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Ask the Experts: How Do You Keep the Garden Thriving in Extreme Heat?

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Ask the Experts: How Do You Keep the Garden Thriving in Extreme Heat?

August 6, 2024

This summer has been hot—really hot. Gardeners across the globe have recently experienced the four hottest days ever observed, and by all accounts, this is the new normal. Late last year the U.S.D.A. released their new Plant Hardiness Zone Map, which shifted zones for many locations, including the Midwest and Northeast, into the next warmer half zone. So, what’s a gardener to do? We reached out to a group of garden pros who have experience in gardening in extreme heat to ask them how we can plant and maintain our gardens to better weather hot summers. 

Read on for their advice for picking plants, watering plantings, protecting trees, and more.

N.B.: Featured photograph above courtesy of Refugia, from Garden Visit: Refugia’s Quiet Revolution in Philadelphia’s Suburbs.

1. Stop planting.

After a particularly dry, hot summer in 2022, Claire Davis, an ecological garden designer in the Hudson Valley vowed: No more summer planting. “I decided that I wanted a cut-off date for plantings,” she says. “That super-dry, hot summer felt brutal for newly planted areas, and it was pretty miserable as a gardener trying to take care of them.” As a bonus, Davis says that pushing pause on planting has given her much welcome breathing space in her garden maintenance calendar. Instead of giving into the impulse to add annuals for “a little color” in midsummer, make detailed notes about when and where you lacked blooms, so you can make a plan to fill the gaps come fall.

2. Favor native perennials.

The Bosque Garden at NYC&#8\2\17;s The Battery, designed by Piet Oudolf, features native perennials. Photograph courtesy of The Battery Conservancy, from An Interview with Warrie Price, President and Founder of The Battery Conservancy.
Above: The Bosque Garden at NYC’s The Battery, designed by Piet Oudolf, features native perennials. Photograph courtesy of The Battery Conservancy, from An Interview with Warrie Price, President and Founder of The Battery Conservancy.

When you do add plants in the fall (or next spring), all the experts we spoke to placed an emphasis on using perennial plants native to your region because they are naturally adapted to the climate there. “For the most resilience in the heat, use mostly perennial native plants instead of seasonal annuals, because their root systems are more extensive,” says Ginny Stibolt, the author of Climate-Wise Landscaping, who gardens in Florida. She notes that forgoing annuals will also result in less disturbance of the soil, which helps build up humus, so that the soil stays moist even when it’s hot. 

3. Pay attention to soil type.

Succulents prefer sandy, gravelly soil. Photograph by Caitlin Atkinson, from Quick Takes With: Flora Grubb.
Above: Succulents prefer sandy, gravelly soil. Photograph by Caitlin Atkinson, from Quick Takes With: Flora Grubb.

Not only should you seek out plants native to your region, Barbara J. Sullivan, the author of Climate Change Gardening for the South, says to choose native plants suited to your soil type. “Whether it’s sand or clay or something in between, there are plants that thrive in your soil [type] and will have a better chance of surviving the stress of drought and extreme heat than plants that require special soil amendments.”

4. Attune your eye.

Native plants can take extreme heat with little to no water, but it might not be pretty, cautions Christine Ten Eyck, the founder of Ten Eyck Landscape Architects in Austin, Texas. “They will look withered, but they will come back strong with rain,” says TenEyck. “If you can deal with the look of brownish planting, just don’t do anything and feel good knowing that by using native plants you have a resilient landscape, which will rebound when weather improves.”

5. Leave no bare ground.

European Meadow Sedge (Carex remota) provides a durable groundcover-cum-mulch. Photograph courtesy of Greenlee and Associates, from The Garden Decoder: What Is Green Mulch?
Above: European Meadow Sedge (Carex remota) provides a durable groundcover-cum-mulch. Photograph courtesy of Greenlee and Associates, from The Garden Decoder: What Is Green Mulch?

You should add mulch, which is key to maintaining moisture in the soil (it also keeps it cool). Or better yet, Stibolt suggests, plant densely, so there is little room for weeds; close plantings also help conseve water. The best thing to do? Replace traditional mulch with “green mulch,” says Sullivan. “Consider using native groundcovers rather than mulch as a way of retaining soil moisture and benefiting soil organisms,” she says.

5. Water strategically.

If you must irrigate, do it early in the morning rather than midday to reduce evaporation. You should also avoid watering late in the day: “Warm weather fungus loves to form in damp earth and plants at night and can lead to sudden death,” warns Ten Eyck. When you water, aim your nozzle at the earth (not the foliage) and water deeply. “Less frequent, deeper watering is preferable to frequent shallow watering,” says landscape designer Hunter Ten Broeck, the owner of Waterwise Landscapes in Albuquerque, New Mexico. Ten Eyck notes that grouping potted outdoor plants together in the shade will help keep their roots from baking in the pot (it’ll also make it easier to water them if they’re close together).

