Trees and Drought: How Calgary's Dry Spells Affect Your Yard

Calgary sits in a semi-arid climate zone. The city receives roughly 400 millimetres of precipitation annually, significantly less than most Canadian cities. While we are accustomed to dry conditions, extended drought periods are becoming more frequent and more intense. For the trees in your yard, drought is one of the most serious threats they face, and its effects can linger for years after the rain returns.

What Happens to Trees During Drought

Trees lose water constantly through their leaves in a process called transpiration. Under normal conditions, the roots replace this water as fast as it is lost. During drought, the soil dries out beyond the roots' ability to extract moisture, and the tree begins to experience water deficit.

The first response is stomatal closure. The tree closes the tiny pores on its leaves to reduce water loss, but this also shuts down photosynthesis, which means the tree stops producing food. If drought continues, the tree begins shedding leaves to reduce its transpiring surface area. This is a survival mechanism, but it comes at a cost: fewer leaves means less energy production, and the tree starts drawing down its stored reserves.

Prolonged or repeated drought pushes trees into a weakened state where they become highly vulnerable to secondary stresses like insect attack and disease. Many trees that appear to die from pest infestations or fungal diseases were actually fatally weakened by drought first.

Which Trees Are Most Vulnerable

Trees with shallow root systems, high water demand, or those planted in poor soil conditions are most at risk during Calgary dry spells:

• Birch: Shallow-rooted and moisture-loving, birch trees are among the first to show drought stress. Drought-weakened birch are prime targets for bronze birch borer.
• Spruce: While spruce are adapted to Calgary's climate, their shallow, spreading root systems make them sensitive to extended dry periods, especially in compacted urban soils.
• Recently planted trees: Any tree planted within the last three years has a limited root system and is extremely vulnerable to drought.
• Trees in clay soil without mulch: Clay soil shrinks and cracks when dry, physically tearing fine roots and cutting off water pathways.

More drought-tolerant species include bur oak, green ash, Manitoba maple, and most native poplar species, though even these have limits during extended dry periods.

Recognizing Drought Stress

Drought stress symptoms can appear quickly during a heatwave or develop gradually over weeks of below-normal rainfall:

• Leaf wilting: Leaves that droop during the day and recover overnight are in early drought stress. Leaves that stay wilted overnight are in serious trouble.
• Leaf scorch: Brown, crispy edges on otherwise green leaves indicate the tree cannot supply water to the leaf margins.
• Early fall colour: Leaves turning yellow or orange in July or August instead of September signal drought-induced early senescence.
• Premature leaf drop: The tree sacrifices leaves to survive, sometimes dropping 30 to 50 percent of its canopy.
• Reduced twig growth: Shorter annual twig extension compared to previous years.
• Bark splitting: In severe cases, the trunk bark may crack as inner tissues dry and shrink.

What You Can Do

Water Wisely

During drought, prioritize your trees over your lawn. A dead lawn recovers in a season. A dead mature tree takes 20 to 40 years to replace. Focus deep watering at the drip line using a soaker hose or slow-running garden hose. One thorough soak per week is far more effective than daily light sprinklings.

Mulch Generously

A 3 to 4 inch layer of wood chip mulch across the root zone dramatically reduces evaporation and keeps soil cooler. During drought, this retained moisture can be critical. Extend the mulch ring as wide as practical, ideally out to the drip line or beyond.

Avoid Pruning During Drought

Pruning stimulates new growth, which demands water the tree does not have. Hold off on all but emergency pruning during drought conditions. Also avoid fertilizing, which can push growth the tree cannot sustain.

Reduce Competition

If water restrictions limit your overall water use, remove or reduce grass competition around trees. Expanding the mulched area at the expense of lawn directs more of your available water to the tree.

The Long-Term Perspective

Drought damage often shows up a year or two after the event. A tree that survived a dry summer may show dieback, reduced leaf size, or increased pest vulnerability the following spring. Monitoring for two to three seasons after a drought year helps you catch delayed damage and respond before the tree enters irreversible decline.

Calgary's climate is trending drier, making drought resilience an important consideration for any tree planting or care plan. Choosing drought-tolerant species, maintaining healthy soil, and committing to consistent deep watering during dry periods gives your trees the best chance at long-term survival.