Whether you know it or not, microclimates exist all over your property. Formed by houses, fences, trees, shrubs, water features and paved surfaces, microclimates create subtle but very real differences in the temperatures and conditions around your home. For example, one area is shaded, but a spot just a few feet away is in full sun. As a result, the temperature at midday between the two spots might vary by as much as 10 or 15 degrees. That can ultimately determine which plants thrive and which don't.
Once you identify microclimates in your yard and garden, you may be able to expand the number of plants you grow. Master gardener Paul James has been able to cheat the USDA's plant hardiness zone system by one entire zone. For example, in his Zone 6 garden he can grow plants better suited to Zone 7, possibly even the northern reaches of Zone 8, by providing protection in the winter. By finding sites that offer protection from the hot summer sun, you can grow plants normally suited to northern climates (lower zones) in warmer climes.
A willow oak, for example, is not dependably hardy in Zone 6, but if it is planted against a south-facing stone wall, it might receive the protection it needs to survive. The stone wall will absorb the heat of the sun during winter days and slowly release it at night. Although that placement may increase the surrounding soil and air temperature by only eight or 10 degrees, that's the temperature difference between one zone and another. If you don't have a stone wall, you can accomplish the same result by using stones or river rock as mulch. Plant small hardy plants near paved surfaces to achieve the same effect.
A Canadian hemlock, which hates temperatures above 95 degrees, seems to be doing fine in the moist, shady site that James has selected for it. Even though the ambient air temperature in this area often climbs above 100 degrees--sometimes for days--the spot offers enough protection to help the plant survive.
If you don't have a suitable microclimate for the plants you want to experiment with, consider creating one. These tips can help:
- To create shade, put up a fence on the south side of your property.
- If you've got nothing but shade trees and want a spot for sun-loving perennials, thin the canopies of your trees to let more light through.
- Two important resources James uses are the USDA Hardiness Zone Map and the American Horticultural Society's new Heat Zone Map. The two indicate whether a plant will survive the winter cold (USDA map) and summer heat (AHS map) in your area. Both maps divide North America into 12 zones--the USDA map according to average minimum winter temperatures and the AHS map according to the average number of days per year that the temperature remains above 86 degrees Fahrenheit.
If you experiment with microclimates and arm yourself with these zone maps, you may be pleasantly surprised by how many new and different plants you can grow, regardless of which zone you garden in.
