How to Protect Gardens From Heat Stress

• What is Heat Stress?
• Watering Do’s and Do Not’s During Heat Stress
• Common Symptoms of Heat Stress in Plants
• Helping Your Lawn
• Beat the Heat!
• Things to avoid

What is Heat Stress?

Any time the thermostat soars above 90°F (32°C), your garden begins to gasp for relief! So do we, but we’re able to hide in the air conditioning or move into the shade. We can wear shades and sunscreen. 

Your landscaping isn’t so lucky!

Those dog days of summer can leave your plants wilting and shriveling all day when temperatures get too hot, but it isn’t just the sun. Lack of water, high humidity and even drought and hot winds can wreak havoc on your garden.

During the heat, some plants can go into a mini-dormancy period to conserve energy. Leaves wilt or roll up, plants can shrivel and waste away into nothing. 

Both metabolism and growth slows down or stops completely. Flowers, fruit and leaves drop to conserve moisture loss and limit evaporation, and leaves can actually get sunburn or sunscald. Fruit and stems can blister! 

Plants that are mature, well-established and planted in highly organic soil with mulch and supplemental watering, will survive when the temps get high for a day or two. But when you have prolonged heat, with no rain and no break - then you have problems with some plants that aren’t able to take the heat!

Here’s what to watch for and how to help your plants through these harsh times.

Watering Do’s and Do Not’s During Heat Stress

Knowing when and how much to water is half the battle, whether you’re dealing with heat or not. 

When gardening in the summer, water in the early morning is best, or if absolutely necessary, late in the afternoon when the hottest part of the day has passed. 

Also, be sure to only water right at the roots. Sprinklers or aerial watering systems, or even a garden hose spraying all over the leaves; just won’t get the moisture where it’s needed most, which can lead to fungal and bacterial issues. Plus those water droplets will just evaporate into the dry parched air, or sometimes they can actually burn holes and brown patches into the leaf surface like magnifying glasses if present when the sun comes out!

Knowing if your plants have very shallow surface root systems means knowing you need to water more frequently. These kinds of plants also really need that extra layer of mulch protection. Tap rooted trees and plants won’t need water as often since they go much deeper. 

New plants and ones trying to establish, or fruiting plants that you are salivating as you watch your harvest ripen, both need to be watered frequently in the heat. Check moisture every morning using the ‘finger test’ method by sticking your finger into the soil at the roots up to the 2nd knuckle and feel if there is moisture available still.

Common Symptoms of Heat Stress in Plants

Problem: Bolting

Anyone who’s planted spinach or lettuce and then the summer heat snuck up on them before they had their first salad knows, bolting is a common issue for cool-weather crops. Bolting is a plant ‘speeding up’ their growth process to try and get their seeds out there as soon as possible, the sudden heat tricking them into thinking the end is nigh! Sometimes other plants can bolt, sending up their flowering shoots fast and even stunting the rest of their growth.

Solution:

There are bolt-resistant and heat-resistant crops and of course, providing smaller plants some afternoon shade and a soil-cooling bed of mulch to grow in. Water well as needed, preferably in the morning, and cover with shade cloth to reduce this issue. Sometimes just moving that patio umbrella out into the garden is a temporary solution!

Problem: Leaf Wilt

The first sign your plants are in trouble is of course when the leaves wilt and droop. Water is evaporating from the leaves and stems faster than the roots can replace it. Plants are about 90% water, and the water in the cell walls helps hold them up and out (the technical term is ‘turgid’), so once that moisture has escaped, they can, well… deflate! 

This can happen often during the hottest parts of the day, then at night the roots are able to catch up with demand and begin pushing moisture into the aerial parts of itself and they look perfectly normal by morning. 

Anyone that has had a Peace Lily or Hydrangea shrub they forgot to water knows the magic that happens when the plant does get that much-needed drink; you can almost watch the leaves return to their healthy state before your eyes!

