Most gardeners are aware of what planting zone they are in. Knowing your planting zone lets you select plants that are hardy in your area but every gardener should also know about microclimates and how they impact the plants in your garden.
Microclimates are small areas within a larger planting zone that have different characteristics than the zone as a whole. They may be warmer or colder. And microclimates can also refer to whether the area is dry or wet, and whether it is sheltered from the wind. You may have several microclimates on one piece of property. A large city may constitute a microclimate within a planting zone and within that city different yards may have different microclimates.
Microclimates are created when some feature of the landscape changes weather conditions. It could be lots of pavement and buildings which trap heat or a low, moist area that cold air sinks into. It could be a courtyard or a wall or solid fence or a pond, things that affect the air and soil conditions. Shade and exposure to sun and wind also define microclimates.
To give your plants the best possible growing conditions a gardener needs to be aware of microclimates. Gardeners in northern areas are generally happy when they find a microclimate spot that lets them grow plants that might not be quite cold hardy in their area- they like warmer microclimates. Those in warmer areas may like microclimates that allow plants that don’t like heat to be happier.
The areas near a large body of water are generally cooler in the spring and warmer in the fall than the surrounding areas. This may be helpful to keep some things like fruit trees from blooming too soon and extending the time in the fall that fruit has to ripen. The cherry industry on Michigan’s west coast takes advantage of that microclimate along Lake Michigan.
How do you find microclimates on your property? Careful observation and educated hunches are your tools. Spots where snow or frost doesn’t melt until long after other areas are cold microclimates. Soil still frozen after other areas have thawed indicates a cold area. Areas where water pools in the spring and fall are generally cooler areas. Hollows, dips, valleys, ditches, all lower spots are generally cooler than the surrounding area. The north side of a slope/bank is cooler. Watch for plants that emerge later or bloom later than those in other areas.
Areas where plants emerge and bloom sooner than surrounding plants may be warm microclimates. A dark colored house or house foundation, stone walls or dark pavement in full sun absorbs heat and stays warmer through the night. You may see dandelions or chickweed blooming there long before they bloom in more exposed areas. Raised beds, and elevated, well drained areas are often warmer than other places. Nooks and alcoves, courtyards, islands in paved areas
Summer microclimates can be important for some plants as well as winter microclimates. Areas that are very hot and humid in the summer may not suit some plants as well as areas that may be just as hot but drier. And some plants need and thrive in high humidity. Some nurseries have attempted to define summer hardiness zones as well as winter hardiness zones. If you find plants labeled with heat hardiness ratings pay attention and do your homework to see if your type of heat is suitable for that plant.
Lighter colored house walls or fences may reflect so much sun back onto plants that they burn them or dry the soil out quickly. Areas surrounded by pavement may get very warm in summer. Rocky, sandy soil heats up and dries out quickly. Some plants will not thrive in that microclimate. Too much heat can be as bad as too little in many cases. But careful plant selection can generally find a compatible plant for the conditions.
If you are a gardener who longs to grow a plant that’s not supposed to be winter hardy in your area you may want to look around your property for a warmer microclimate. Don’t expect to grow oranges outside in Michigan; at best you will probably raise the survival rate by one zone, unless you build a heated greenhouse. For example if your planting zone is rated zone 6 and you want a plant whose hardiness is rated zone 7 to overwinter in your yard, you could plant it against the south wall of the house in full sun in light, sandy soil. You may need to mulch some plants (lavender doesn’t like heavy mulch) deeply to help them survive. Or you could put up a hoop house or unheated greenhouse to protect plants a bit.
A food gardener who has an exposed, low area for a garden spot may want to enclose it with a solid or slatted fence and use raised beds to provide a better microclimate. That’s especially true if the soil is heavy clay. Most food gardens in wide open areas can benefit from some kind of windbreak on the north and west sides. Creating a favorable microclimate gets you gardening faster in the spring and lets your crops have a longer growing season.
Soil moisture levels also define microclimates. Areas that don’t drain well, are boggy or swampy can occur within property that is generally well drained. In those areas you’ll need to select plants whose root systems can thrive in wet soil. This can bring some interesting plants into your garden. Or you can improve the drainage in some way and eliminate the odd conditions. Remember wet spots are often cooler than surrounding areas too.
Remember that microclimates can change over time, fences get removed, windbreaks grow tall and provide too much shade, drainage patterns may change and so on and you may need to change the plant species that grow there too. Sometimes a less than hardy plant for your zone will survive one or two winters because the temperatures were warmer than normal- we’ll see this more as the climate warms- but then die when we have a “normal” winter. In that case you didn’t really have a microclimate.
Not everyone has a spot on their property to “cheat” a growing zone. When selecting plants whose hardiness is less than your growing zone rating be prepared to lose it. It doesn’t hurt to take a chance, as long as you can afford the loss. If you suspect your garden may have a bit of a cool microclimate choose plants whose hardiness rating is one or more zones lower than yours. For example if you are in zone 5 you may want to choose plants that are hardy to zone 4.
When a new plant or plant variety comes on the market the zone hardiness may not be precisely defined yet. It may take growing it in many locations by many people for a few years to define its hardiness. Experimenting and reporting on your experiences can help other gardeners make better choices.
Kim Willis lives near Clifford, Michigan on a small farm that she shares with her husband and numerous animals. She worked at the Lapeer County MSU Extension office for many years but is now retired. She is a freelance country and garden writer. Her book Complete Idiots Guide® to Country Living was published in November 2008. Her best selling chicken book, Raising Chickens for Dummies® was published in 2009. She also wrote Knacks Canning, Pickling and Preserving (2010) and Cooking with Beer (2012).