Preparing Your Trees for Calgary's Summer Heat

People often think of winter as the hardest season for Calgary's trees, but summer can be just as punishing. Extended stretches of 30-plus-degree heat, dry winds from the prairies, water restrictions, and intense UV at our elevation all take a toll. Trees that are not properly prepared going into summer can develop drought stress, sunscald, and pest vulnerability that carries into the following year.

The good news is that most summer tree damage is preventable with some straightforward spring preparation.

Water Is Everything

Calgary averages only about 400 millimetres of precipitation per year, and most of it comes as rain in June and snow in winter. July and August are typically dry. For trees, especially those planted in the last five years, supplemental watering during summer is not optional. It is survival.

How Much Water Do Trees Need?

A general guideline is 10 litres of water per week for every 2.5 centimetres of trunk diameter. So a tree with a 20-centimetre trunk needs roughly 80 litres per week during hot, dry weather. That sounds like a lot, and it is. Your lawn sprinkler is not doing the job. Lawn irrigation wets the top few centimetres of soil where grass roots live, but tree roots are deeper. Trees need slow, deep soaking.

Best Watering Methods for Trees

• Soaker hose: Coil it in a circle at the drip line (the outer edge of the canopy) and run it for 30 to 60 minutes. This puts water where the feeder roots are.
• Tree watering bag: These slow-release bags wrap around the trunk and deliver water over several hours. Great for young trees.
• Bucket method: Drill a small hole in the bottom of a 20-litre bucket, fill it, and set it near the base. The water trickles out slowly and soaks deep.

Water early in the morning when evaporation is lowest. Avoid watering in the evening, as prolonged surface moisture overnight can promote fungal growth.

Mulch: Your Tree's Best Friend in Summer

Mulch is the single most effective and cheapest thing you can do to help your trees through summer. A 7 to 10 centimetre layer of wood chip mulch around the base of your tree:

• Reduces soil temperature by up to 10 degrees Celsius
• Retains moisture and reduces evaporation significantly
• Suppresses grass and weed competition for water and nutrients
• Feeds the soil biology as it slowly decomposes

Spread the mulch from about 10 centimetres away from the trunk out to at least 60 centimetres, or as far as the canopy extends if practical. Do not pile it against the bark. Mulch volcanos cause bark rot, harbour pests, and are one of the most common tree-care mistakes we see in Calgary.

Prune Strategically (But Lightly)

Heavy pruning in spring removes the foliage that shades the trunk and branches from sun exposure. If you are going to prune before summer, do it early in the season and keep it moderate. Never remove more than 25 percent of the canopy, and be especially conservative with trees that are already stressed.

That said, removing dead and damaged branches before summer is important. Dead branches do not photosynthesize, provide no shade, and attract boring insects. Getting them out before the heat arrives keeps the tree focused on healthy growth.

Protect Young Trees From Sunscald

Thin-barked species like maple, mountain ash, linden, and young fruit trees are vulnerable to sunscald on their southwest-facing bark. In summer, the intense afternoon sun can heat bark to the point of cell damage. You will see the damage show up the following spring as cracked, peeling, or discoloured bark.

For young trees in exposed locations, consider wrapping the trunk with a commercial tree wrap through their first three to five summers, or plant a companion shrub on the southwest side to provide some shade.

Hold Off on Fertilizing Stressed Trees

If a tree is already showing signs of drought stress, adding fertilizer can make things worse. Nitrogen fertilizer stimulates leaf growth, which increases the tree's water demand at the worst possible time. It is like telling someone running a marathon to sprint faster when they are already dehydrated.

If you want to fertilize, do it in spring when the tree is actively growing and water is still available, or in fall after the leaves have dropped. Deep root fertilization done by an arborist is the most effective method for delivering nutrients to the root zone without encouraging surface root growth.

Know the Signs of Heat Stress

Even with preparation, some summers push trees to their limits. Watch for these warning signs:

• Wilting: Leaves that droop during the heat of the day but recover by morning are mildly stressed. Leaves that stay wilted overnight are severely stressed.
• Leaf scorch: Brown, crispy edges on leaves, especially on the south and west sides of the canopy. This is a classic sign of not enough water reaching the foliage.
• Early leaf drop: If a tree drops its leaves in July or August, it is in survival mode, shedding foliage to reduce water loss.
• Branch dieback: Dead tips on branches, starting from the top and working down, indicate the root system cannot supply enough water to the full canopy.

If you see these symptoms, increase watering immediately and consider having an arborist assess the tree's root zone and overall health.

A little preparation in spring goes a long way. The trees that sail through Calgary's summers are the ones with deep mulch, adequate water, and owners who pay attention. It does not take much time, but it makes all the difference.