Understanding your new plant hardiness zone

The USDA just updated the plant hardiness zone map for the first time since 2012 and there’s a very good chance you may find yourself in a different zone than you thought you were gardening in. So there’s only one thing to do: Panic.

I’m kidding, of course, but I wouldn’t blame you if you got a little caught up in all the chatter about the zone adjustment. Let’s be honest, it’s autumn and gardeners need something to talk about. 

If your zone changed (and you should consult a zoomed in map in addition to the zip code finder because the map is far more detailed than previous versions and some zip codes will have more than one zone), what does this mean for you? Well other than creating a bit of an identity crisis for some gardeners, it probably means less than you think: The conditions in your garden haven’t changed. (P.S. Canada friends, here’s a link to your map.)

Let me clarify: They may have changed in the 11 years since the last zone map was released, but they didn’t change this week. So you already know what the conditions in your garden are and have been.

Here’s the interesting thing: the Agricultural Research System, which is responsible for creating the maps, says the change in zones is more reflective of the increase in data they now have to make the map, rather than significant changes in temperatures, which I was surprised to read. They explain it this way:

“Climate changes are usually based on trends in overall annual average temperatures recorded over 50-100 years. Because the USDA PHZM represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming.”

The map is also much more detailed, because it uses GIS mapping technology and many more sources of data. Data from 13,625 stations was used to create the map, which is a significant increase from the last map in 2012, and the finer data points mean that temperature changes in cities and areas near bodies of water are better reflected.

WHAT IS THE MAP?

I remember when the last map came out and the reaction from gardeners was much the same: part enthusiasm to grow more plants and part distrust, with references to recent extreme weather events. 

Both are natural reactions and both are probably wrong when you better understand the data that goes into making the map. The most important thing to understand is that it is based on the AVERAGE annual extreme minimum winter temperature from 1991 to 2020. In 1996, the temperature in Milwaukee, much of which is now considered zone 6a, got down to -26 degrees Fahrenheit (that’s -32 for you sensible Celsius folks). That’s a zone 4a temperature, but that doesn’t make Milwaukee a zone 4a because it is based on AVERAGE minimum temperatures.

Nothing about the map suggests that you won’t experience dramatic temperature differences from what the lowest average temperature in your zone is. This is a great reason not to change a lot about what you grow in your garden. A handful of years ago I bought Cornus kousa x ‘Venus’. I knew it was a zone 6a plant but I wanted it so badly that I bought it anyway. And it lived for about three years, then just when it started to show signs of its potential, we had a winter cold spell and it was toast. That is still going to happen in gardens, and although I’m not a climate expert, I know enough about what’s happening to suggest that it’s probably more likely to happen than ever as weather events get more dramatic.

The Plant Hardiness Zone Map isn’t all that useful for people in warmer growing zones, where minimum temperatures are of little concern, but factors like chill hours (the amount of hours in a year below 45 degrees Fahrenheit) do matter. Gardeners in these areas can benefit from using the Sunset Climate Zone map. 

NOTHING WILL CHANGE … EXCEPT THIS

When I told Mr. Much More Patient that the new hardiness zone map showed us a half zone warmer (from 5b to 6a) than we were before, he said “Well that’s it! Rip out all the plants, we need new ones.” He was kidding, of course, and very little will change about what I grow in my garden, and probably what you should grow in yours.

The difference that will happen is that plants hardy to warmer zones will start showing up at your local garden centers, particularly if a large part of your area “zoned up.” In fact I’m sure that garden centers are salivating at all the new plants they’ll be able to tempt you with, although I suspect that plant warranties are going to be less common. 

In addition to possibly having access to plants that might not have been sold in your area before, I think gradually gardeners are going to get a little bit bolder in experimenting with a bit of zone pushing. Many of us have been doing it for awhile and this will likely embolden more cautious gardeners. 

HOW SHOULD GARDENERS RESPOND?

How should gardeners approach this new zone identity? It’s an interesting question because I’ve been seeing some angry gardeners out there. In fact a whole group of Minnesota gardeners expressed their dismay on Instagram of being moved to a zone 5a, as they have long prided themselves in growing great gardens in an area akin to the tundra. Others are chomping at the bit to pull a Mr. Much More Patient. Here’s my completely unasked for and mostly unqualified advice: If you moved into a new number zone like I did, from a 5 to a 6, take care. Since plant hardiness is often rated by the main zone number rather than the sub zone letter, you could be looking at 10-degree minimum temperature difference, and that’s pretty big. 

Go ahead and buy those tempting new plants, but site them carefully. Put them in a protected area or a warmer microclimate in your garden. Make sure you’re not fudging the other requirements for the plant in terms of amount of sun and soil conditions. And if they die, don’t go complaining to the garden center that the healthy and robust plant you bought didn’t make it when you planted it in your garden. 

MY NEW GARDEN ZONE

I am one of the many gardeners who find themselves in a new zone. My garden went from a zone 5b to a zone 6a, although the zip code finder doesn’t reflect that. I fall into a very skinny strip right along Lake Michigan that extends just a mile or so inland (exactly how far is difficult to judge). I have always been a little bit of a zone pusher (see the ‘Venus’ dogwood story). In some cases that’s worked out well as I’ve had better luck growing perennials on the edge in recent years after losing them in the past (although my gardening practices, particularly leaving perennials stand for winter, have changed and that could account for more success). 

Sadly I took very few photos of my ‘Venus’ dogwood when I had it, but this one of a flower (technically a bract) just opening is just a hint at it’s potential beauty

The main lesson that I take away from my previous zone pushing experiences is that I’m much more comfortably trying plants that are hardy only down to zone 6 when they are perennials, or possibly shrubs. More than the cost of the plants (although certainly that is a factor), it’s the time that goes into growing something like a tree or shrub that makes it harder to stomach potentially losing it.