You’ve probably seen those maps showing contrasting stripes of yellow, green, blue and pink. You may have wondered why growing zones matter to you.
Those pretty stripes contain valuable information that will aid you in growing a garden you are happy with. They will tell you which plants are best adapted to your climate and help you narrow down what kinds of plants you should grow.
If you want to save money and time, you shouldn’t waste them trying to grow plants that won’t thrive outdoors. Even though I’d like to grow oranges, they wouldn’t be suitable for my Kentucky farm.
What Are Planting Zones?
The United States Department of Agriculture (USDA) conducts research on agricultural planting zones for gardeners to use as planting guidelines. It is called the USDA Plant Hardiness Zone Map (PHZM). The zones on this map are also sometimes referred to as planting zones, growing zones, or hardiness zones.
Growing zones in North America are divided into thirteen groups, each of which has an “A” and “B.”
A and B zones are further divided by an average of 5°F difference between them. The zones are divided by ten-degree increments based on winter averages.
Hardiness zones are determined based on the average temperatures over a thirty-year period. It’s all about averages – not extremes. Some zones may get much colder than average from time to time.
As an example, I live in zone 6, which has an average winter temperature ten degrees warmer than zone 5. In my area, the low temperature has fallen to the -30s, despite zone 6 having an average low of between -10 and 0 degrees.
How Often Are Growing Zones Adjusted?
A map of climate zones is regularly updated as necessary. One major change occurred in 1990 when USDA scientists began looking at data and the effects of climate change. Another change occurred in 2012 due to a change in average temperatures around the country.
Many of you may have noticed that your own planting zone has changed over time. In fact, if you look at the USDA map from 1990 and the USDA map from 2012, you will see many subtle changes.
Despite the fact that we might have changed the hardiness zones in your area, your plants are not concerned with that. If they’re happy, you’re happy. However, as you plant new things, you may want to consider whether the climate is changing.
Does Climate Change Affect My Zone?
If you’ve been gardening for a long time, like myself, you’ve probably noticed how climate change works. November is often warmer than usual, instead of the chill of the past.
Last November, my peach tree began to make buds, and I pleaded inwardly “stop that, you are using up the energy you’ll need for spring”!
When I was young, it was customary to plant tomatoes after the first of May. Now I usually plant tomatoes in mid-April, sometimes even earlier since I like to push the boundaries.
Hardiness Zones are Shifting North
There has been an increase in temperature of up to 3 degrees at 95% of USDA’s data collection spots in recent years. The growing zones are shifting northward.
It’s possible to go online to Climate Central and find a variety of maps showing how the climate has changed and how zones have changed in different parts of the country.
There is evidence supporting the shift in growing zone designations. In 2010, the Forestry Service published a study titled Study Suggests Tree Ranges are Shifting Due to Climate Change which pointed to tree species that are expanding further north.
This study notes that “spaces north of where a particular plant species previously did not flourish may now be suitable, and thus the range of the species will have ‘moved,’ with more seedlings growing and thriving beyond what they did before.”
There is a concern that some trees will disappear from certain planting areas and become more common in planting areas further north. The sugar maple, which makes maple syrup, is one of the species at risk. According to the forestry department, sugar maples may move to Canada, causing economic losses to farmers in northeastern states and Ohio.
Climate Change and Invasive Species
Some of the most problematic plants we may have on our property are those that have taken advantage of the warming climate. Kudzu, honeysuckle, and wild mustard are just some of those species that are benefiting from a warmer climate.
You may have pests moving into your planting zone and destroying the plants in your garden. Not only should you watch out for new plants, you should also be on the lookout for insects and mammals making their way into your area.
Earlier this year of 2021, the Department of Fish and Wildlife in western Kentucky announced that an armadillo population had been discovered – typically a Texas mammal!
More Accurate Tools to Monitor Data
In addition to better technology, the maps themselves have been revised. Temperature data is measured all over the United States at 244 stations. This information is then compared with previous years. As conditions change, the maps are revised, too. With advances in technology, we have become more capable of detecting even the most minute changes in temperature.
As you can see from the map, the USDA’s system takes into account similar microclimates such as bodies of water, as well as elevations, which is why you’ll see smaller circles with a number and arrow pointing to them.
Microclimates
Consider the possibility that your area may have microclimates – or even your own yard. Certain aspects of your area may produce a particular climate, like elevation, wind direction, and the amount of sunlight.
Your yard most likely has microclimates. Do you have an area near a brick wall where you enjoy gardening? Or maybe have a low spot in your garden. These are likely to stay warmer during the growing season.
In some microclimates, the temperatures are cooler/warmer than in surrounding areas. That means rosemary may live all winter long when, for instance, my neighbor next door may not. My friend Susan lives three miles away from me without the benefit of a fence. She gets blossoms on her trees about 7-10 days before I do. The daffodils and strawberries in her garden always bloom before mine.
She’s not the best gardener, but she lives on a ridge and the heat of early spring rises to her location.
These microclimates aren’t reflected on the map. The USDA cautions that all PHZMs are guidelines, and you shouldn’t use them as a guide. You’ll need to experiment thus.
The winter is an excellent time to observe your garden for heavy frost. You can also note where snow melts first. These clues can show you areas of your garden are warmest on a sunny day or areas that are cooler.
If you use those experiences to plan your garden, you will gain a lot of useful information.
Using Growing Zones
What do you do with this information? To begin with, any plants you choose must be able to survive the zone’s lowest temperature. For instance, if you live in zone 7, your plants should be able to survive 0°F.
It’s possible that you can experiment with this a bit depending on your microclimate by planting something that usually cannot be grown in your area in a warmer, more protected area of your yard.
One of the best things about gardening is stretching plant growth seasons. You can do a lot to extend growing seasons, and I am always adjusting my garden’s environment in order to get a longer harvest.
Imagine a perennial favorite in your backyard: the tomato. The tomato is from Central and South America, but we grow it throughout the United States as an annual. Even in zones 3 – with greenhouses – backyard gardeners can grow tomatoes.
Think about whether a plant can mature in your growing season even if it is out of your zone. Look at your frost dates and note if a plant can mature within your growing season.
Our gardens are often improved by using materials such as garden fabric, greenhouses, and shade cloth.
Getting Extra Help
If you are having trouble understanding this, worry not. There are always ways to seek local assistance.
It is always a good idea to get information from your local extension office, since they are going to have plenty of information and be able to offer advice on what grows well where you live. Many extension offices offer classes. You can also ask at your local nursery for more ideas.
Take Away
USDA growing zone maps are useful tools but you will still need to know other details to make informed choices in gardening. Knowing the dates of the first and last frost and the amount of rain expected are also important for a successful garden.
As my mother used to say, “don’t put all your eggs in one basket.” This holds true for gardening. Plan for a variety of scenarios and utilize several tools so that you can grow the perfect plants for your family.