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How to care for plants and trees in drought, extreme heat

Toronto, Oakville, Burlington, Mississauga, Milton and basically all of southwest Ontario have been suffering a drought in summer 2016. In an average year, Toronto gets 72mm of rain in June; this year, we received only 26.4mm. July and August have fared no better; we are getting only one-third of the rainfall we normally would. We have not had significant rain in six weeks. So, if you have a lot of annuals, perennials, shrubs, and trees, you have been busy watering because the alternative is to replace a lot of your plant material next year.

The second whammy all of southwest Ontario has been enduring is: extreme heat. For almost two weeks now, we have been seeing temperatures that can give Dubai a run for its money. With the humidex, we have had temperatures in the low 40s Celsius.

Signs of extreme heat stress in plants and trees: Stems become limp, leaves become wilted with brown edges, brown flowers, or a complete lack of flowers altogether. When plants are stressed, they are in shock. In extreme heat and drought, for example, a lawn turns completely yellow and stops growing altogether.

Established trees are better able to withstand drought than newly planted ones. Trees cope with severe drought by curling their leaves, to reduce evaporation. Watch for curling leaves, burned edges, wilting, and yellowing leaves on the tree overall or specific branches.

The biggest problem with extreme heat and drought is that it puts enormous stress on plants, making them susceptible to disease and insect infestations.

Under severe and prolonged drought and heat stress, a tree may actually go dormant. The tree will drop its leaves first, but they won't be in beautiful fall colours. The dropped leaves will be brown and brittle, typical of a drought.

Next, the tree may cut off what little water remains to secondary branches. Trees are able to compartmentalize and prioritize which branches get access to the little water that is available. Because of this, the effects of severe drought may not be felt for one or two growing seasons; you may be pruning large branches off that tree next year, as they will have died.

Even if your tree has dropped all its leaves, keep deep watering its root system. It may survive next season; you won’t know until the following spring, with the next bud-set. If you don’t water the root system after the leaves have fallen, the tree will die for sure.

Large evergreens such as pines, spruces, balsams and hemlocks suffer a lot of stress during drought and extreme heat. Although their needles do not need quite as much water as leafy, deciduous trees, they typically have a very shallow root system. After the top few inches of soil dry out very quickly, most of their roots then have no access to water. To survive, the evergreen will eventually start dropping needles. Many evergreens will be unable to regenerate needles even after normal rainfall resumes. The tree will have some bare limbs that will need to be removed. If the evergreen is young enough, you may be able to allow tip growth to “cover” the bare limbs, but a mature evergreen never seems to generate enough new growth to cover the bare limbs. Soak a wide area under the evergreen. Move the sprinkler around the evergreen to soak the entire root area, allowing it to run up to an hour in each position.

Plants in hanging baskets and containers: Put these in shade and keep them well watered. If the basket or container is very small, you may have to water twice a day. Mulch will help keep the plant cooler. These plants appreciate a dousing of cool water from your garden hose at the end of the day, as it helps to absorb the heat.

Plants in garden beds: Keep watering them, even if you cannot provide shade. Again, mulch helps a lot to keep moisture in the ground and keep the sun off the roots; you need at least a 3-inch layer of mulch to help your plants in extreme heat and drought. Tomatoes and geraniums tolerate the heat, but extreme heat actually causes these to grow too fast and actually degrades the plants.

CAUTION: Don’t fertilize stressed plants! Adding fertilizer to already stressed plants just adds to their problems: it burns them even more. Keep them watered and in the shade as best as you can. Let them recover. Give them at least two or three weeks to recover and don’t add fertilizer until the temperature drops back down to the 20s, or even high 10s.

Do deep watering of your shrubs and trees. Keep watering any trees and shrubs that were planted this spring. Water once a week and water deeply. Doing deep watering encourages the plant to develop deep deep roots, and so it is better equipped to handle drought and heat in future seasons. How to do deep watering: Take off the sprinkler head and adjust the flow of water so it is about the diameter of your thumb. The idea is that all the water that’s coming out of the hose should be absorbed at the base of the tree, and not run into the gutter. Place the hose at ground level at the bottom of the shrub or tree. For a mature tree, place the hose about 6 feet away from the trunk. Allow water to flow for 30 minutes; move 2 or 3 times, clockwise around the base of the shrub. Move 4 to 6 times (or more), clockwise, for a tree depending on its size. Until the drought is over, do deep watering weekly for all your shrubs and trees.

Water your lawn: Again, do deep watering. This means putting on the sprinkler for 30 minutes per area once a week. Water early in the day or later in the day, to reduce evaporation. Deeper watering, fewer times per week, is better to encourage grass to develop deeper roots, to better withstand the next drought.

Plants that dislike heat: Pansies, trailing lobelia, delphiniums, impatiens, and begonias dislike heat. These tend to wilt even if well-watered. Move these plants into the shade, if possible.

Plants that tolerate heat and are more drought resistant: Blanket flower, celosia, coreopsis, echinacea, mandevilla, pentas, petunias, Rose of Sharon, rudbeckia, salvia, sedum, sunflowers, verbena, zinnias.

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