How Heat Affects Plants

When it gets hot for a day or two, plants can wilt and require some extra attention. That’s pretty easy to understand. After all, I spend less time outside in the heat of the day because I wilt. I also drink more water. After more than a week of temperatures hovering near or above 100° F, our plants are showing other signs of heat stress that we less often consider.

jimson weed datura new mexico
Jimson weed, or datura, loves heat and drought. Then again, it does have the word “weed” in its name.

You would think that living in the Southwest and having mostly xeric plants means plants can tough it out dor a few hot, dry (windy!) days. That’s partially true. But these temperatures are way above normal, at least for an entire day/week and more. Normally, July 1 marks the beginning of monsoons, where clouds build and drop at least some rain in the afternoon, giving the plants and me a break. Not this year. And it’s causing problems for nearly every living thing in our garden. Here are some reasons why:

Heat can reduce bee populations

We noticed about midway through our current heat wave that we seem to have fewer bees in our garden. Of course, I thought I was imagining that. Nope. Bees are especially sensitive to temperature and its regulation. Temperature particularly affects young bees, which are important for keeping colonies going. They tend to keep their hives at 95° or lower, and stress when the temperatures exceed 98°. Bees also need water, and we’ve had two weeks with no measurable rain and consistently high temperatures, even for our evening lows. Here’s a great explanation from Tufts University on how honey bees keep cool (and warm). Despite the fact that we have plenty of flowering plants for bees beginning in spring, the population is lower this summer. I was pleased to see a few survivors on our lavender this weekend, but this is a brand new worry!

bee buzzing on lavender stalk
Lavender loves dry heat, and bees love lavender, so there’s that…

Lack of flowering or fruiting

Those bees! They also help pollinate our ornamental plants and especially our vegetables. So, there’s one concern in heat. But I recently was reminded how heat also affects flowering and fruiting. It turns out that pollen loses its effectiveness in high heat. Even though we’ve hit the 100° mark before, I can’t recall it lasting for days and weeks since we’ve been growing tomatoes and other edibles in New Mexico. What’s more, drought lessens the chance that pollen can stick to a female flower.

tomatoes green on plant
These tomatoes got started before the heat wave, and I think the plant is doing all it can to keep them nourished.
Another plant is failing to produce. I'm hoping there's still a viable tomato behind each dried up bloom.
Another plant is failing to produce. I’m hoping there’s still a viable tomato behind each dried up bloom.

Wilt

This is the heat issue we best recognize – leaves wilting on a hot day. Plants “sweat” to cool themselves like we do. With transpiration, water travels up from the roots and to the leaves, where it evaporates through tiny pores under the leaves. So on a hot, dry day, the soil, roots, and entire plant dry out more quickly. Add some wind just for fun, and a plant can wilt or stress. Plants can take more water, but only up to a certain point. Some plants have plenty of moisture and experience what’s called “incipient wilt” during peak heat each day. If they recover by morning, the plant probably is getting plenty of water.

yellow squash leaves wilting in heat
The big leaves on our yellow squash tend to wilt each day, but the plant looks healthy when it cools.

Sunburn and sun scald

Plants can get sunburned. The sun can scorch leaves of plants that typically need a little more shade or lower temperatures. And many plants can burn on super-hot days. The sunburn often leads to brown spots or browning and dying of entire leaves. Sunburn of a few leaves shouldn’t be a problem, but a sign to shade or move a plant if possible. Sunscald is similar damage to bark in high temperatures.

When weeds and wildflowers wilt, it's just too hot and dry. We have some that also look burned up. Woohoo!
When weeds and wildflowers wilt, it’s just too hot and dry. We have some that also look burned up. Woohoo!

Entirely new or different insects

Aphids, which can be controlled organically, tend to love cool, moist weather. Every type of insect is different, but they’re cold-blooded creatures. Insects can’t internally regulate their temperatures, so they rely on what’s happening in their environment. I’m certain that both weeds and insects change each year depending on weather. Cabbage loopers and leaf miners like heat. Ants? Don’t get me started. And dreaded grasshoppers thrive in drought. When a plant already is stressed by heat or drought, it’s more vulnerable to insect damage or insect-borne diseases.

grasshopper in patio chair
Sure, grasshopper, pull up a chair. In fact, take the shady spot. I’d rather see you resting than eating on our plants.

Some plants adapt better to heat than others. However, young plants still might need more frequent watering in high heat. That’s the part that’s getting to me most. We have some new plants we put in the ground not long before the heat wave hit, and they’re struggling. We’ve even had to water a few xeric and waterwise plants that typically thrive with no water at all. It’s all part of the fun of gardening in the Southwest, I guess. See my previous post for tips on helping plants survive extreme heat.