Gardening With Microclimates

Audrey Hillman
Adams County Master Gardener

I do a lot of long distance driving. One of my pastimes to help with the monotony is to play 'Plant ID' of the various plants I pass by. After awhile you begin to notice that you see the same plants growing together. Then you notice that the areas where you see them together share some of the same physical conditions. Some are shady and moist, others are sunny and dry. While some plants are very specific in their requirements, others are not as fussy and do well in a variety of situations. In nature, the associations of plants are referred to as plant communities.

Each plant community has its own particular factors that influence what plants are part of that community. These factors include temperature, moisture, light, soil, terrain and wind exposure. The unique combination of these factors in a specific area are referred to as microclimates. A microclimate can be as large an area like coastline where temperatures may remain warm while a few miles inland a frost occurs. But usually, when one refers to a microclimate, one usually is referring to a smaller area, even as small as just a few square feet.

So what does this have to do with us as gardeners? Plenty! If you look around your property, you'll find different microclimates. By identifying and understanding just how all the variables of that microclimate combine, you can put the right plants in the right places, and in doing so, develop ecologically sound and sensitive landscapes.

To understand your garden's microclimates, you need to look at all those variables individually. When I evaluate a site, the first thing I do is determine where North is. The basic microclimates will be dependant on the sun's exposure: North, South, East and West. Areas with a northen exposure can experience shade year round, as a result, their soil is often moister. Northern exposures are the last to warm up in the spring and first to cool down in the fall. Difference in Northern and Southern exposure is greatest in the winter, where long shadows cast from structures block the sun, causing persistent snow on walkways and drives. However, a northern exposure can provide a cool outdoor living space in the summer.

A Southern exposure receives more sun and is thus warmer, thereby increasing soil evaporation and earlier snow melt, resulting in drier soils and increased plant transpiration. They warm up earlier in the spring. As a result, spring bulbs bloom several weeks earlier on the southern exposure then on a northern exposure. Early blooms however can be a problem, especially for plants that can be affected by a killing frost. The spring blooming Magnolia is more likely to break dormancy and develop buds earlier in a Southern exposure then in a Northern exposure, resulting in more bud loss to that killing frost. Because they stay warmer in the fall, Southern exposures have a longer growing season, which allows you to enjoy the beauty of your garden longer.

Eastern exposures receive sun in the morning all year. They provide relief from the hot sun on a summer's afternoon and evening, as such, the soil tends to stay moist. I find that plants that like moist soil and part sun do well with an eastern exposure. Eastern exposures also tend to be protected from winter winds and temperatures.

Western exposures mean shade in the morning with sun in the afternoon at a lower angle. In the summer these areas can get hot and the soil is usually drier. But it's the warmth of the afternoon sun in the spring and fall followed by cold nights that can damage plant tissue. This can be observed as cracks in the bark of young trees, or blackening of newly formed leaves and stems. This is the area for hardy plants that like dry soil and warm temperatures. Because most winds in our area are out of the west, this is also the exposure where you'll want to plant wind breaks.

But the sun's exposure is not the only thing that helps to create your microclimate. Other factors that contribute included the location and density of trees and shrubs, existing structures and paved surfaces, presence of water, and variations in topography.

Plants that grow in full sun in a cooler climate can be grown in warmer climates given some shade. Plants that are zoned 7 or 8 can often be grown here given protection to keep the winds away and soil warmer in the winter.

The effects of sun and shade can be observed in such structures as fences, walls, and rocks. In addition, air temperatures can be influenced by materials and colors of nearby structures. For example, dark colors can collect and hold heat releasing it after the sun goes down while light colors reflect daytime heat and light back onto plants. The color of paved surfaces and mulch will also have the same effect. Dark mulch will warm the soils earlier in the spring, and lighter mulch and surfaces are great for those pants like lavender that love the reflected heat in the summer.

Variations in topography can also be challenging. If you garden on a slope there are some things to remember that will help you with plant selection: 1) Warm air rises, cool air sinks creating frost pockets. If this is a problem allow for air drainage just as you would for water. 2) The soil is usually rockier and more course along the top of the slope. Because of this, it drains well and holds less moisture. The steeper the slope the drier the soil. 3) Conversly, soil at the bottom of a slope will be moister, and because of runoff and erosion from the top, finer and higher in nutrients. Retaining walls can help level slopes and reduce runoff.

It all seems like a lot to consider. But with practice you can learn to evaluate your conditions and microclimates. Once you do, it will open opportunities for plants that you might not have considered. It may also help you to understand why that plant you refuse to give up on just doesn't appreciate your efforts. With a little practice, you can improve it's well-being by identifying and moving it to a more compatible site. Resist the urge to buy that gorgeous flower until you can study its requirements a bit more than what the plant tag tells you. When you are sure that you can accommodate it needs, then buy without guilt and save yourself the frustration of losing your prize!

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