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All Things Garden

6 Ways to Protect Garden Plants in a Heat Wave

“96 degrees in the shade… real hot!” Where are my Third World reggae fans at? Even if you aren’t familiar with the song, I think you all know what a heat wave is. Unseasonably warm weather and extreme temperature swings can cause a lot of stress and potential damage to your garden. Even plants that prefer warm weather aren’t big fans of sudden and drastic change. However, plants are far more resilient than we often give them credit for! With a few preventative and protective measures, you can easily help your garden survive a heat wave with minimal impact. 

Read along to learn six ways to protect plants during a heat wave. We’ll talk about steps to take when you see unusually hot temperatures in the weather forecast, along with best practices to proactively employ in your garden year-round – to make plants more tolerant to heat stress and drought in general. 

What temperature is “too hot” for plants?

Various types of plants respond differently to a range of heat. Vegetables such as lettuce, radishes, bok choy, broccoli, cabbage, and other leafy greens or members of the brassica family generally prefer cooler soil temperatures (in the 50 and 60s). These cool-season crops may temporarily wilt or begin to bolt in temperatures over 75°F.  When a plant bolts, it prematurely forms a flower spike and then goes to seed. When temperatures hit over 90°F, these guys will likely fry and die. Learn how to prevent cool-season crops from bolting here

Meanwhile, summer garden crops like tomatoes, peppers, and beans will thrive in that 75 to 90°F range! That’s their jam. Yet even heat-loving crops may become cranky, less productive, or damaged in excessively hot temperatures. For example, tomatoes may experience flower drop and issues with fruit development when daytime temperatures are regularly over 95°F, especially combined with high humidity and sustained warm temperatures overnight. Similarly, zucchini and squash plants may slow production when it’s over 85°F for an extended period of time. 

No matter the variety, young plants are more prone to heat stress and damage than mature ones. Their tender nature and small, shallow root systems makes them far less heat-hardy, so keep a close eye on seedlings! Move any seedlings that are still in containers indoors or into the shade during heat waves if possible. Also, avoid transplanting new plants outdoors right before or during a heat wave.

A two image photo collage, the first image is a red tomato attached to a cluster of fruit on a tomato plant has signs of sunburn and damage on half the fruit. It is blistered and wrinkled with a large, round, light in color discoloration amongst the red fruit. The second image is a green bell pepper that is still attached to the plant, half of the fruit has turned white and papery due to sunburn, leaving only small amounts of green on the outer edges of the fruit. Protect your plants from a garden heat wave so your fruit won't succumb to sunburn.
The effects of strong sun: sunburned tomatoes and bell peppers. Providing shade and protecting fruit with foliage can help prevent this. Photo courtesy of Utah State University

Quick fixes versus long-term adaptations to heat

Thankfully, the issues with pollination, production, or wilting that some plants experience during short-lived heat waves usually results in only a temporary setback. They simply slow down a bit, like we all do when it’s uncomfortably warm. Use the tips below to protect your garden during a heat wave, and the plants should bounce back once the weather cools off and returns to normal. 

On the other hand, gardeners in areas with expected and prolonged excessive high temperatures may need to implement longer-term solutions to combat the heat. For instance, folks living in Arizona, the deep South, Las Vegas, or Hawaii may adjust their garden seasons and avoid growing tomatoes or squash during the hottest summer months altogether, and grow them during the spring and fall seasons instead. Other long-term adaptations include careful selection of plant varieties, planting certain crops in partial shade, using in-ground beds rather than containers (which more easily dry out  and succumb to temperature swings) or erecting shade structures that will stay up for several months. 

A raised garden bed sitting on a hardscape of gravel contains many types of herbs growing from parsley to sage. It is affixed with three black stakes on each sides of the length of the bed, a black piece of shade cloth is attached to the top of the stakes to create a lean-to of sorts to protect the plants from sunburn.
Shade cloth is a fantastic season extender to grow heat-sensitive and tender plants into late spring and summer, and also comes in handy during heat waves! This awesome shade cloth kit is from Gardener’s Supply.

6 Ways to Protect Plants During a Heat Wave

The following six tips can help your garden survive during a heat wave. Wondering when to act? A good rule of thumb is that if it’s suddenly going to be 10 to 15+ degrees warmer than it has been and usually is, or if the weather networks issue an “excessive heat warning” for your area, implement a few of the steps below to help your garden survive the heat wave. It may not be reasonable or necessary to apply every single protective measure listed, so feel free to pick and choose as you see fit!

