Sustainable Gardening

How to Use a Microclimate in Your Garden

Brick wall creates a microclimate for this Echinacea

Gardening is like real estate – Location, Location, Location. It’s also like a business – if at first you don’t succeed, try, and try again. Gardening is about following rules as well as breaking them, about learning from experts, and exercising self-expression. Finally, gardening is about microclimates and how they can assist you in all of the above.

Know Your Gardening Zone

When choosing plants, it is important to know your zones. USDA hardiness zones are based on the average annual minimum winter temperature and tell us which plants are most likely to survive and thrive a winter in our location. There are 26 hardiness zones (from 1a to 13b), which shift at five-degree increments. The Triangle is in USDA hardiness zone 7b, which is based on an average extreme minimum temperature of 5 to 10 degrees Fahrenheit.

There are also heat zone maps put out by the American Horticulture Society (AHS) that estimate heat tolerance of plants. Heat zones count the number of “heat” days with temperatures above 86 degrees Fahrenheit. There are 12 heat zone ranges from one, with zero heat days, to 12, with 210 or more heat days. We reside in zone 7, which has 61 to 90 heat days. The zones provide a rough picture of our current climate, however, it may shift in the future due to climate change.

Some ornamental plant labels include information on both rating systems and show the ranges for heat tolerance as well as winter hardiness. This information can be helpful in plant selection. For example, Hydrangea paniculata can tolerate winter in zones 3 through 8, and is heat tolerant to AHS zones 1-9, whereas H. quercifolia has slightly different requirements tolerating USDA zones 5-9 and AHS zones 3-9.

Using a Microclimate in Your Garden

The zones are a starting point, as microclimates also play a role. Microclimates are areas that have atmospheric conditions that differ slightly from the surrounding areas. Microclimates can be small (a few square feet) or large (several square miles). Factors that affect microclimates include both natural features like bodies of water and topography and man-made features such as areas of pavement and buildings. Factors that define a microclimate include temperature, moisture, soil conditions, elevation, and ground-level winds.

Astilbe

Astilbe by a downspout / Cynthia Sollod

You can take advantage of microclimates in your yard to increase the gardening potential of your landscape. For example, you can use structures and protected areas in your yard such as the south side of your home, to extend the growing season of frost tender plants perhaps even pushing the zone classification to an 8a, allowing you to plant something that might not otherwise survive. The bricks from your home absorb the warmth of the sun later releasing it into the evening, thus raising the air temperature. They also protect plants from wind. Other structures that can absorb and reflect heat include fences, sheds, a notch near a chimney, or even a large driveway. Another trick is to use dark mulch in the winter to absorb some solar heat. Tall structures and even other plants can act as windbreaks to protect more tender plants.

You can also use a microclimate to cool things off. Some sun-loving plants may want a break from intense Southern heat. In this case, a structure can cool the surrounding air by providing shade. Growing plants on the east side of your house will provide a shadier and cooler afternoon microclimate. Again, using other plants can also create a microclimate. Sun-loving early spring bloomers typically like a little afternoon shade once temperatures start to climb. Plant them strategically where other plants can provide late day shade as needed. Save the full sunny spots for plants that can take the afternoon heat.

Water availability throughout your yard also creates microclimates. Low areas tend to stay wetter, as do areas that have heavy clay. Use these areas for plants that tolerate or enjoy wet feet – think ferns, Astilbe, water iris, and lily of the valley, among others. Don’t have a low wet area? Try planting near a downspout, or better yet, your air conditioning drip line. This is a great spot for summer-long moisture, even during droughts.

Experimenting with microclimates is a great way to exploit variation in locations, break the rules about what may grow in our zone, try and try again new plants, and most of all express your own gardening vision.

Featured image: The brick wall makes a microclimate for this Echinacea / Beverly Hurley.

Cynthia Sollod has always loved plants, hence a B.S. in botany and a PhD in Plant Pathology. She has volunteered with the Wake County Master Gardener program since 1995. She enjoys painting, illustrating plants and writing about gardening.