Hot news: The new USDA garden zone maps

The USDA has just updated its map of gardening zones for the first time since 2012, sending American gardeners rushing to the Internet to find their zone. (Online is the only way to get the information, the days of printed maps are over.) For a lot of us, community gardeners included, it’s a big deal. Knowing your zone isn’t simply a matter of utility, it’s a marker of whether or not you qualify as a “real” gardener. You know, you strike up a promising conversation about homegrown tomatoes when the other person pauses, gives you a cool look, and asks “So, do you know your zone?”

You’d better check. About half of the US has seen a jump in zone this time around. Surprise, surprise – things are warming up. Here in Charlotte, we were in zone 7 a few years back. Now we’re in zone 8.

Technically speaking, we’re talking about USDA’s Plant Hardiness Zone Map (PHZM). The map establishes 13 zones, very roughly running in belts from east to west across the US. As you go from Zone 1 in northern Alaska south to Zone 13 in Porto Rico and Hawaii, conditions get warmer and warmer. Each “step” from zone to zone represents a range of 10 degrees Fahrenheit. Zones are further divided into “a” and “b” sections, each step 5 degrees F. The number used to determine zones is a 30-year-average of the lowest temperature each year. (See for yourself by visiting the zone map https://planthardiness.ars.usda.gov/)

In everyday terms, the lower your zone number, the colder your annual low temperatures. Some plants – white spruce, say - can take the cold while others - bananas - can’t. Without getting into the weeds (which grow everywhere, no problem), the USDA calculation lines up reasonably accurately with how well a given plant will grow where you live. That’s why gardeners like it.

It also means, however, that very different and distant regions can oddly be in the same zone. Here in North Carolina, we’re in zone 8a on the new map. My brother in Sequim, Washington, thousands of miles west and north of us, is in a WARMER zone, 8b (due to the influence of the ocean.) Do we grow the exact same things (speaking of tomatoes)? No way! Nevertheless, we are both in zone 8. Does that mean PHZM is another useless bureaucratic exercise? Not at all. In fact, USDA hardiness zones are very useful, especially for selecting fruit trees, berry bushes, and native woody plants.

With vegetables, community food gardeners may actually stand to benefit, since warmer conditions (no place is getting colder) mean we can grow vegetables for a longer period of time. That said, there are a few things to keep in mind. The warmer average temperature doesn’t take into account sudden cold snaps that can wipe out tender crops. In fact, one additional risk from global warming is less predictable conditions and more intense weather events. So, I’d keep those row covers and frost protectors handy (and maybe also be thinking of shading and other strategies for excessive heat.) Don’t forget that milder conditions might also worsen insect pests, diseases, and weeds (they’ll remind you...)

Even more importantly, as zone map experts including USDA emphasize, do not simply rely on the PHZM. Consider ALL the environmental factors vegetable crops need to thrive: Sunlight, water, the timing of the first and last frost, your garden's site-specific microclimate, and so on. (There are also lots of other kinds of maps beyond PHZM, but that’s for another blog or two – a “heat zone” or ecosystems map may be at least as useful as a hardiness zone map based on cold.)

Global warming, as reflected in the new zones, has already changed the ways we garden. The USDA, in a classic example of “precision newspeak”, insists that its map update is not a “reliable indicator of global climate change.” Chris Daly of Oregon State University, who partnered with USDA in creating the map, is less obfuscatory. Speaking with NPR’s Julia Simon, Daly said “We will expect to see a slow shifting northward of zones as climate change takes hold.”

In a practical sense, this means we can’t simply follow our old habits, even with veggies. Here in Charlotte, for instance, setting out tomatoes on Tax Day, April 15, has become a ritual. Looks like we might need to slide that in the direction of April Fools.

Rachel Patterson, a Florida gardener told Simon that the new zones made her feel “…like I’m not crazy. ..The sweet little grannies here are just heartbroken, they can’t grow their tomatoes. It’s so much hotter, the tomatoes burn.”

For other popular community gardening projects, including food forests and pollinator patches, the impacts are potentially greater. A change in zone can make a significant difference in your selection of trees and bushes (but, as USDA itself emphasizes, there is no need whatsoever to dig up your existing established plants based on the new maps! They’ll probably be fine.)

The original zone maps, dating back decades, focused on woody landscape plants. Unlike vegetables that grow to harvest in a single season, trees and shrubs, such as apple trees or blueberry bushes, are very much affected by the cycles of cold over many years. In addition to hardiness – whether they live or die - fruit crops are also affected by “chill hours”, the amount of time at a required low temperature during dormancy. This poses special challenges for those starting food forests, where existing or historical orchard varieties may no longer be well adapted to a given site. Additional heat effects from urbanization make such problems even trickier, and so do new pest pressures.

Native plant and pollinator patches are a beneficial project for community gardens, since they help support a more sustainable planet and model good gardening practices. However, again, as the new zone maps show, conditions have changed and are changing. Selecting native plants for a given location based on history, geography, or wildlife habitat may not be sufficient. We may face the challenge of having to design new ecosystems that can cope with the changes human behaviors have already caused to the climate and Earth’s life support system.

As Rachel Patterson, our fellow gardener in Florida, reminds us, the updated USDA zones illustrate how human activities that impact our planet are often easy to see in our gardens. Our hard-earned past experience and even Grandma’s favorite varieties and techniques may no longer work as well as they used to. If we’re considering fruit trees, berry bushes, or native plant patches, we need to be doubly careful to match our selections to changing conditions, while keeping a careful eye on established plants. Our veggies may like the longer and milder conditions, but so will plenty of pests and diseases.

Around the world, community gardeners, like the rest of life on our planet, have to cope with climate change caused by human activity. We need to face that fact and do all we can to prevent things from getting even worse. It’s not just our USDA Zone: We all need to push beyond our comfort zone, stop being in denial, and take action. Global warming isn’t cool.