1) Water deeply and routinely (all season long) 

Start now! This is a tip to preemptively protect your garden from heat waves, and to develop healthy, strong root systems – which is alway a good thing! When you routinely water your garden throughout the growing season, provide plants with long and slow water that will saturate deep within the soil. That is better than watering for quick bouts and more often, or only wetting the top few inches of soil. 

The more moist the soil is deep within your garden bed, the more it encourages roots to grow deeply too. Did you know most plants can grow roots underground equally as long as the plant is tall above ground? That means a tomato plant can grow roots over 4 feet deep, peppers and eggplant around 2 feet deep, squash over 1 foot deep, and so on. 

Having deep roots enables plants to access more nutrients and water, and makes them more resilient to drought and varying soil temperatures. Plus, deeply damp soil maintains more steady temperatures and is less likely to dry out. So, set your plants up for success with routine deep watering from the start! 

Given all the unique variables in every garden or climate, it’s hard to say exactly how much to water and how often. However, all plants benefit from a consistent watering schedule, be it twice per week or four days per week. A good goal is frequently enough to maintain the soil modestly damp at all times, but not soggy and never fully dried out. When you water deep, you can water less often.

Read more tips and information about DIY, non-toxic and efficient garden irrigation options here. Or, come learn how to set up automated drip irrigation for raised garden beds here.

A 4x8 foot wood raised garden bed full of small plant seedlings, with four rows of black irrigation tubing running between the plants down the length of the garden bed. The soil appears to be moist and rich.
Our new raised bed drip irrigation system makes watering a breeze! Each unique garden will have different watering needs depending on the climate, plants, and watering system used. We usually run this system for about an hour twice per week, but sometimes longer or more often during a heat wave.

2) Water before a heat wave – but resist the urge to overwater 

Give your garden a good deep drink of water either the evening or morning before a heat wave is due to hit. Providing water well before the peak heat of the day will give them ample time to soak up the water, and also keep you out of the sun too!

However, resist the urge to continue to pour water onto your plants all day long. Plants may wilt under high heat and strong sun, but that doesn’t necessarily mean they need more water.  Even here where temperatures rarely reach the 80s, our summer zucchini and collard greens can look pretty sad and limp in the middle of an average warm afternoon – but they always perk back up that night! Yet if they look droopy in the morning? Then they may be thirsty.

Before offering more water, check the moisture content of your soil by probing an inch or two below the soil surface. If it feels moist or if you just watered in the last day or two, it probably doesn’t need more water yet. 

A large squash plant is succumbing to the heat of the day with all of its leaves hanging limp. The surrounding area is very green and grassy, you can almost feel the heat and humidity by the image alone.
Wilting squash plants on a hot summer day. Image from Peaceful Valley (one of the many places we love to buy seeds), where they do a great job at explaining why this doesn’t always mean plants need more water!

3) Avoid wetting plant leaves 

Avoid overhead watering and wetting your plant leaves during hot sunny conditions. You know how humans can still get a decent sunburn (if not worse) in foggy conditions? Moisture amplifies the sun like a magnifying glass! Similarly, lingering water droplets on plant leaves in direct sunlight can magnify the intensity and heat of the sun, and increase the chance of sunburn or scorching the leaves. As a best practice, we always try to water the soil around the base of the plant rather than the plant itself, but especially so during heat waves. 

A watering can that isn't visible is watering the soil below the canopy of collard greens so the water isn't getting on the plant's leaves. The water resembles that of a rain shower spray. Protect your plants from a garden heat wave by watering your soil, not your plants.
Water your soil, not your plants! We like to make sure the soil between plants gets water too, not only immediately around the base of each plant.

4) Don’t skip the mulch!

Did you know that plants are more irked and influenced by temperature swings in their root zone and soil than they are by ambient air temperatures? For instance, a plant can usually survive and rebound after getting a bit frosty or fried above ground, but are far less forgiving if their roots are distressed. So, be sure to mulch your garden to offer a protective layer for the soil and roots! 