Flowers can turn brown or drop, and leaves may develop brown tips if the wilting isn’t caught or water is given to your plants in time. Usually, this is not a permanent problem unless it is allowed to continue past the ‘permanent wilt’ stage.

Solutions:

Wilt is easily prevented by providing a thick 3-4 inch layer of mulch over the surface of the soil to help hold in more moisture right at the roots where they need it most. And be sure to regularly water and keep a close eye on your plants throughout these harsh times.

Weeding is important to reduce competition for moisture and nutrients. Weeds can be tenacious and seem to grow faster and deeper than ornamental plants, beating them to that precious resource.

Problem: Losing Flowers & Immature Fruit

A sudden early spring, or badly timed heatwave, or prolonged drought, will wreak havoc on fruiting and flowering trees. A plant will protect itself by dropping all its flowers which are moisture hogs. This act of self-preservation only interrupts flowering briefly and once the worst of the heat is over, new blooms usually replace those lost. However, if this is a fruiting plant, you’ve lost a significant amount of your crop.

During summer heatwaves and times of drought, fruiting trees will also dispense with fruit, and you’ll find your harvest on the ground rotting before it had a chance to ripen. Fruit of course takes lots of water to produce, so as a last resort, your plant will get rid of its offspring as a last-ditch effort to save itself. Since the entire purpose of a plant's life cycle is to have the next generation, you know things are bad when they’ve been discarded.

Solution:

• Plant trees and shrubs that are drought-resistant 
• Choose plants generously rated for your growing zone
• Plant these in highly organic soil (which naturally holds moisture better and releases it slowly back into the soil)
• Ensure your plants have been planted at least 6-8 weeks before harsh summer temperatures kick in for your area
• Install drip irrigation or soaker hoses around trees and shrubs to deliver water at the roots, or provide supplemental watering during these times
• Mulch the root system very well, extending out to the drip line of your plant.

Problem: Leaves Curling or Rolling Up

By doing this self-defense maneuver, a plant actually tries to limit the surface area exposed to the sun and heat. They also close their stoma and stomata (the pores on the upper and lower leaf surface) to keep them from losing precious moisture. Unfortunately, they don’t always unroll when the heatwave passes, causing your plant to look stunted and diseased. But don’t fret, your plant will grow new leaves once the danger has passed and will not have permanent damage.

Solution:

There’s not much you can do once temperatures and sun become this intense, just follow the steps above to ensure your plant was given the right start to support it in times of need: mulch, water, organic soil, supplemental water, and hope for the best. If plants are small enough, there is shade cloth and other shade-producing measures you can install to give them a bit of relief. Removing any weeds competing for moisture from around the root zone, that way your plant has it all to itself.

Problem: Environmental Damage

Damage to your plants that is permanent to the foliage such as prolonged heat exposure, repeated wilting, and environmental contaminants like smoke and bad air quality can cause what’s called ozone damage. Usually appearing to most gardeners as possible insect or disease damage, there can be browning and spotting between the leaf veins and margins, and drying up or browning edges. This always seems to be amplified when the heat is on.

If your plant is an evergreen or broadleaf evergreen, those leaves are there to stay, but deciduous plants will luckily get a new wardrobe next spring.

Solution:

Again, there is not much you can do in the uphill battle with climate change, environmental factors and pollution. Choosing plants that are rated highly for urban conditions and that have a good start as mentioned above - Mulch, water, organic soil, supplemental water, and cross your fingers.

Problem: Keeping Up With Container Plants Watering Needs

Smaller plants in pots can be difficult to keep watered throughout the day during summer's heat, let alone during a heatwave. Those contained root systems are usually smaller and don’t have the earth's protective mass to keep them hydrated and cool. They dry up exponentially faster and soon you’ll see the soil pulling away from the sides of the pot.

When you attempt to water overly-dried-out potting soil, the water usually just pours down the gap that forms between the pot and soil, and then right out the bottom; never able to absorb into the soil nearest the central root systems. When this happens, it takes a significant amount of sustained watering to rehydrate that entire pot to the point where it won’t wilt in an hour anyway.

