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How To Understand Growing Zones & Hardiness Maps To Plant Smart

If there is one thing that seems to baffle gardeners, it is trying to understand growing zones and hardiness maps.

It’s easy to see why it can all be so confusing. What might be a perennial in one area can only be grown as an annual in others. While someone living in a certain growing zone might be able to successfully grow the same plant that someone in the exact same zone can’t.

How in the world do you make sense of it all? Well hopefully today’s article can help shed some light on this oft-confused subject. And at the same time, help to make these maps a better guide and resource for future plantings.

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Understanding Growing Zones – How The USDA Creates Their Hardiness Zone Map

One thing is for sure, knowing your growing zone is extremely important for all types of gardening. And that includes vegetable gardens, annual flowers, perennials, and even trees, shrubs and bushes as well.

To start, it’s first important to understand how and why the USDA creates their Hardiness Zone Map & Growing Zones.

The map is created to help determine which plants can survive and thrive in specific climatic areas. Within the map, individual growing zones are separated by 10 degree Fahrenheit increments for each zone.

knowing your growing zone
Knowing your growing zone helps you determine what plants are likely to survive your winter, and which aren’t.

These zones then correlate to which plants can survive in each climate area. Or more precisely, which plants can survive the harsh range of low temperatures through winter.

The zones on the U.S map range from 1 to 13, with 13 being the warmest winters of all, and 1 being the coldest. Each zone is given a number relating to the average coldest temps in that specific zone.

In some cases, zones are separated even more with a sub-zone. These are indicated with an “a” or “b” beside the zone number.

This is done when there is an even slighter variation within a specific zone’s temperature range. For instance, if you live in growing zone 5a, your plants need to be able to weather temps that can get to -15 to -20 Fahrenheit. 5b is slightly warmer with a range of -15 to -10 Fahrenheit.

Knowing & Understanding Growing Zones

So how does the USDA comes up with the temperatures for the separation of zones?

understanding your growing zone
The USDA creates their hardiness maps by taking an average of the lowest extreme minimum temperatures. Although some plants can survive a hard freeze, for many others , it can spell the end of the road.

They create their zone hardiness map based on the averages of extreme minimum temperatures over a long period. 30 years to be exact. Another words, just because you may live somewhere where the temperature once fell to 20 below, you will not be in the -20 growing zone.

By using an average of the extreme minimums over a lengthy period, they can then create maps that show the most likely issues plants will face from freezing temperatures. It is very important to realize it is an average over a long period of time – not a single season.

Benefits of Knowing Your Growing Zone

From there, a gardener can, with fair confidence, select plants suited for his or her growing zone. But, of course, it is not a perfect science for sure.

It’s important for gardener’s to realize that the zones on the map are not exact. Instead, they are guidelines to grow what is most likely to succeed in your area.

growing zones
Some plants need extra protection to survive an extremely cold night in their growing zone.

Extreme cold can hit any area beyond the norm. And without some protection, plants can and will freeze out.

Obviously, a palm tree rated for zone 9 will not thrive in Zone 3. But it could very well be the case that a plant suited for zone 6 can still be grown in zone 5 with added winter protection.

But there is one other little factor that can play a big part – and it is known as a microclimate.

Understanding Microclimates within Growing Zones

Microclimates can easily exist within specific growing zones. And these can actually change the hardiness zone for just that specific area. Believe it or not, even within the space of your own backyard!

A micro climate can exist within a space as little as a 1/4 acre backyard.

So what are Microclimates? They are tiny variations within a climate that occur because of an added condition. A great example of this are hills and valleys. Valleys naturally trap cold air as the warm air rises. They can sometimes be 5 to 10 degrees colder than the peak area of the hill.

It also exists in some plantings near brick or concrete. These surfaces absorb heat and then release it back into the space nearby. Because of this process, they can make a specific area warmer.

Working With Microclimates

Believe it or not, these small changes can allow for plants that can or can’t be grown in an area to thrive – or in some cases, freeze out. Rhododendrons and rose bushes are both notorious for working well in one area of a yard, and failing elsewhere due to microclimates. See : Growing Rhododendrons

Microclimates create growing different growing conditions within a small geographical area.

So how do you account for these variations? Unfortunately, only time and experience growing in your specific area will really tell the tale. But with that said, understanding your growing zone is the best way to select plants that will work best from the start.

As a wonderful free resource, the USDA Zone Hardiness Zone Finder on their website. Simply put in your zip code, and you instantly find your exact growing zone. You can’t beat that!

Here is to growing with success by better understanding your growing zone!

This Is My Garden is a website dedicated to spreading the love and knowledge of gardening around the world. We publish two new garden articles each week. This article may contain affiliate links.