By Winston Gamache

Microclimates are small spots within a space that differ in their conditions from the surrounding area. Both living and non-living structures can create or define microclimates. They can be small and simple, like a rock in a flower bed that affects the frost in the ground. They can also be massive, like a shelterbelt of trees or a ridge that shades and shelters the valley below. 

When we talk about microclimates in Alberta, we generally think about how these spaces are affected by sun/heat exposure, snow cover, and frost in the ground or air. Edmonton is Zone 4a (see Know Your Climate Zones) but many desirable plants aren’t hardy in that zone. Microclimates can help us grow many things that wouldn’t otherwise be hardy here.

When we think about annuals and microclimates, we are usually aiming to grow something that needs more heat and/or warmer soil than what we normally get. For plants like peppers, eggplants and dahlias, the goal is to get that plant as warm as possible. We want to get these tropical plants into a warm temperature range for a longer period of time.

Warmer soil earlier in the season can be achieved by growing plants in raised beds or in black pots. By lifting the planting soil higher up to the sun and air, the soil will warm up weeks ahead of the ground. The raised soil will also reach a warmer temperature than the ground ever would. 

Brick and rock walls and other hardscape structures, like patios and concrete, also create warmer microclimates. These structures hold the heat from the sun and thus help keep the nights a little warmer. The type and number of plants also affects the microclimate by adding humidity and cooling the surrounding area. A hot patio could be a great place for potted peppers and tomatoes. South and west exposures capture the most amount of sun and heat. 

Shelter from strong winds is another important factor to consider for many annual plants. Strategically placed fences, shrubs, trees, and even walls can help shelter young tender plants or plants prone to breakage. Dense plantings of annuals can also shelter each other from wind and create a more secure microclimate.

When we talk about microclimates for herbaceous perennial plants in Alberta, we are generally talking about protecting these types of plants throughout the winter more than improving their growth in the summer. Create sheltered microclimates where the ground will stay insulated and frozen to protect the plants. The repeated freeze-thaw cycles that we experience in Alberta in the fall and in the spring are why many of our perennials die.

Snow cover is our best friend. Pay attention to where your snow drifts! Include structures such as shrubs, rocks, snow or wattle fences, and walls to catch drifting snow. The temperature below the snowline is much more stable and warmer than our coldest winter temperatures. Many perennials can survive -15 C under the snow on a -40C day but if the crown of the perennial is exposed, the perennial will be much much colder and most likely damaged. Snow helps keep our ground temperature at a moderate -3 to -5C when it is frozen. This is an acceptable temperature for many perennials, even up to zone 6. Snow also helps reflect the sun’s rays and this keeps the ground frozen. If the snow cover is sparse, shovel some of the snow that you do have onto the crowns of your perennials to help protect them.

One example of a winter sheltered microclimate is how the ground on the shady side of a boulder may stay frozen longer into spring than the ground on the sunny side of the boulder. The shady side of the boulder will also help keep the plant dormant until night time temperatures are above freezing. The snow may even drift deeper along the boulder and further insulate the plant. 

Another way to create a perennial microclimate is to use a thick layer (6-8 inches) of organic mulch. A layer of organic mulch can keep the frost in the ground which will keep plants dormant longer. Organic mulch is not a substitute for good snow cover but can further insulate your perennials against those cold autumns when snow doesn’t fall until Christmas. Organic mulch basically extends the protective blanket that the earth provides. Rock mulch tends to have the opposite effect because it heats up in the sun, melts snow, and can cause extra freeze-thaw cycles. For plants sensitive to crown or stem rot (like lavender), be careful that the organic mulch doesn’t get too wet in spring and damage the plant. For these types of perennials it’s best to use chunky bark or wood chips versus leaves or finer wood shavings. 

Much like microclimates for perennials, microclimates for trees and shrubs have a lot to do with shelter and keeping the plants dormant. However, there are a couple of additional things to keep in mind. Consider the damaging effects of wind and winter sun on our woody plants in the garden.

Some part of every tree and shrub is exposed to cold air, wind, and sun all winter. Alberta is known for its very sunny, cold winters. This can cause sun damage to the bark of a tree and lead to it breaking dormancy too soon. On a cold day the air may be cold, but the sunny side of a  tree trunk can defrost. This repeated cycle of freezing and thawing can lead to serious damage on young trees. White trunk guards or painting the sunny side of a tree trunk, with 1:3 water:white latex paint or plaster, can help protect the young bark until the tree itself becomes woody enough to shield itself. 

Cold air and wind can cause extra damage to trees and shrubs in the winter. Be sure to use hardier species of trees and use structures as windbreaks to create sheltered locations. This is also great for wind protection in the summer as some ornamental trees cannot handle a full strength prairie winds without breaking. 

For trees that may be on the edge of our growing zone, a spot that receives winter shade is ideal. Winter shade keeps trees and shrubs dormant for a longer period of time and prevents sunscald. This type of microclimate is great for fruit trees such as pear trees and stone fruit trees. Both these types of trees are notorious for blooming too early which results in frozen blooms. A heavy layer of mulch also helps keep trees and shrubs dormant longer, much like with perennials. 

There are many shrubs that die back to the snowline every year. One can take advantage of the microclimate beneath the snow by bending and weighing down the branches of shrubs to below the snowline. This works well for shrubs like elderberries, smokebush, weigela, and even more tender shrubs like blackberries.