Have You Ever Watered a Plant… and It Still Wilted?
Have you ever watered a plant, found the soil still damp, yet watched the leaves wilt anyway?
That plant may not be thirsty– it may be suffering from heat stress.
As Okanagan summers become hotter and heat waves last longer, gardeners are discovering that watering alone isn’t always enough. A plant also needs to be able to tolerate prolonged periods of high temperatures. And that’s where Heat Zones come in.
Rudbeckia ‘Goldsturm’ and a variety of sedum in the OXA demonstration garden during the 2023 extreme heat. Photos by Judie Steeves
Hardiness Zones vs. Heat Zones
Most gardeners are familiar with Plant Hardiness Zones, which tell us how well a plant can survive the coldest temperatures of winter. A lack of cold hardiness in plants generally results in dramatic winter kill.
But cold isn’t the only stress a plant has to survive– summer heat leaves its own mark, just less obviously. The effects of heat stress on a plant occur at the cellular level, resulting in wilting, dieback, and flower damage. The effects often mimic drought stress but with wet soil.
Fewer people know about Heat Zones. Developed by the American Horticultural Society (AHS), Heat Zones are based on the average number of days per year when temperatures reach or exceed 30°C
While hardiness tells us whether a plant can survive winter, heat zones help us understand how well it can cope with prolonged summer heat. Plants may wilt, leaves may scorch, flowers may fade, or stems may die back– even when the soil still contains adequate moisture.
That’s because extreme heat affects how plants function, making it difficult for them to keep up with water loss during prolonged hot weather.
Plants chosen with heat tolerance in mind for the full sun setting in the UnH2O demonstration gardens
Why Heat Zones for Plants Matter in the Okanagan
Kelowna’s summers are getting hotter, and now there’s a way to measure it. AHS Heat Zone 5 is defined as 30 to 45 days per year above 30°C, and Kelowna’s recent summers are trending into that range.That doesn’t mean every plant rated below Heat Zone 5 will suddenly fail. It does mean that heat tolerance is becoming an increasingly important consideration when choosing plants for Okanagan gardens.
How OXA Rates Heat Tolerance
Plant growers and horticultural references may provide heat zone ratings to indicate a plant’s tolerance to summer heat. Rather than asking gardeners to interpret those numerical ratings, OXA has translated them into simple, easy-to-understand categories in our Plant Database: Poor, Fair, Good, and Excellent.
These simplified ratings make it easier for Okanagan gardeners to compare plants and choose species better suited to today’s hotter summers. They are included in the Notes section for individual plants.
Every plant in OXA’s Plant Database is rated to withstand at least AHS Heat Zone 6, which gives a buffer for the Zone 5 designation.
Plants with a Poor heat-resistance rating may already struggle during our hottest summers, while those rated Fair, Good, or Excellent are generally better equipped to cope with prolonged heat.
Article prepared by Okanagan Xeriscape Association, with contributions from Mark Godlewski
Learn More
Heat tolerance is just one of many factors to consider when selecting plants. Soil conditions, exposure, irrigation, and winter hardiness all play important roles as well.
Explore OXA’s Plant Database to compare heat resistance ratings, or return to our companion guide, Okanagan Heat: Helping Your Garden Thrive, to learn more about creating climate resilient landscapes for the future.