Managing Microclimates

Photo Courtesy of Art Today

Managing Microclimates

Most of us are familiar with the USDA zone hardiness maps. While it is the most useful tool we as gardeners have at our fingertips when it comes to determining which plants are suited to our growing areas, it is at best a blueprint and at worst a generalization. You see, the hardiness map cannot possibly take into consideration all of the variables that determine how successful a plant will survive in an individual's garden. The truth is, many of us have plants that are growing in our yards and gardens that defy the zone hardiness ratings, sometimes by a zone or two. For example, I have successfully planted a Southern Magnolia in my zone 5b garden. But, there is more to it than that; I realize that on my property, there are four distinct growing zones. While the majority of my property would fall within the description of zone 5b as determined by the USDA hardiness zone map, I know for a fact that I have areas that are cooler, into zone 4a, and areas that are warmer, into zone 6b and that spot where the magnolia grows, zone 7a. How did I go about determining the different warm and cool areas of my property?

There are many variables that determine what the different microclimates are that exist on a given property. I determined the different areas of zone variation by taking into account these variables:

Exposure:  How protected or exposed a spot is in a given yard or garden can help to determine how well a plant can survive in a sheltered or non-sheltered location. For example, my rhododendrons and azaleas do poorly in the sunniest, most exposed areas of my garden. Those open areas tend to get cold, drying winter winds. These plants can suffer from both the effects of wind and from sunscald. Rather than siting these on the southern or western exposed areas of my property, I have planted them in the protected northern and eastern areas of my garden.

Direction:  Hand-in-hand with exposure is compass direction. Some plants are better suited to early morning sunlight than hot, afternoon sun. My roses all do better in an eastern or southeastern location. They also do poorly in areas of little direct sunlight, such as to the north or northeastern areas of my property.

Buildings, pavement, walls, and other structures:  Darker buildings, areas of pavement, and patios serve as heat resevoirs, absorbing sunlight and radiating heat to the surrounding areas. Lighter buildings reflect and intensify sunlight in the immediate areas adjacent to them. These principles hold true for stone walls and wooden fences as well. If these areas receive sunlight, they will release heat. Conversely, the sides of these structures that do not receive sunlight can become traps for cool air pockets. The sunless side of a building, fence, or wall will warm up slowly and tend to trap frosts. Plants that are there will break dormancy later in the season. The good aspect of the sunless side of these structures is that plants that cannot tolerate prolonged sun will do better in these locations. Some examples are ferns and hostas. The other good point of having plants on the cooler sides of fences, etc., is that the soil will dry out less quickly in these locations than on the sunnier sides of these same structures. The up side of planting on the sunnier sides of these same structures is that they stay warmer, longer, and sometimes the radiant heat given off by, say, a stone wall can mean that a marginal plant will not be nipped by a late spring frost.

Slopes and valleys:  Every property has them. Slopes and valleys can determine how frost pools in a yard. Since cold air is more dense than warm, cold air tends to fall away from higher elevations. Study your yard in the winter and spring. You will notice that frosts tend to arrive first and stay longest in the little valleys and at the bottom of slopes. Also, north-facing slopes stay cooler throughout the growing season while south-facing slopes warm up more quickly during the early spring and stay warmer later in the fall.

Raised planting beds:  Raised planting beds dry out and warm up more quickly in the spring. Unless raised plantings are in a valley or at the base of a slope, they can also escape many late frosts.

Bodies of water:  This effect of microclimate is dramatically evident along the shores of the Great Lakes, particularly along the eastern and southern shores. If prevailing winter winds are from the west or northwest to the east and southeast, the heat given off from bodies of water such as Lakes Erie and Michigan will cover the immediate areas of land adjacent to the lakes, keeping them more moderate than areas only a few miles further inland. A quick look at the zone hardiness map will bear this out: The narrow bands of land to the southeastern, southern and eastern shores of Lake Michigan and the southern shores of Lake Erie all are in zone 6b. While smaller bodies of water don't offer such dramatic impact on the climate of an individual's property, even a pond or small inland lake will have a moderating effects on gardens planted adjacent to these areas.

Soil type and moisture:  Often overlooked, soil type and moisture also help to determine how well a plant will perform in a particular location. If a plant has the proper soil type in regards to acidity or alkalinity, fertility and structure, it will perform its best when grown in the soil conditions to which it is adapted. The amount of moisture is a factor that must also be taken into consideration. Plants that thrive in moist areas, such as ferns, will winter over in better condition than ferns that are stressed going into the winter by being planted in an area that lacks adequate moisture during the growing season.

City versus country:  Those who live in urban areas have an advantage over those who live in the country. Buildings, pavement, and the heat given off in a city helps to moderate temperatures. Trees tend to break dormancy sooner in the spring in town than they do out in rural areas. Conversely, fall frosts tend to arrive later in the season in urban areas than "out in the sticks".

Trees and windbreaks:  This goes along with exposure. The presence of woods or windbreaks can keep properties free of strong winds and offer shelter and protection to plants to some degree. Again, if a patch of woods or a windbreak is along the northern or western edge of a property, the property will be protected from the drying effects of summer and winter winds.

The best way to determine your microclimates is to observe your property during the year. See which areas remain the warmest, see which dry out first, see where frost lingers. Jot this information down in a notebook. You can then use the information you gather to your advantage, and perhaps try to push the envelope a bit by taking advantage of these microclimates. You might not be able to plant a banana tree, but maybe you can plant a holly or an azalea, or perhaps even a Southern Magnolia like I have, by discovering and managing those microclimates in your yard!

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