How Can I Keep Plants Cool During Scorching Heatwaves?

Move potted plants to cooler locations during the most dangerous periods of a heatwave.

“It has been hotter than it’s ever been on record. How can I protect my plants from the severe heat?” Angie of Walla Walla, Washington

Answer: It’s scary when you live in an area that typically has mild summer temperatures, and you unexpectedly experience super high heat. Your tender plants are as used to it as you are, which means they’re going to struggle. From what I have read, your average June temperatures have highs around 80 degrees F. Being faced with temperatures well over 100 and even 110 degrees F must be really awful. Here are some ways to help protect your plants, especially those that are less heat tolerant. Specimens planted in full sunlight during the hottest times of the day (around 2:00 pm to 4:00 pm) need the most protection.

Four Ways to Cool Plants in High Heat

  1. Irrigate plants very well in the early morning. If temperatures exceed 100 degrees F, water again in the evening after the sun has fallen.
  2. Protect plants that are in full sunlight with floating row covers fitted with white shade cloth. These covers are easily placed over plants and removed. You can also simply purchase shade cloth and drape it over highly exposed plants. Both methods help. Gardeners in your area have also informed me that they have been moving patio umbrellas around their yards to shade their most prized plants. (One even said that her neighbor did not, and it resulted in some substantially fried rhododendrons and hostas.)
  3. Move containers into shaded areas or indoors during the most dangerous heat and water twice daily.
  4. Opt for lightweight, light-colored mulches. A cooling layer of straw around vegetable roots will really protect plants from heat, and long-fibered sphagnum peat moss makes a good, cooling cover for shade beds and containers. (Click here to learn about more mulch options.)

I also want to note that pets and wildlife also feel the stress. Leave water out for the animals and keep your pets from going outdoors for extended periods of time and walking on hot surfaces (grass only!). I also recommend that you read the following two garden articles for more helpful tips.

Cool Gardens: Designing for Summer Temperature Control

Nine Water-Saving Garden Tips to Fight Drought

Stay cool and happy gardening,

Jessie Keith

Black Gold Horticulturist

Cool Gardens: Designing for Summer Temperature Control

There are certain design and planting features that will greatly reduce summer heat in a garden, and they go way beyond just providing shade. By tapping into the power of the air, water, stones, shade, and cooling plants, you can create a pleasing outdoor garden space that will help temper the high heat of summer.

Here are some of the best tools in the design toolbox for creating cool (and cooling) garden spaces.

Increased Airflow

Leave open spaces along garden edges to encourage airflow.

Keeping areas of your garden open, to facilitate airflow and catch prevailing winds, will not only cool your garden but dissuade flying insect pests, like mosquitoes. Allowing for some open spaces around your garden for welcome breezes is also pleasing to the senses. You can even plant a few fragrant plants, such as gardenias (Gardenia jasminoides), pots of sweet alyssum (Lobularia maritima), jasmine tobacco (Nicotiana alata), and fragrant roses, upwind for the additional pleasure of natural aroma.

Cooling Walls

These outdoor standing planter walls offer airflow while also creating an herb-filled cooling enclosure for a patio.

If an enclosed garden is what you have (or want) then green walls and/or shaded stone walls don’t radiate heat. Trellised or clambering vines, like climbing roses, golden-flowered Carolina jessamine (Gelsemium sempervirens), scarlet honeysuckle (Lonicera sempervirens), or espaliered fruit trees, cool sun-drenched fences, or hardscaped walls because they don’t absorb and release heat. They also look lovely. (Click here to read more about great garden honeysuckles.)

Plantable green planter or pocket walls will also do the trick, and these can be filled with herbs and edibles for culinary gardeners. Water-permeable, felted wall pockets on a freestanding support make this very easy. Florafelt living wall systems are easy-care, quality options that help homeowners install plantable walls in no time. Tall, outdoor planter shelves or freestanding trellis walls are two other options. Both also facilitate airflow.

Shaded stone walls or patio stones are also greatly cooling because they hold the lower temperature of the night and emanate it during the day, which effectively reduces the temperature of any shaded patio.

Green Canopies

Virginia creeper (Parthenocissus quinquefolia) is a fast-growing vine for pergolas that turns brilliant red in fall.

Pergolas, arbors, or arbored tunnels covered with vines will stave off the summer heat because they don’t absorb heat, and they release cooling moisture into the air. Grapes (Vitis spp.) are one of the best vines for the job. Not only do they produce fruit, but they are long vining, tolerant of a wide array of weather conditions, and have very large leaves that provide good cover. Other good long-vined candidates include brewer’s hops (Humulus lupulus), if you make your own homebrew, as well as native Boston ivy (Parthenocissus tricuspidata) and Virginia creeper (Parthenocissus quinquefolia), which both turn brilliant hot colors in fall.

