With the recent change of “Plant Hardiness Zones” from the USDA, I have received a few questions about how that may change what happens in one’s backyard or back pasture. While changes in the boundaries of a national map may not affect the weather around your place, knowing the climate specific to your piece of ground may be of great interest.
While the new USDA map shifted some of the zones around, the map does not know that you have a south-facing brick wall that absorbs heat during the day and radiates heat at night. The map does not know that you have large oaks and pine trees in the front yard but have an open, sunny backyard.
Every landscape has a uniqueness that may help or hinder selected plants.
Folks who live on top of a hill will generally be warmer than their neighbors whose home is at the bottom of the hill where the colder air settles. Folks in town surrounded by more paved surfaces, and large buildings, will hold heat better than a lone house in the middle of the country just a few miles away.
All that information I listed above is climate data, the general conditions over a fairly large area. Microclimates are smaller areas with features that cause local conditions to deviate from the average. Consider how the temperatures in downtown Gettysburg are higher than the temperatures just a few miles outside of town. Gettysburg’s buildings paved surfaces, and lack of trees hold heat, making the town warmer. A gardener in town has a growing season that is a few days longer than if she were gardening even five miles away.
But we can find microclimates that are more localized, even within one’s backyard, in or out of town. Being a gardening nerd, I keep track of temperature with two thermometers: one in the shady front yard and one in the sunny back yard. There is a considerable difference between the two, depending on the amount of sun on a particular day, how dry it is, the direction of the wind, and several other factors. I’ll bet if you spend time really looking and sensing your yard’s environment, you’ll find there is more than one microclimate in your yard, too.
Most gardeners already know about the warmth of the side of a building or a rock wall. Whether it’s the buildings in town or your yard, stonework, and masonry store heat. If you’re impatient to get your tomatoes earlier or you want to grow plants adapted to a southern climate, you put your botanical treasures along a rock wall or the side of the house, a warm spot. But we also know that warm soil dries out much faster, so your special plants should be placed in the soil with a lot of organic matter mulched to hold moisture, and watered regularly in hot and dry weather. Shade cloth adds protection on hot, sunny days, too.
Moving air is a condition that is tough on plants. During the winter many sensitive plants suffer more from desiccation than cold, and your prized specimens need some sort of shelter from the winter winds. But even in the summer, a windbreak retains humidity, helping plants hold valuable moisture and keeping the ground from drying out.
Check the location of shade through the gardening seasons. The sun is not in the same place now that it will be in June and shadows change position through the year, too. This year the Summer Solstice is on June 21, and from our position on earth in Adams County, PA, that’s the day the sun will rise and set in its most northerly position and will be as high in the sky as it reaches at our latitude. From now until the summer solstice, shadows move and get shorter until June 21, when they are the shortest. Then the sun starts moving lower in the sky and towards its southernmost extreme on the Winter Solstice.
What this means to a gardener depends on what is being grown. The gardener needs to know where to put plants that need full sun, part shade or shade. The position of the shade and how long a spot is shady varies through the year because of this apparent movement of the sun. On April 8 the sun will be in the same position as on September 3, the Saturday of Labor Day weekend. Of course, there is less shade under trees in April, since they haven’t leafed out yet. Keep that in mind as you evaluate your yard.
One of my tricks is using tall plants to create a microclimate for shorter, shade-tolerant ones. To keep short, leafy plants cooler on hot days of summer, I put them in the shade of taller plants. Lettuce planted between tomatoes is a useful pairing, along with short, shade-tolerant plants beneath trellised green beans. Vining squash grows well within the cover of a block of corn, where it shades the ground and holds moisture.
Consider areas of your yard that are prone to frost, too. Cold air collects in low areas and can have a light frost when areas uphill are frost-free. Tall plants help protect short neighbors in this situation, too. Since low areas are slow to warm in the spring and will be the first to be affected by frost, it may not be wise to put vegetables with a long maturation time in this area.
Spending time getting to know all your yard’s environmental quirks can reveal gardening situations that make your specimens thrive or languish. You can make your trip to the garden center armed with information to make great choices. You can also know, within your yard’s limits, what you can do to make a particular choice work.
