most fruits, tomatoes have a fairly narrow optimal temperature range for ripening. Here’s how it works:

· Nighttime temperatures below 54F/12C slow ripening and overall plant growth. Fruit may be less likely to set.

· Overall temperatures between 60-75F/15-24C are optimal for tomato growth and tomato ripening.

· Once daytime temperatures reach about 80F/27C, pollination becomes more difficult and tomato plants tend to produce fewer fruit, and the fruit they do set tend to be smaller. Green tomatoes start to ripen more slowly as well.

· When things get hotter, real problems start to occur. Nighttime temps above 70F/21C combined with daytime temperatures above 90F/32C effectively ends tomato pollination and fruit production. The fruits produced at these temperatures are generally smaller and take longer to ripen or may not ripen at all.

We think of tomatoes as a heat-loving summer plant, and they are—from the point of view of Europe or the Northeastern US. In hotter parts of the world and in a changing climate, summer months may no longer be peak time for tomatoes without a bit of help.

Protecting Tomatoes from the Heat

So how can we help our tomatoes thrive in hot weather? The answer lies in controlling growing conditions as best we can by moderating the heat, light, and water our tomato plants receive. Different garden spaces will have different needs, but here are some general ideas that might work for many of us:

· Grow heat-tolerant cultivars: Some kinds of tomatoes are better at dealing with the heat than others, and while even heat-tolerant tomato cultivars aren’t invincible to hot weather they do make it easier to keep them alive. Heat-tolerant cultivars also tend to have fun heat-related names: “Heat Master”, “Solar Fire”, “Phoenix” and “Heat Wave II” being some examples. Some old favorites like “Arkansas Traveler” and “Green Zebra” can also handle a fair amount of heat as well.

· Water deeply and regularly: watering really does help keep plants and soil cool. Both are important for healthy tomato plants during hot weather, so we need to make sure that we’re doing the right think in watering our plants deeply, at soil level, once or twice a day.

· Protect the soil: A good layer of mulch, grass clippings, or or leaves can help protect soil from sun and heat, which makes life a lot easier on a struggling tomato plant in hot weather. Combined with regular watering, this can help keep soil temperatures low which in turn ensures that the plant can more readily absorb water and nutrients.

· Provide some shade: Container plants can be moved into the shade—and off hot hard surfaces like stone or concrete! If natural shade isn’t readily available, a tarp, shade cloth, or other form of cover during the hottest part of the day can help a lot. Make sure to mount any such cover high enough over the plants to ensure air flow—a blanketed tomato plant will overheat and die fairly quickly in the heat of summer!

None of these is foolproof, either alone or in combination. They do however give our tomatoes the best chance they have of surviving an unusual hot summer or a heat wave. For many gardeners, this is how we’ll keep your plants as healthy as possible until the temperature drops a bit and they can start producing fruit again.

Tomatoes and Hot Summer Gardens