On January 26, The United States Department of Agriculture released a new map of hardiness zones in the continental US. As most gardeners know, hardiness zones are based on the coldest temperatures normally expected during winter. We rely on this type of map to tell us what will and won’t survive in our particular area. In the USDA system, the colder the winter, the lower the zone number. Zones can also be subdivided into a (colder) and b (warmer). In the continental US, the zones range from 10b in Florida to 2a on the northern borders. The last official USDA hardiness map was released in 1990.
The new map, not surprisingly, has the US looking warmer. In the New York area, we’ve gone up about a half a zone. So, what has the new map changed?
In one sense, of course, the map changes nothing. It’s descriptive, not prescriptive. Although it’s natural to think that the new map reflects changes induced by global warming, that’s not necessarily the case. For one thing, the 1990 map was based on 13 years of data; the new map takes 30 years into account, including some overlap with the previous map. But as anyone who has gardened over the past decade knows, the map merely reflects what we have already observed: our winters are less severe, on average, and we can grow a wider variety of plants than we could in the 1980s and 1990s.
Want more proof? David Wolfe, a Cornell plant and soil scientist, points out that, “In fact, in 2002, the New York Botanical Garden planted a test garden in the Bronx of plants hardy only to a zone warmer than its zone on the 1990 map. These plants are all doing fine.”
So, the good news is, we can push the hardiness envelope with our plantings, and expect better performance from some of the tender plants that have been iffy in the past. The bad news is, the new USDA map reflects what a consensus of climate scientists has already concluded: the climate is getting warmer. Gardeners can see the result of a degree or two of difference. Unfortunately, so can pests like fire ants and kudzu.
As a gardener, you can help address the problem of climate change directly. Here are a few suggestions from the National Wildlife Federation:
- Reduce the use of gasoline-powered yard tools that put carbon pollution, the root cause of global warming, into the atmosphere.
- Remove invasive plants from the garden and choose an array of native alternatives.
- Reduce water consumption, which will improve the resiliency of your garden during droughts and heat waves, reduce energy consumed to transport water; and reduce runoff of fertilizer into waterways.
- Plant lots of native trees and native grasses to absorb carbon dioxide from the atmosphere and serve as long-term carbon storage.
For an added treat, check out this nifty interactive version of the new map, courtesy of the Washington Post.