6. Maximize your rainfall.

Ten Eyck turned to an ancient technique for conserving water when she built check dams in tiered garden beds (shown) in her front garden in Austin, Texas.  Photograph by Matthew Williams for Gardenista, from Curb Appeal: \10 Landscaping Ideas for a Low-Water Garden.
Above: Ten Eyck turned to an ancient technique for conserving water when she built check dams in tiered garden beds (shown) in her front garden in Austin, Texas.  Photograph by Matthew Williams for Gardenista, from Curb Appeal: 10 Landscaping Ideas for a Low-Water Garden.

Rather than rely on irrigation, Ten Broeck prefers creating passive rainwater harvesting methods, such as swales, basins, rain gardens, French drains, pumice wicks, and soil sponges (post holes filled with organic matter), all of which move water from impermeable surfaces to where trees and plants can use them. “One inch of rain running off from 1,000 square feet of hard surface can produce over 600 gallons of usable water,” he explains. “Even if you use rain barrels or cisterns they will overflow. Passive rainwater harvesting saves that.”

7. Baby your trees.

“The speed at which we’re transitioned from spring to plentiful rainfall to intense heat and no rain these days is devastating to trees, and especially for young trees,” says arborist Basil Camu, one of the owners of Leaf & Limb and founders of non-profit Project Pando in Raleigh, North Carolina. To help trees through extreme heat, Camus suggests gardeners add 3 to 12 inches of arborist wood chips extending from the trunk to the edge of the canopy. Then continue to build the soil come fall: Feed the soil in this area under the tree with compost and compost teas and leave the leaves. Consider planting a “soft landing” to further promote healthy soil. Ten Eyck adds, “Trees need infrequent, deep watering during the summer. Too many times I see trees being watered on the same irrigation as small perennials.”

8. Plant shade trees—but not right now.

“Plant deciduous trees on the south, southwest, and west sides of houses, patios, driveways, and recreational areas,” advises Ten Broeck. They will naturally cool your living spaces in the warmer months and let in the sunshine in winter.

9. Give bees a drink.

Above: A bird bath carved out of rock by Cleve West at the Chelsea Flower Show 2016. Photograph by Kendra Wilson, from 10 Easy Pieces: Bird Watering Holes.

“Remember to provide water for birds and other wildlife,” says Ten Broeck. “A little goes a long way.” Many people add native plants to feed hummingbirds, butterflies, bees, and other pollinating insects, but they forget to provide a drink. The Xerces Society recommends a shallow source of water (a bird bath or a bowl with stones on which bees and wasps can perch safely). Change the water often to prevent mosquitoes from breeding in the water.

10. Give transplants a fighting chance.

If transplanting is unavoidable, Stibolt offers some ideas to help the plant along. “Build a berm of soil outside the planting hole, so that all the additional irrigation stays to water the plant,” she advises. Then water every day for at least a week. “Water early in the morning when the plant is beginning to transpire and thus will be moving water through the plant.” Once the plant is no longer wilting, dial back to watering a few times a week and then eventually only in really dry spells. And don’t prune for at least a year!

11. Get to know your microclimate.

A mix of sand live oak, Sabal palm, saw palmetto, southern shield fern, rusty lyonia, and false rosemary in Seaside, Florida. All these native plants thrive in sandy soils with minimal water needs. Photograph by Brittany Godbee, courtesy of Dept. Landscape Architecture and Urban Design, from Ask the Experts: \1\1 Favorite Native Plant Combinations.
Above: A mix of sand live oak, Sabal palm, saw palmetto, southern shield fern, rusty lyonia, and false rosemary in Seaside, Florida. All these native plants thrive in sandy soils with minimal water needs. Photograph by Brittany Godbee, courtesy of Dept. Landscape Architecture and Urban Design, from Ask the Experts: 11 Favorite Native Plant Combinations.

The experts are all quick to point out that no amount of wisdom gleaned from an article can replace your own deep knowledge of place. For example, full summer sun in Vermont is different from full summer sun in the Southern coastal plain. “Very few plants will thrive with ten hours of full, direct sunlight plus drought when temperatures are in the 90s for weeks on end,” Sullivan says. “So, even if the label says ‘full sun,’ you may need to give your plants a position with some dappled or afternoon shade.” Likewise, Stibolt suggests leaning on local knowledge. “Gardening in Florida, I’ve found that general gardening advice may not apply to your climate.” 

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