Growing Outside of Your Zone: Creating Microclimates
Written by Patricia Lunn Adsit, Guilford County extension Master Gardener Volunteer
“You can’t grow lemon trees here. They are tropical plants, not meant for our zone.” This was a recent statement made by a friend. If you are an adventuresome gardener like me, you recognize a challenge when you hear it.
When you go to a nursery or garden center and select a plant, most selections will have a tag that shows the United States Department of Agriculture (USDA) Hardiness Zone. The USDA created Hardiness Zones in order to guide growers and gardeners in the States with an easy way to “determine which plants are most likely to thrive at a location.” The Zones are defined by “the average annual minimum winter temperature (over a 30-year period), divided into 10-degree F zones.” The Zones are then subdivided into sections A and B, based on 5-degree F increments. Remember: these are not the coldest temperatures ever experienced or likely to be experienced in an area. If you visit the Agriculture Research Service (ARS) webpage, you can simply enter your zip code to find your Zone. For reference, I live and garden in Hardiness Zone 7b, 5-10 degrees F. And that Meyer Lemon Tree I mentioned is indeed a Zone 8-10 plant.
In addition to the USDA Hardiness Zone, other things that must be taken into consideration when choosing plants include:
- Amount of Light: To succeed, consider the amount of light a plant receives during its entire growth cycle, factoring in winter light. A plant designated as a shade plant will not thrive if placed in a full sun location.
- Amount of Moisture: Plants rated hardy for your zone can be injured if they endure periods of “moisture stress.”
- Soil Type: Plants that thrive in acid soils will suffer in higher pH soils, and vice versa. For success, always test!
- Care throughout the Growth Cycle: Proper planting and pruning practices often determine the success or failure of plants. And paying attention to the how the plant responds to the annual frost-free cycle (from average last date of frost to the average first date of frost) in your area is important.
- Environmental factors: Plant success can also be determined by other factors such as wind, pollution, and localized microclimates.
This last “consideration” includes microclimates, which are defined by the USDA as “…fine-scale climate variations (similar to) small heat islands—such as those caused by blacktop and concrete—or cool spots caused by small hills and valleys. Individual gardens also may have very localized microclimates. Your entire yard could be somewhat warmer or cooler than the surrounding area because it is sheltered or exposed. You also could have pockets within your garden that are warmer or cooler than the general zone for your area or for the rest of your yard, such as a sheltered area in front of a south-facing wall or a low spot where cold air pools first. No hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.” [The information is from the USDA; the emphasis is mine.]
Is there more than one type of microclimate? Yes. According to Cornell University, “(m)icroclimates may be quite small – a protected courtyard next to a building, for example, that is warmer than an exposed field nearby. Or a microclimate may be extensive – a band extending several miles inland from a large body of water that moderates temperature.”
Large microclimates, such as those created by lakes and reservoirs, urban asphalt, cold valleys, and windy hilltops should be taken into consideration when planting for success. However, there is little that a gardener can do about the growing environment that these large-scale microclimates produce, other than use them to our advantage.
Still, we can enjoy the smaller microclimates that are native to our property and that we can fashion for better gardening success when planting. Considerations and cautions include:
Focus on structures: Your house, garden shed, and other buildings create many small microclimates throughout your property. The structures absorb heat during the day and radiate it back at night. They can block chilling winds from the north or northwest, creating warmer, more sheltered microclimates on the south and east sides of the structures. Just like your house or shed, fences, walls and large rocks can protect plants from wind and radiate heat, creating sheltered spots. Remember, if structures block cold air in one portion of your property, the cold air can pool behind the structures, creating a circumstance of localized frost damage on near-freezing nights, which is another type of microclimate.
Focus on hard surfaces: Driveways, sidewalks, patios, and paved paths can absorb heat during the day and reradiate it at night, moderating night-time temperatures. If impervious, these areas can’t absorb water, and may create wet spots if the water that flows off of them goes into one, concentrated area. The same can be applied where water flows out of downspouts and gutters or off of roofs.
Focus on height, spread, and orientation of trees and current plantings: Tall trees can create excellent microclimates on your property by shading and protecting plantings, but they can also prevent rain from reaching the ground and provide too much sun protection in the summer, too little in the winter, especially if deciduous. Another consideration about trees: competition for water and nutrients created by the roots may make it problematic to grow less-competitive plants around the base.
Focus on season extenders: While we cannot easily arrange and rearrange structures, hard surfaces, and trees on our property, we can incorporate a number of so-called season extenders to add productive days to the normal frost-free growth cycle for your Zone and create microclimates to our advantage:
Utilize raised beds and containers to not only promote early growth, since they warm up sooner in the spring than the rest of the ground, especially if one edge of the bed or pot faces south, but also to provide fewer challenges to growth than native clay soils.
Use row covers. “Woven polyester row covers have the advantage over clear plastic covers because they allow air circulation around the plants. Structures that surround plants…help store heat during the day and release it at night to protect plants from frost,” said George Dickerson, horticulture specialist with New Mexico State University’s Cooperative Extension Service.
Incorporate mulches to create desired microclimates by warming the soil (black plastic), cooling the soil (alfalfa and/or wheat straw), controlling weeds, and conserving moisture.
What does this mean in practical terms? Let’s say you want to have lettuce for your sandwiches and spinach for your salads outside of the “normal” growing season for these vegetables. You can tap into the benefits of creating microclimates in your vegetable garden to accomplish this goal. By utilizing south-facing raised beds in full sun you can get your seeds planted a week or two earlier in the late winter. By planting in containers you can move to a shaded area in the summer, you can extend the productive period for these cool season crops. And, by using winter-weight row covers in the fall, you can protect these tender plants from the falling temperatures…often extending well past the first frost.
Or, maybe you want to grow a Meyer Lemon tree (Citrus limon ‘Meyer’, Zone 8-10) in Zone 7b. Creating a microclimate by using a container that can be moved to a full-sun location in summer and an enclosed porch in winter just might be your answer. It was mine.
Gardening by using the concept of microclimates is, of course, a risk. There may come to your Zone 7 property an extremely cold winter that cancels all the benefits gained by planting that fragrant gardenia (Gardenia augusta rated for Zone 8 and above) in the microclimate created by a south-facing brick wall. Then again, that gardenia may thrive in its microclimate and produce blooms for many years to come.
All gardening, in the end, involves risk. And, I happen to agree with the following:
In the real world, we garden in microclimates, not hardiness zones.
– Charlie Mazza, Senior Extension Associate,
Cornell University, Department of Horticulture Gardening Resources, Microclimates. Accessed on 2/7/2014 from http://www.gardening.cornell.edu/weather/microcli.html
New Mexico State University, Microclimates and Mulches Give Garden Crops an Early Start. Accessed on 2/7/2014 from http://newscenter.nmsu.edu/5885/
USDA Plant Hardiness Zone Map, 2012. Agricultural Research Service, U.S. Department of Agriculture. Accessed on 2/7/14 from http://planthardiness.ars.usda.gov