How to protect your garden from a heat wave

If you are expecting temperatures above 90-95F, you might want to help keep your garden cool. High temperatures can cause flowers to drop, prevent fruit from settings, and turn certain vegetables starchy or bitter.

You can help keep your garden cool by making sure the soil stays moist, using mulch to reduce heat, misting the fruits & vegetables during the hottest part of the day, and providing shade (by using shade cloth).

The Midwest and Southeast are about to see a heat wave that lasts an entire week starting around June 20-21, 2025. I'm in the Chicago area and we are forecasted to see six days with highs around 95F and lows only dropping to 80F.

First, you'll learn how this affects different types of edible plants in your garden. After that, we cover the actions you can take to keep your garden cool and protect your fruits and vegetables from hot weather.

How does this impact your garden?

Wilting leaves

The most obvious thing you'll notice is that plants will be hot and some will naturally have limp leaves. The droopy leaves expose less surface area to the sun, allowing them to conserve water. This is really obvious on zucchini, summer and winter squash plants and is normal and expected.

But leaves can also turn limp and wilt due to water stress - this is bad, especially during a heat wave. Make sure the plants are very well watered during high temperatures.

Flower drop & fruit set

Many plants also fail to pollinate and flowers drop particularly above 90-95F. This happens to tomatoes, zucchini, and peppers, among other fruit and vegetables.

Furthermore, some plants won't set fruit when nighttime temperatures are above 75F, including peppers and tomatoes.

This is a bigger problem for gardens with fruits and vegetables that are in full bloom. If pollination fails, a large portion of your crop will be lost. New flowers that form will pollinate just fine - so it really depends on the stage of your plants and if they continuously flower or not.

For example, beans will fail to pollinate in these high temperatures, but since they continuously flower, they'll pick back up once temps cool. In six weeks you might have a lull in harvest, but the week after that would be back to normal.

Bolting

A lot of lettuces and herbs bolt in hot weather from a heat wave. When they bolt, they 'go to seed,' which means they send up flowers and stop producing the leafy greens we are after.

Dill, cilantro, parsley, and chervil are the most easily affected. Basil is must more heat-tolerant but can also bolt above 95F. Hardier herbs like rosemary, sage, oregano, and bay can handle the heat. Thyme is also fine in hot weather.

Bitterness & other flavor problems

A heat wave can also turn several types of vegetables, herbs, and lettuces bitter.

Cucumbers develop more cucurbitacin, a bitter compound, that is concentrated at the stem-end and the skin
Lettuce leaves turn bitter, even mild sweet varieties
Basil turns bitter, and I notice it slightly in parsley too

Extremely high temperatures (above 90-95F) also affect the flavor and texture of several vegetables.

Snap peas turn tough and starchy, less sweet
Beans turn more fibrous, starchy, less sweet, and can be smaller and malformed
Carrots and parsnips turn starchy, but if you wait to harvest, they can convert some of that back to sugar

How to keep your plants cool

There are several strategies, each one helping a little bit. You'll get the most benefit if you do all of them, but that's not always possible based on your time and budget.

Water the garden
Add mulch
Mist during hottest temps
Add shade cloths

1. Keep soil moist

The easiest thing you can do to help protect your garden during a heat wave is to make sure the beds are well watered. The best time to water is late at night so the water can soak in and not evaporate.

Your fruits and vegetables need more water to handle higher evaporation rates (just like we do when we sweat in hot weather). The water also helps keep soil a little bit cooler which keeps the plants cooler. Think of when you're standing on wet soil vs hard-dry soil... the former is cooler.

2. Add mulch

Cover your garden bed dirt with wood chip mulch, straw, or even grass clippings. The keeps the sun directly off the soil, which keeps the soil cooler. Avoid plastic mulches which can heat soil up significantly.

Mulch also slows evaporation and prevents the soil from radiating even more heat back at the plants at night.

3. Mist during hottest parts of the day

Normally you want to water during cooler parts of the day to conserve water, since the water evaporates quickly when it's hot and sunny. However, during a heat wave, you want to harness that evaporation to cool the plants.

Think of walking under a mist-sprayer at a theme park on a hot day. It really helps cool you down, right? Your plants also cool down from a similar treatment.

A Master Gardener at Washington State says, "using misters or micro emitters for 5 to 10 minutes 2 or 3 times during the hottest part of the day can also lower the heat inside the structure by another 10° for 30-60 minutes." (pdf source).

4. Add shade

Lastly, the more labor intensive and potentially costly option is to provide shade with shade cloths, or anything you have lying around.

As the Washington Master Gardener points out, "my personal experience with shade cloth is that the light tan color reduces the temperature under the cloth by about 13° as long as the structure is open to a breeze."

White and light-colored shade cloths reflect more light, ultimately keeping your garden cooler. Black and dark colored shade cloths absorb the sun, radiating some of that heat back on to your garden. Think about wearing white vs black on a hot, sunny day... a white t-shirt keeps you cooler.

This isn't the easiest thing to do unless you have a structure that already allows you to attach shade cloths to. (I have trellises throughout my garden that make this pretty easy).

In a pinch, or to save money, you can try using other materials to create shade, like an old painting drop cloth, old bed sheets, anything to help. I've even been eyeing a giant cardboard box in my basement to see if I can lean it up (and secure it) against a bed for some shade.

I haven't tried these alternative options yet (but I plan to this year). I haven't found any research testing these out either, but it seems like they should help, right? Also keep in mind that these alternative options will hold water if they get wet from misting or from rain, weighing them down.

Make sure the garden still gets air flow - this helps keep it cool.

Products to protect your garden from extreme temperatures

This is the first year I'm protecting my garden, but I've talked with a bunch of experts and read through some papers on heat-stress for different fruits and vegetables.

The products I'm suggesting are based on this research, but I have not personally tried them. I do get a small commission if you buy something from Amazon, since these are affilliate links and it helps keep the blog running. But I always like to be up front and let you know what products I like and have personally tried, vs ones that I haven't.

I did buy some shade cloth on Amazon that arrives soon, so I can't give you a review or endorsement yet. I also bought some remote thermometers to see how cool I can keep the garden.

Shade cloth on Amazon - full list, a reflective option I almost bought, and a beige version and white version that I did buy.
Remote thermometers: I also bought some remote thermometers so I compare temperatures and see how it's doing throughout the day. This 3-pack is shockingly cheap. I'll let you know how it performs.
Trellis options on Amazon