When summer heat waves hit, most people can seek out air conditioning. Plants can’t.
“They’re rooted in place, so they have to deal with whatever comes along,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “Stretches of severe heat can be hard on them.”
How well a plant handles hot weather depends largely on the climate in the place it originally came from. Many of the annual flowers we use in gardens and containers, such as petunias, zinnias and begonias, as well as vegetables such as tomatoes, peppers and cucumbers, come from subtropical regions of the Americas, Africa, or India, and can handle heat — as long as they have enough water. But most of our trees, shrubs and perennial plants come from temperate regions of the world, and aren’t as well adapted to living through waves of 90-degree days. “They need our help,” Yiesla said.
Here are some tips from the Plant Clinic.
Water. Plants cool themselves by allowing water to evaporate from tiny holes in their leaves, and they need a steady water supply to do that. “All the life processes of a plant depend on water,” Yiesla said. “Water makes up 80% to 90% of the mass of most plants.” Plants need moisture throughout the top 6 inches of the soil, where most of their roots are. To check whether you need to water, use a trowel to dig down at least 2 to 3 inches and feel whether the soil is moist.
Don’t depend on rain. Even though the year had a wet start, we can’t assume that rainfall is taking care of plants’ needs. “Sometimes rain just wets the surface but doesn’t penetrate down to where the roots
can absorb it,” she said.
Watch the forecast. If temperatures in the 90s are predicted, water thoroughly the day before. “That will make sure the plants are ready, with enough water in their systems to withstand the heat,” she said.
There will also be a reservoir of moisture in the soil for the plant’s roots to draw on.
Don’t wait for plants to wilt. “Wilting means the plants are already stressed,” Yiesla said. “You want to water them in advance to keep them from getting stressed in the first place.” If you do spot plants
drooping or wilting, water them right away.
Mulch. A layer of mulch over their roots helps plants in two ways: It insulates the soil against extremes of temperature, and it keeps moisture from evaporating from the soil. You can apply mulch any time, so go ahead and spread it if you haven’t already. Make a layer 1 to 2 inches deep on perennial beds and 3 to 4 inches around trees and shrubs. Spread the mulch out evenly, rather than piling it up against the
trunk or stems. Keep mulch clear of the bark of a tree or shrub.
Water deeply. Don’t just lightly sprinkle the soil surface. Water long enough so the moisture sinks several inches down. “If you find you have to water in-ground plants every day or two in normal weather, you’re not watering deeply enough,” Yiesla said. In very hot weather, the plants will use up the water in the soil more quickly as they try to cool themselves. “Check to see whether the soil is dry and you need to water,” she said.
Soak containers often. Since pots don’t hold much soil, they dry out quickly. “In really hot weather, you may need to water containers more than once a day,” she said. Smaller containers, which contain a smaller volume of soil, dry out faster than larger ones.
Give new plants extra help. Any trees, shrubs or perennials you installed this year, or trees planted within the last three years, will need even more watering. “New plants don’t have much of a root system, so they can’t absorb water as readily as established plants,” she said. “That means they need to be watered more often.”
Help mature trees too. A big old tree with a well-established root system can survive a few days of temperatures in the 90s. But in long periods of hot, dry weather, water your mature trees as well as your young ones. “A big tree has tens of thousands of leaves, and they all need to be full of water to keep the tree cool and healthy,” Yiesla said. Apply water to as much of the root zone as you can — at least as far as the tree’s branches extend.
Cut your plants some slack. When it’s intensely hot, “plants are focused on surviving, not blooming or fruiting,” she said. Tomatoes, for example, won’t be pollinated or set fruit in temperatures over 90 degrees, and temperatures consistently in the 90s can prevent them from ripening. “Just keep watering to help them hang in there until days get a little cooler,” she said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
