What’s the deal with hardiness zones?

Peonies and irises combine to make a bold addition to any garden. Photos by Lyn Des Marais


BRANDON — I am often asked about what plant should be planted where. Let’s start with the basics: hardiness zones of plants and shrubs, soil type, water needs, and the amount of sunlight needed are key facts to know before you plant.  

Sales of perennials are upon us. I find that sales of plants are so hard for me to resist. I often say I’m not buying another plant only to come home with 10-12 crammed into the back seat of a car. 

But before you buy or plant, it helps to know what conditions your garden has that will affect whether that gorgeous sale plant will thrive.

Most plants sold around mid-Vermont are appropriate for Zone 5a. But there’s more to a plant’s survival than the US hardiness plant zone map.

What is the US hardiness plant zone map?

The first map of hardiness zones traces back to Boston, Ma, and the Arnold Arboretum, in 1927. The map was a guide to show interested people various perennial plants and shrubs and their ability to survive in different parts of the United States based on the lowest average cold temperature, and it was a hit.

The USDA created a map of 13 hardiness zones in 1960 with data from hundreds of US weather stations. It updated that map in 1990 and again in 2012.

The 13 zones cover the entire United States, with each zone encompassing a ten-degree change in Fahrenheit. Every zone has an ‘a’ or ‘b’ designation that splits the zones in half. Brandon’s zip code,  05733, is designated Zone 5a.

What does this mean?  

It means that the average minimum temperature of this region is between -15 and -20 degrees Fahrenheit in winter. That sounds very cold. Some might say, “But it’s not that cold for very long.” Others might say, “the wind chill makes it even colder.” Still, others say “plants can freeze at  25 degrees in the spring.”  

All of these statements are true—many factors,  including temperature, will affect the longevity of your perennial plants.

That means gardeners in Brandon (and we are lucky there are so many talented gardeners here) can plant all sorts of glorious perennials that typically thrive in our zone: tulips, daffodils, iris,  peonies, echinacea, daisies, hosta,  daylilies, hydrangea, hyacinth, turtlehead, lupin, blueberries, raspberries, and lilacs to name only a few.

Looking at all the different gardens and different combinations of flowers when walking around town is a treat.

What other factors affect whether your perennials will survive?

So many: soil type, pH of the soil, altitude, proximity to water, snow cover, wind, summer heat, rapid changes in temperature, water needs, and exposure to the sun are a few important ones.


Brandon has sand, gravel, loam, and clay as soil types. Soil sampling is an inexpensive way to help you determine the acidity of your soil before you buy plants and plant them.

Once you’ve determined soil type, you’ll know what additives or amendments to add to your garden. For example, our soil is primarily clay, so I add compost and sand to make it more granular and less compact and hard. Then there’s pH—some plants, like peonies,  prefer sweet or basic soil. Others, like blueberries, love acidic soils.

Sunlight and Water

Hostas look sharp around the base of a silver maple tree

Once you’ve got a handle on your soil type, record the hours of sun and shade in your garden and know where water runs off and where it pools. This should be done ideally the season before planting. Keep notes and grab ideas from gardens you love. Write down what you want. 

Do you want all one plant? All one color? Contrasting colors? What heights? Do you have favorites you can’t live without?  Do you have mainly shade or sun where you want your garden?

Save pictures from the internet, magazines, and newspapers. Dream, plan and think (where is the closest water source?). You’ll be happy you did when you need to water the new plants or seeds. 

Once you’ve determined your zone ( be conservative—I assume I’m Zone 4 because we have lots of wind here that dries in summer and drops the temperature in winter), soil types, water, and light, you are ready to plant. The planning takes time, but the planting takes very little time.

You can grow nearly every flower and vegetable from seed. I recommend trying at least a few: daisies, poppies and cornflower, basil, mint, lettuce, cherry tomatoes, spinach, and most squash. It’s simple and so satisfying. Plant starts are great also. If you like a fully grown plant,  head for those perennial sales. 

For the daredevils among you who say, “What if I choose a zone 6 perennial and put it in my garden anyway?” I say, ‘Why not? Be brave. Try it; it may survive. If it does not, there’s another name for it—an annual.

To paraphrase Helen Keller—life is a glorious adventure. To quote Warren Kimble, “Just have fun!”

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