Except when it isn’t.
Sometimes, knowing the hardiness zone isn’t quite enough, because your fruit garden’s unique topography, natural or manmade structures, and other factors conspire against you to trap cold air in a microclimate significantly colder than the zone. There are few garden chores more depressing than having to dig out a young fruit tree that didn’t survive the winter, in spite of a zone rating that should have given it no excuse to fail. Building your awareness of the factors that can affect your gardening zone can give you the best possible chance for success in growing hardy fruit trees and plants.
HOW NATURAL TOPOGRAPHY AFFECTS YOUR HARDINESS ZONE
Arguably the biggest factor affecting your gardening zone will be the natural topography of the land – the landscape and features such as hills, valleys, and other features not made by human hands. Weather is a natural phenomenon, after all, and it works in sync with other natural features of the globe. For the gardener trying to coax fruit trees and plants to survive a cold-zone winter, there are a few things to be aware of when analyzing whether you have a microclimate deviating from your zone’s official rating:
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- Cold air sinks. Valleys tend to become “puddles” of colder air that linger, forcing temperatures lower for longer.
- Hills, ridges, and other geographical structures can block sunlight for many hours of the day, particularly in the winter when the sun hangs lower on the horizon. This forces temperatures lower.
- Hills, ridges, and other geographical structures can also serve as windbreaks OR wind tunnels. Depending on placement, they may block harsh winter winds from reaching your fruit garden, OR they may funnel them directly across your tender plants.
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HOW MAN-MADE STRUCTURES AFFECT YOUR HARDINESS ZONE
Believe it or not, your proximity to population centers does affect temperature. Cities and urban centers have a disproportionate amount of concrete and other structures that act as a natural heat-sink; the sun’s warming rays are absorbed by cities, but they don’t release that heat right away. This, combined with high energy of cities and some other factors, creates a phenomenon referenced to as an Urban Heat Island, where temperature is measurably different than surrounding areas. For the urban gardener, you should keep this in mind, particularly when you have patio trees that need a certain number of chilling hours to thrive.
Similarly, manmade structures such as walls, buildings, and others can create a microclimate within your hardiness zone by blocking sunlight and extending the number of shaded hours in the day; blocking or redirecting winds; and other means. However, some of this you can turn to your advantage, by planting a more sensitive tree or shrub in front of a brick wall that acts as a heat sink, or in a corner where the wind rarely reaches.
HOW BODIES OF WATER CAN AFFECT YOUR HARDINESS ZONE
Large bodies of water have an odd positive-negative affect on growing conditions in your garden. Where I live, I’m in the impact area for heavy lake-effect snowfall catalyzed by the Great Lakes in the northeast. That snow is a bear, crushing fragile twigs on my berry bushes and giving rabbits and deer a survivalist motivation to gnaw the bark off my fruit trees. (Read more about how heavy snow affects fruit gardens.)
On the other hand, close proximity to large bodies of water can effectively increase the temperature of your area. Just like urban centers, water acts as a heat sink. If you live near an ocean or close enough to a large enough lake, some of that latent heat will act as a buffer, helping keep your temperatures higher than your zone might otherwise allow.
On a smaller scale, features such as rivers, streams, and ponds tend to throw off a lot of fog in spring and fall. If your area tends to have rapid flash-freeze temperature swings, I would be nervous about young plants susceptible to ice injury. Freezing fog is a real thing, and although you probably won’t run into it if your nearby body of water is small, it is something to be aware of.
The factors listed above are some of the biggest wild cards that can create a gardening microclimate that completely throws off your hardiness zone – but don’t take this as an exhaustive list. There are many, many factors (large and small!) that can impact your fruit garden, even when you believe you’ve selected the best possible cold-hardy fruit trees and plants.
My personal rule of thumb is to look for cultivars that give another zone’s worth of winter hardiness; back when gardening catalogs used to insist that I was in zone 5, I always looked for cultivars tolerant to zone 4 — and I’m glad I did, because it wasn’t long before the maps were corrected to show what I already believed, that yes, my property is in fact considered a in a zone 4 hardiness zone. Today, if given a choice, I look for plants tolerant to zone-3 temperatures, but it’s tough unless you’re looking for the most basic types (think apple trees. I have enough, thank you). The bottom line is you should be aware that your zone “on paper” isn’t always reflective of your microclimate’s actual winter weather, so make close observations, build your knowledge, and be prepared to adapt your gardening strategy when your microclimate deviates from your hardiness zone.
Not sure what fruit trees will work in your zone? Check out my article on how to find cold-hardy fruit trees for zone 4 and colder.