Riding the Heat Wave

by Sarah West

green tomato

We don’t want to, but we know the heat wave drill. After the last three weeks, we definitely know—shut in our houses with the curtains drawn, the box fan on high, dreading dinnertime and its fire-breathing stove—that all we can do is ride it out. Some people claim to love the heat, as if it were imbued with restorative properties in which they rarely get the chance to braise. But at a certain point, we all wilt, drooping and dripping through our day, finding excuses to go to the grocery store and stroll the freezer aisle.

Plants aren’t so different. Some, like my spring kale, start to go limp once the Fahrenheit reaches a sunny 76, perking up again each evening when the sun gets low. Others wait for the heat, sitting stubbornly in 65-degree cold until they see the kale wilting and know it’s safe to strut their stuff.

But plants have their limit, too, and for many it culminates at 115-degrees, the cell destroying temperature for the majority of species. We are familiar with a plant’s most visible response to heat stress: wilting. Just like we produce sweat, plants release water vapor through tiny holes in their leaves in an attempt to regulate the temperature around them. If a leaf releases too much water, it looses turgidity (wilts) and, if conditions don’t improve, eventually turns crispy-brown. Between perfect conditions and death-by-heatwave lies a whole spectrum of largely invisible reactions to rising temperatures, some of which may be of interest to hungry gardeners.

Take tomatoes, a common garden plant that we associate with hot summer days. It’s true that tomatoes largely shut down below 50-degrees, refusing to grow again until the weather begins to more closely resemble their native jungle habitat. However, anyone with a few plants out back may be wondering why, despite an unusually warm June, their tomatoes seem to have entered a holding pattern. Market shoppers may have noticed that even though the summer feels like it’s already deep into August, tomato selection remains limited.

This is due to a few temperature-sensitive quirks of tomato physiology. While tomato foliage remains sturdy in the heat, their flowers and fruits only thrive in the rather narrow range between 70- and 85-degrees. Tomatoes self-pollinate, meaning their male and female parts are located in the same flower. Wind, insects, and your fingertips are all sufficient pollinators, as it takes no more than a little jiggle to help the pollen hop from stamen to nearby pistil. Tomato pollen is usually available for hopping between the hours of 10am and 4pm, though temperatures above 85-degrees render it gummy and immobile. If, after two days, the pollen is unable to fertilize the ovary, the flower, admitting defeat, falls to the ground—a phenomenon known as blossom drop.

But let’s say that our heat wave sleeps in one morning and the pollen makes it. Now the ovary begins to swell into a fruit. Tomatoes require a specific foliage-to-fruit ratio to achieve proper ripening—drawing not only on the plant’s energy reserves to infuse the fruit with sweetness, but on leaf-synthesized flavor compounds that will add complexity and fragrance to the mix. Because leaves are essential to tomato development, the plants must continue growing more foliage to meet the demands of newly set fruit.

This puts our heat-wave tomatoes in a conundrum. In the best of scenarios, the foliage-fruit balance requires a certain amount of precision. In the heat, when a plant is sometimes faced with life-threatening conditions, it abandons nuance for subsistence. Above 85-degrees, a tomato uses most of its energy just to keep from drying to a crisp. At 94-degrees, photosynthesis becomes sluggish at best, cutting off a plant’s ability to replenish much needed energy reservess. Staying cool costs the plant all the energy it would have otherwise invested in ripening fruit.

Add to this equation one final limitation. Most tomato varieties rely on two pigments for their signature ruby-ripeness: carotene and lycopene. When exposed to temperatures above 85-degrees, these pigments stop synthesizing. Almost-ripe tomatoes entering a heat wave will hang on the vine with that I’m-nearly-there yellow-orange color, inspiring unrequited thoughts of caprese salads and gazpacho until the heat breaks.

With sufficient water and any but the most pitiful soils, tomatoes will whistle right along until the thermostat hits 85, at which point they’ll start putting the breaks on fruit production. At 95, they enter survival mode, a temperature at which I, too, have little else on my mind.

While gardeners have few tools at their disposal with which to convince tomatoes, and most other flowering and fruiting plants, to mature in the midst of a heat wave, they can prepare their plants to better cope with a week (or three) of heat, and recover more quickly once it’s over. Irrigation is key; water deeply before a heat wave, and use drip irrigation when possible to ensure the water is going straight to the root zone, where your plants need it most. A thick bed of mulch helps to moderate soil temperature and protect it from drying sunshine and winds. Resist fertilizing before or during a heatwave, which will promote tender new growth that is completely unprepared to withstand the harsh conditions.

And finally, summer is not a good time to prune. A healthy head of foliage helps a plant provide its own shade as well as photosynthesizing replacements if and when cell death does occur on the front lines. If you feel the compulsion to remove something, take a load off your tomato plants and cull a few green fruits. Those left behind will ripen that much faster.