This is the time of the year when backyard gardeners start seedlings or scan seed catalogs as they eagerly plan for spring. I’m actually writing this with potting soil under my fingernails from prepping a tray of buttercrunch lettuce and two new tomato varietals. But before you make those final decisions about what you put in the ground, you might want to check the latest USDA Plant Hardiness Zone Map, because most people will find that they’ve moved by at least one half-zone, and some have moved even more. 

If your gardening experience extends beyond occasionally grabbing a pack of marigold seeds at the hardware store or murdering a tomato on the balcony, odds are you already know this. The 2023 revised map came out two months ago and the shocking difference between that map and the previous version has been a big subject of discussion in gardening circles.

The maps are made by examining the average annual low temperatures in an area over a decade. Each half-zone change represents a 5-degree (Fahrenheit) shift, so most of the time the changes between one generation of the map and the next are small. That’s not true this year. Some locations have advanced by a full 10-degree zone and changes are almost universal.

This does not mean it suddenly got warmer where you live and you can expect 2024 to be that much warmer than years past. It means that climate change has already made it warmer and the USDA is just catching up. 

How will the changes in the map affect your spring planning? It probably won’t. For one thing, you’re already aware of what grows and does not grow at your place, and the changing temperature is probably something you’ve been adjusting to from season to season. 

Besides, most of the plants the home gardener plants are annuals. They sprout, give up their fruit or vegetables over the summer, and die in the winter no matter how warm that winter may be. The increasing temperatures may mean that the growing season for your garden has been extended, so if you want to try slipping in those watermelons that never managed to ripen before the first frost, or get a little more adventurous in starting a second crop after some mid-summer vegetables run out … go for it. But consider that an experiment.

The bigger effect of the zone hardiness map is on perennials — the plants that live on year after year. If you’ve been making your choices on what kind of shrubs, ornamental plants, or fruit trees you can have at your house based on the old map, there’s a good chance your opportunities for some of those things that seemed just out of reach have expanded. Though it’s best not to plant right to the limits of the map. That’s a formula for suffering when an unusual cold snap hits three years in and wipes out those blueberries that were just starting to produce fruit.

For gardeners in the northern half of the country, who have spent decades scowling over all the things their southern neighbors got to try while they were stuck with the same-old same-old, the change in the maps may seem like a boon. But this is a direct measure of the worsening man-made climate crisis intruding into an area of life that many people treasure. It’s a sword that cuts two ways.

And don’t forget, plants don’t just have minimum growing zones. They also have maximums. For example, many of the most popular apple or pear trees will only grow well in zones between 4 and 7. That means that about half the continental U.S. is now too hot for these trees. 

I’ll be considering that before I put another tree in the ground. On the 1990 map, my location near St. Louis was in zone 5b. The 2012 map shifted me half a zone to 6a. On this new map, I’ve moved all the way to 7a. That’s a 15-degree shift in average annual lows. Were I to plant one of the more sensitive fruit trees now, there’s a good chance I’ll be in 7b, or even 8a before it begins to produce. Because that’s how the USDA trends have gone — up, up, and up over the last thirty years. NOAA has a series of maps that show how things changed in USDA maps between 1970 and 2010. Past changes were relatively small, but not this latest shift. The trend isn’t just toward warmer, it's getting warmer faster.

But don’t worry. Those who want to scoff at climate change have already latched onto an excuse. This time around, the USDA greatly expanded its number of monitoring sites, making this a much more detailed, high-resolution map than in previous cycles (go put your zip code into the map and zoom in on your location). “They changed how they did it!” has already become the battle cry of the it’s-not-really-getting-warmer crowd.

Across the lake from me, there’s a guy who planted a number of cold-hardy banana palms a decade ago. I thought he was taking a big chance at the time since those trees maxed out at zone 6a. Of course, they’ve done great, shading his yard with tropical splendor. Meanwhile, I’ve lost an oak or hickory to heat stress almost on an annual basis.

But anyway … I have half a dozen different peppers to get started and an almost infinite number of tomatoes yet to try. So time to get my fingers back in the potting mix. There will be a lot more sunlight in my garden this year, now that the big old pignut hickory that stood off to one side has fallen.

Republished with permission from Daily Kos.