Planting Zones and Hardiness

What Does “Hardy” Mean?

In searching for that perfect Tree, you’ve probably heard the terms “Planting Zones,” “USDA Zones,” or “Growing Zones.” These terms all refer to plant hardiness ratings.

What does “hardiness” even mean? Like many, you may have assumed that a Tree labelled “hardy” means that it’s easy to grow, fast growing, drought tolerant, or resistant to pests and disease.

Actually, the word “hardy,” as applied to Trees and other plants, means none of those things! A hardy plant is simply one that can tolerate a certain low temperature.

How low? Each plant is assigned a number from 1 to 13. That number is code for the lowest temperature—within a 10-degree range (Fahrenheit)—the plant can withstand. The lower the number, the greater the cold tolerance. For example, a Tree hardy to Zone 6 can handle a winter low of 0° to -10°, while a Zone 5 Tree should be able to survive -10° to -20°.

To be more precise, each zone is further divided into two five-degree sub-zones, “a” and “b.” A Zone 5a Tree can tolerate -15° to -20°, while a Zone 5b Tree can only tolerate -10° to -15°.

What’s My Zone?

You may not know offhand your area’s coldest average annual temperature. Luckily, the USDA publishes a Plant Hardiness Zone Map, illustrating where each zone falls. You can find your zone by looking at the map or by entering your zip code.

Rest assured that when you enter your zip code on the Bower & Branch™ website, that step is already done for you. We only suggest Trees that are hardy in your zone.

Sometimes you’ll see a range of zones listed, as in “hardy to Zones 3-7.” That second number indicates a plant’s heat tolerance. The USDA doesn’t publish a heat zone map, but growers often specify a zone indicating the plant’s southern limit. Paper Birch, for example, is cold-hardy to Zone 2 (well into Canada), but it struggles in the South and is “heat-hardy” to only Zone 6 or 7.

What Doesn’t the Planting Zone Map Tell Me?

The Planting Zone Map is a valuable tool. However, you should know that it doesn’t tell the whole story. Sometimes a Tree is actually more or less hardy than its zone rating. Sometimes the conditions on your site don’t quite match the numbers on the map. Be aware of these situations when considering a Tree (especially a borderline-hardy Tree) for your home:

• Trees are fully hardy only after they’ve had time to acclimate to the cold. Frigid weather that hits unusually early can damage Trees that haven’t completely shut down for the winter.
• The Plant Hardiness Zone Map is based on average annual low temperatures. If your Zone 6 area has a rare “Zone 5” winter, the more tender Zone 6 Trees may be in jeopardy. Newly planted Trees will face greater risk than established Trees.
• The hardiness ratings are for Trees planted in the ground. A Tree sitting above-ground in a container is significantly less cold-hardy than its tag says.
• You may have micro-climates on your property. Some areas may differ a half-zone or more from what they’re “supposed” to be. A low spot may be a chilly “frost pocket.” On the other hand, a south-facing wall may provide enough extra warmth for borderline-hardy Trees planted nearby to flourish.
• Snow acts as an insulating blanket, moderating temperatures in the rootzones of Trees. Reliable snow cover is a good thing! A generous layer of mulch in winter can have a similar benefit.

Now you won’t have to “zone out” when the talk turns to plant hardiness! If you have any questions about Tree hardiness or Planting Zones, please don’t hesitate to ask our experts here at Bower & Branch.