You invested time and money in planting a new tree. You watered it faithfully all summer. It put on healthy growth and looked great heading into fall. Now comes the real test: surviving a Calgary winter. Young trees, generally those planted within the last three to five years, are significantly more vulnerable to winter damage than established trees. Their bark is thinner, their root systems are less extensive, and they have fewer energy reserves to draw on. Here is a step-by-step guide to giving your young trees the best chance of coming through winter strong.
Step 1: Deep Water in Late Fall
This is the most important step and the one that makes the biggest difference. Young trees with shallow, still-developing root systems are especially vulnerable to winter desiccation. They need adequate moisture in the soil heading into freeze-up because they will continue to lose water through their bark and any remaining foliage throughout winter, even though the ground is frozen and they cannot take up new water.
Water each young tree deeply in late September or early October, and again if we get a warm spell before the ground freezes hard. Let a hose trickle slowly at the base of the tree for 15 to 20 minutes, allowing the water to soak deep into the root zone rather than running off the surface. The goal is to saturate the top 30 centimetres of soil around the tree.
Step 2: Apply a Generous Layer of Mulch
Mulch is your tree's winter blanket. A layer of wood chip mulch three to four inches deep over the root zone insulates the soil, moderates temperature extremes, and retains the moisture you just applied. For a young tree, spread the mulch in a circle at least one metre out from the trunk in all directions, though wider is better.
The critical rule with mulching is to keep it away from the trunk. Pull the mulch back four to six inches from the base of the tree, creating a slight donut shape rather than a volcano. Mulch piled against the trunk holds moisture against the bark, encouraging rot and providing a hiding spot for mice and voles that will chew the bark over winter.
Step 3: Wrap the Trunk
Young trees with thin bark are highly susceptible to two winter injuries: sunscald and frost cracking. Both are caused by temperature differentials in the bark. On sunny winter days, the south-facing side of the trunk warms up while the rest of the tree remains frozen. When the sun drops or clouds move in, the warm bark refreezes rapidly, killing the active cells and creating dead patches or vertical cracks.
Calgary's chinook winds make this problem worse than in most cities. A chinook can push air temperatures from minus 20 to plus 10 in a matter of hours, creating exactly the kind of rapid warming and cooling that damages bark. Wrapping the trunk with commercial tree wrap or white plastic spiral guards reflects sunlight and prevents the bark from warming unevenly.
Start wrapping at the base of the trunk and spiral upward to just below the first branch. Overlap each layer slightly so there are no gaps. Secure the top with tape or a small clip. Apply the wrap in late October or early November and remove it in April when temperatures stabilize. Leaving the wrap on during summer can trap moisture and harbour insects.
Step 4: Install Rodent Protection
Mice, voles, and rabbits feed on tree bark during winter when other food is buried under snow. They work at and just below the snow line, chewing a ring of bark around the trunk. If they remove bark all the way around the circumference, a condition called girdling, the tree will die because the connection between roots and canopy has been severed.
Young trees are targeted because their bark is thin and tender compared to mature trees. Protect them with a cylinder of hardware cloth or a commercial plastic tree guard around the base of the trunk. The guard should extend from ground level up to at least 60 centimetres above the expected snow depth. In areas of Calgary that accumulate deep snow, this might mean protecting 90 centimetres or more of trunk.
Step 5: Remove or Adjust Staking
If your tree was planted with stakes and ties, check them before winter. Stakes that are too tight can girdle a growing trunk. Ties that have loosened can rub bark raw in the wind. Most trees should have their stakes removed after one growing season. If the tree still needs support, make sure the ties are loose enough to allow some trunk movement, which is essential for developing taper and strength, and that they will not cut into the bark as the trunk grows.
Trees left staked for too long develop weak trunks that cannot support themselves when the stakes are finally removed. If your tree can stand on its own, remove the stakes. The gentle flexing of an unstaked trunk in the wind is actually what stimulates the tree to develop structural strength.
Step 6: Skip the Fertilizer
It may seem counterintuitive, but you should not fertilize young trees in fall. Fertilizer stimulates growth, and the last thing you want heading into winter is a flush of new, tender growth that has not had time to harden off. Soft, late-season growth is extremely vulnerable to frost damage and can set the entire tree back.
If your young tree needs nutritional support, apply a slow-release fertilizer in early spring after the soil has thawed and growth has resumed. During the establishment years, the tree's energy is better spent developing roots than pushing top growth anyway.
Step 7: Protect from Snow Damage
Young evergreen trees with flexible branches can be damaged by heavy snow loads. Gently brush snow off branches after a heavy snowfall using an upward sweeping motion with a broom. Never shake or hit the tree to remove snow, as frozen branches are brittle and can snap with rough handling. For small evergreens in particularly exposed locations, a loose burlap cage provides protection without trapping excess moisture.
The Payoff
Young trees that are properly winterized come through to spring with minimal damage and are ready to put their energy into growth rather than repair. The first five winters are the critical period. After that, the tree's bark has thickened, its root system has expanded, and it has built up the energy reserves to handle Calgary's climate on its own. The winterizing effort decreases each year as the tree matures, and eventually you can retire the tree wrap and guards entirely.