Mulching is always a fantastic idea (a must, if you ask me!) and especially important to protect plants in heat waves. Mulch is a layer of material that goes on top of exposed soil, which helps to buffer against temperature extremes and promotes more steady soil temperatures overall. It also reduces evaporation and runoff, protects the living soil food web, prevents soil from drying out, and reduces your need for water!  

Apply about 2 inches of organic mulch material (even more for light mulch like straw) on the soil surface around the base of plants or over the whole garden bed. Examples of organic mulch include straw, small bark or wood chips, shredded leaves, pine needles, and aged compost (our favorite). Some gardeners use a layer of newspaper, cardboard, or plastic sheeting. Pop over to this article to read more about the pros and cons of 8 different types of garden mulch, or this guide on mulching best practices. 

A close up of a pitchfork piling up a thick layer of yellow straw mulch around the base of two medium size tomato plants, about 2 feet tall and bushy.

5) Provide shade

Shade is an incredibly effective tool to keep things cool and protect plants during a heat wave. By reducing the intensity of the sunlight and heat beating down on plants, the soil stays cooler, retains moisture, and generally reduces the impact of excessive heat. Even more, shade can prevent sun scalding or sunburning of fruit, commonly seen as whitish yellow patches on tomatoes or peppers. The damage caused to the skin often causes the fruit to prematurely rot. Blocking the hottest afternoon sun is particularly important.

There are a number of ways to create shade in the garden: by draping bed sheets or specialized shade cloth over hoops, stakes, or other supports, using large patio or beach umbrellas, or even erecting large shade canopies over an entire garden area rather than individual beds or plants. Really, anything that will block the sun but still allow good air flow will work! If the shade structure will be up for an extended period of time, it’s best to get shade material that allows some sunlight through (partial UV protection). For a shorter period of time (a day or two), you could completely block the sun using a solid pop-up canopy. Or, make use of shade that already exists in your space – like by moving potted plants into shadier locations.

Four raised garden beds are visible, each bed is affixed with three sets of hoops that have shade cloth attached to protect the crops below. The background is a sea of green with vines, flowering perennials, and fruit trees below and open blue sky. Protect your plants from a garden heat wave by providing shade during extreme weather.
An example from our garden: using shade cloth on wire hoops over beds of leafy greens and young brassicas. Shade cloth material (black, green, or white) can be set higher over tall plants with the support of stakes, archways, arbors, or poles – as shown in a previous photo from Gardener’s Supply. Curious to learn more? Read our guide to using hoops and row covers here. It digs into more detail about shade cloth FAQ such as shade cloth colors, various UV ratings, and more.
A row of bok choy shown from underneath the canopy of shade cloth to protect the plants from a garden heat wave.
Keeping bok choy cool on a random 90 degree spring day. We love our wire hoops from Gardener’s Supply for shade cloth, but primarily use them to support insect netting to protect plants from pests and birds!

6) Promote overall good plant health

The final way to help your garden survive a heat wave is to grow the healthiest plants possible! Like the first tip regarding routine deep water, this is something you can work on all season long. Stressed or compromised plants are more sensitive to the heat, just as some older people or those with health issues may be (no judgement – I get sick in the heat too!). Also just like humans, plants have immune systems that help them respond and rebound to various environmental stressors, including pests, disease, drought, frost, heat, toxicity, and more. 

In our garden, we encourage the most robust and hardy plants possible with organic inputs such as worm castings, mycorrhizae, homemade aloe vera fertilizer, well-aged compost and/or compost tea. All of these goodies feed the living soil food web, enable plants to better utilize nutrients, and improves their resilience to pest pressure, heat, and more. That’s not to say our plants don’t still struggle with miscellaneous ailments from time to time… but these things sure do help!

DeannaCat is crouching in between a couple raised garden beds, she is tossing up a large radish with one hand as she stares at it while it is suspended in air. There are a few more of the harvest daikon radishes laying on a nearby garden bed that contains four large growing cabbages. The raised beds in the background contain cauliflower and brussels sprouts plants.
Cheers to happy, healthy, productive gardens – and the gardeners that tend them!

And that is how to protect plants during a heat wave.

Alright friends, I hope this leaves you with plenty of ideas and methods to help your garden survive the next heat wave that rolls in! As you can see, there are several things you can do now and always in preparation – and a few tools or materials you may want to keep on hand too. Please let me know if you have any questions in the comments below. Or, if there are any great ideas that I forgot to mention. Stay cool and comfortable out there!

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