USDA Planting Zones Simplified

It may seem like a little thing, but zone numbers may make the difference between a plant’s success or failure in your garden. But why? I’m in Zone 7a. But what does that mean? I’m here to give you the simplified version of planting zones and why you need to know yours. I’ll try to keep it short and simple.

The United States Department of Agriculture has divided every location into 13 plant hardiness zones based on their average minimum temperatures. They range from the Northern tip of Alaska zone 1a (-60°F average minimum - brrrr!) to Puerto Rico Zone 13b (65°F average minimum - ahhh!). Each zone is divided into 10° increments, with the ‘a’ and ‘b’ breaking it down further into 5° increments. So I live in Zone 7a which means my average low temperatures are 0 to 5°F. You can go to this map to enter your ZIP code and find your zone. USDA Plant Hardiness Zone Map

But here’s a big BUT! If you talk to any experienced gardener in Utah they’ll tell you to go down a number! Remember that the numbers are based on average minimums, and sometimes we get some random cold snaps. So even though my zone 7a is an average of 0 to 5°F, I know we got below 0° within the last 10 years. Depending on how long these cooler temperatures last, they may damage some of your plants rated for your current USDA zone. So what I recommend is to plant for one or two zones lower than what is recommended on the USDA chart for your more expensive plants like trees and shrubs. Then feel free to push it a little bit on perennials that are easier to replace if damaged or lost to cold. I plant trees and shrubs for zone 5 or 6 in my area.

When you purchase annuals such as petunias or tomatoes, these types of plants are only meant to be enjoyed for one season. They will grow in almost any zone with a long enough summer growing season, so you’ll just want to make sure you plant them in the ground after the danger of frost has passed.

Now I’m going to make this a little more complicated. You may also have a microclimate! A microclimate is a small area of your yard that may vary from the rest depending on certain factors. These factors can be based on things like topography, walls, buildings, wind, etc. A few examples would be a low spot at the bottom of a slope that tends to collect frost or a planting bed next to a brick wall that reflects collected heat onto plants slowly. You can also create microclimates by adding windbreaks or row covers. Just be sure to look for these microclimates in your yard that may affect how well a plant may do in that area and maybe try a higher zone plant next to the brick house, but a colder zone perennial where it’s in a frosty low spot. You can also help your plants out by mulching the roots in winter, putting a blanket over them during a cold spell, or bringing something inside during the winter. And on the other end, planting something in the shade of a large tree can lower the temperature and create a cooler microclimate for something that doesn’t like the heat. Deciding how fussy to get with microclimates is all up to you! You can push the limits, or plant for the averages and not worry about your plants as much.

(L) Monkey Puzzle tree is unique and Zone 7 to 11, (R) Abutilon is Zone 8 to 10 and both are looking pretty great in this Zone 5 yard in Ogden, Utah.

But not all plants want to be toasty warm! There may be some upper zone limits where a plant may thrive. They may need to have a cold spell in the winter to force them to go dormant, or they just may get too much heat at lower temperatures. A good example for Utah is the Quaking Aspen rated for zones 2-7. They may thrive in our mountains at zone 2 but they can easily scorch and not be happy at all on the valley floors in zone 6 or 7. Perennials are more versatile plants and Moonshine Yarrow is rated for zones 3 to 8 meaning it can do well in cooler mountain areas as well as thrive in the 100° temperatures we may get in the valleys during the summer.

Aspen at Zone 4 and Moonshine Yarrow at Zone 6.

So you can see why this can get complicated. That’s where it is helpful to shop at local nurseries that grow and raise plants locally and in our climate. They can give you great advice on what will do well for you, but they will most likely as you where you live because they are going to go by your planting zone! Some states like Ohio have 2 planting zones, but with the mountains and valleys of Utah and the lower elevations of St. George, the state covers 6 different plant zones. Just traveling from the East benches to the valley floors can drop you one to two zones within a short distance.

Before you head to the nursery be sure you check your zones because that’s a great place to start. Or you can always have a landscape designer help you choose the plants for your yard - and that’s where I come in… ;)