Large-Leaved Tropical Plants

This decked outdoor room has the benefits of a partially open border of large-leaved tropical plants and a shading umbrella.

Whether grown in pots or the garden, invite a few very large-leaved tropical plants into your outdoor space. The best are rainforest plants that take up and release moisture in high amounts, which makes them perfect for hot patios or deck sides. Tender varieties for large pots or garden spaces are elephant ears (Colocasia esculenta), giant elephant ear (Alocasia spp.), and false banana (Ensete ventricosum ‘Maurelii’). The hardy to semi-hardy Sichuan hardy banana (Musa basjoo, USDA Hardiness Zones 7-10), which can reach 20 feet, is an in-ground option that will live from year to year in southern zones. For potted specimens, choose a potting soil that holds extra water, such as Black Gold® Moisture Supreme Container Mix and Black Gold® Waterhold Cocoblend Potting Mix.

UV Blocking Patio Umbrellas

Obvious cooling features in the garden are umbrellas and canopies that provide cooling shade, but they are even more effective if they block UV rays. Those designed for UV protection are more reflective. Light-colored canopies also absorb less heat, which increases their cooling ability.

Airy Green Borders

Tall, airy borders of switchgrass allow breezes to pass and pair well with other tall garden plants like this sneezeweed (Helenium autumnale).

Some shrubs and tall, airy plants are less dense, allowing them to facilitate more airflow, while also providing pleasing garden borders. Low, airy shrubs, like the 4-foot Longwood Blue bluebeard (Caryopteris cladonensis ‘Longwood Blue’ (expect it to self-sow)) and Grand Cascade butterfly bush (Buddleia davidii ‘Grand Cascade’), with its 6-foot habit and large purple flowers that lure butterflies, are ideal.

Tall grassy garden borders are also pleasingly airy and attractive. Three excellent options include the plumy, 6-foot Cloud Nine switchgrass (Panicum virgatum ‘Cloud Nine) with its blue-green foliage, or the 5-foot ‘Northwind’ with its large seedheads and olive-green blades. The impressive 6-foot Windwalker® Big Bluestem is another beauty with linear clumps of blue-grey foliage, maroon floral plumes, and maroon fall foliage. It’s a real showstopper that flows in the breeze.

Effective Tree Cover

This garden design has multiple cooling features such as open, freestanding trellis walls, partial tree cover, and a pergola just waiting for ample ornamental vines.

High tree canopies make for breezier shaded spaces, but trees offer far more than just shade. They are also natural air conditioners because trees draw up water from the soil and release it from their leaves as fine mist. This process is called transpiration. Broad-leaved, deciduous trees with larger leaves transpire more for more cooling power. During a growing season, a leaf will transpire many times more water than its own weight. In fact, a large oak tree can transpire up to 40,000 gallons (151,000 liters) of water per year!

Water Features

Small fountains are soothing and cooling and have too much movement for mosquito development.

Moving water in the garden is pleasing to the senses and really cools garden spaces. Whether you install a small fountain, a bubbling pool, or a small fish pond, these serene features will improve your outdoor living space. One key consideration is to only install features with moving water or fish to avoid creating mosquito breeding ground.

Mosquito Control

This koi pond is a perfect water feature that won’t invite developing mosquitoes because it has a constant flow of moving water and fish to eat the larvae.

Some of the cooling options mentioned can invite mosquitoes due to increased moisture and shade cover. Here are some solutions that can really help.

If you add a water feature to your garden, remember that still or stagnant water creates the perfect mosquito breeding ground. Moving water does not. Fish ponds, however, are acceptable because fish consume mosquito larvae. You also need to beware of birdbaths–refreshing their water every few days will wash away any developing mosquitoes.

Full sun is not favored by mosquitoes, but shaded gardens invite them. Several non-chemical means of keeping mosquitoes away include the use of citronella candles and burning tikis. Newer options include electronic mosquito repellers, which create a 15-foot deet-free cloud of repellent across an area. Several plants also help to repel mosquitoes. Those for more shaded locations include lemon balm (Melissa officinalis), which emits a lemony scent and will grow in partial shade. The equally citrusy lemon verbena (Aloysia citriodora) will tolerate light shade. (Click here to discover more mosquito repelling plants.)

A mix of these garden design features will help naturally cool your favorite garden enclaves and outdoor spaces, even on the hottest days.