Solution:

The best way to water smaller plants is to fill a large bucket with enough water so when you put that potted plant into the water, it will completely cover the soil and pot. Submerge the entire pot and watch until it stops bubbling (which means it has saturated the soil completely), pull the plant out and let any excess drain away. You may have to hold the plant down if it’s become too dry until it takes on enough water to not float.

Larger planters and containers you cannot move just need to be thoroughly soaked until you see water starting to pool, then let the excess trickle from the drainage holes. It is very important for these larger containers to never let them become too dried out. 

Mulch or moss to cover the soil surface works just as well in planters and container plants as it does on your landscaping! By reducing soil exposure to heat and sun, you’re keeping roots cooler, reducing evaporation and cooling roots.

Containers gardens can benefit greatly from lining the inside of the pots with a thin layer of Styrofoam (even old egg cartons) before adding the soil to the pot creating a bit of insulation from the heat in summer and cold in winter too! This can also make your pots lighter and saves money by not requiring you to fill the entire container with soil.

There are potting soil mixes with moisture absorbent crystals (like the kind found in baby diapers!) that absorb water and then when the soil around them dries out, they slowly release that water, allowing you to go a bit longer between waterings, but just a bit. Having highly organic soil with lots of peat and compost does the same job, holding excess moisture, then releasing it slowly when needed most.

If you know an excessively hot day or heatwave is on the way and you are able - move your container plants into a more protected area where they can find shady relief from the worst part of the day.

Problem: Dry Winds and Reflected Heat

Temperature, sun and humidity aren’t the only problems your plants face out there! The wind is another factor that can make your plants dry out or dry up really fast! Provide a windbreak or barrier that protects plants that are exposed to those dry winds 

Planting near the street or a structure that reflects lots of sunlight or heat back onto plants is another major problem during the summer and becomes a massive issue during heatwaves. It’s like the heat and sun are amplified! A tree near a picture window or metal fence that bounces the sun's rays back onto the plant, or absorbs excess heat can cook a plant if it's planted too close.

Street trees and more delicate leaved plants can show signs as the leaf margins dry up and turn brown, a frequent problem with Japanese Maples and Linden Trees. The leaf surface can scorch and turn tan or brown for the rest of the season.

Solution:

Really, making sure your plant has a touch of afternoon shade and protection before planting is the first line of defense in preventing this from happening. Keep them well watered and hope for a new flush of foliage, once temperatures calm down.

Helping Your Lawn

Watering your lawn is about the same as your plants. Water deeply and thoroughly, but for turfgrass, water less frequently when establishing your lawn, will ensure the roots grow down deep. This is a very important method for training your plants to find water on their own. 

Light and frequent watering keep the plant roots close to the surface where they can dry out quickly and stress the plants sooner.

Beat the Heat!

Plants react to the heat differently for a wide range of factors, even your soil type can make or break a plant's ability to survive heat stress! Knowing your soil, your plants and being an attentive gardener will be the determining factor, being ready with the garden hose can’t hurt either!

Things to avoid

Never fertilize or apply any pesticides or other chemical sprays to plants that are enduring high heat or in the throws of heat stress. 

Avoid pruning during this time as well. Cut surfaces and exposed wood dried out incredibly fast and you can put your plant into shock by removing too much plant material while they’re already fighting temperatures and drought.

Install underground trickle watering systems or under-mulch drip irrigation to deliver moisture straight to the roots and when they need it most. This is especially worthwhile for prized specimens and valuable investments!

And it is certainly the worst time possible to try transplanting or installing a new plant! Wait until spring or fall to attempt any of these kinds of maintenance.

Choosing plants for full sun, plants that can better survive drought, and larger plants that are more mature or have larger more established root systems, are all ideal ways to get ahead of high temperatures. 

Water is the source of all life and you can’t control what Ma Nature does, but you can get your garden off to the right start by ensuring your plants are heatwave-ready. 

With the help of NatureHills.com, we’ll beat the heat!