Last year the US Department of Agriculture released a new Plant Hardiness Zone Map, which is based on average minimum winter temperatures over the prior 10 years. Most locations were given a somewhat higher categorization on this new map to account for global warming. So, because of ongoing climate change, gardeners may be seeing earlier last-frost dates and longer growing seasons as well. What you can expect, of course, depends on where you live. The range of zones in the US is enormous, from Zone 1a in the harshest parts of Alaska (minimum winter temperature -60 to -55°F) to Zone 13a in the mildest regions of Hawaii (minimum winter temperatures 60 to 65°F).
But be aware that the winter temperatures you actually experience in your garden can be quite different than the averages indicated on the Plant Hardiness Zone Map, especially in different parts of your garden. In other words, your garden may have variable microclimates. For example, I know that I can grow fairly large fig trees on the south side of my home because the winter winds come primarily from the north, giving this part of my outdoor garden a winter microclimate that is quite sheltered in winter.
In spring, the microclimate conditions in my garden change because there’s a large city 20 miles north where buildings and pavement heat up quickly drawing in cold air off the ocean to my south. Consequently, my garden experiences strong southerly spring winds coming across the cold ocean waters. These cool winds have significant microclimate effects, causing the flowers in my garden to bloom about two weeks later than those in the nearby village with its more protected coastal location.
By considering microclimates, you can better manage what you grow. For example, if you have a stone wall running east to west and your prevailing wind is northerly, the south side of this wall is an ideal microclimate space where temperatures might be as much as 10°F warmer than only 10 or 15 feet away. Leaning a large piece of glass against this wall’s south side might create an even warmer microclimate where you can start tomatoes or peppers a week or two earlier.
Microclimate considerations also apply to the siting of a greenhouse. By putting a greenhouse in a warm microclimate, you’ll save on long-term heating costs if you use the greenhouse in the winter. There are also microclimates in which you should not put a greenhouse. Examples are in a dip where cold air might accumulate; in an area where the north side will be exposed to cold winter winds; in an open space with no protection from the wind in any direction; and in an area shaded by evergreen trees. This last consideration doesn’t apply to deciduous trees. Deciduous trees can be beneficial to a greenhouse in that they shade it from extreme heat in the summer but shed their leaves in winter, allowing more light to reach the glass. So this is an example where the microclimate changes seasonally. By keeping in mind variations in microclimates and how they can be used advantageously, a gardener’s success can be greatly enhanced.
