A friend from Oregon who moved to the East Coast a few years ago to be closer to grandchildren recently sent me a link to an article from the New York Times in late May that reflected on changes we might see in our gardens as the climate warms. The article written by Nadja Popovich was a little scary because apparently the climate change is already having an effect on the hardiness zones in the United States.
The article cited the National Oceanic and Atmospheric Administration (NOAA) and a map that their scientists have put together to reflect the changes now and into the future.
Traditionally, Tillamook County has been in Zone 8 of the United States Department of Agriculture (USDA) maps, with southern California and the southern tip of Florida being Zone 10. We share Zone 8 with southern Georgia and parts of Texas and Arizona. When our family moved from Western Pennsylvania 18 years ago, I was pleased to be moving from Zone 5 (hot, humid summers and cold, snowy winters) to our current Zone 8. It meant I could grow many more types of perennials and have a longer growing season than I was used to. If we wanted to grow dahlias (and not many people took the trouble), we would have to lift them in early October to beat the frost. Now I have the luxury of leaving them in the ground if I so choose. Some years it’s a gamble, but worth it when I don’t have time to lift them. In Pennsylvania, not lifting dahlias would mean a certain death to them.
The USDA map is based on winter temperatures when determining hardiness, broken into 10-degree differences. For example, minimum temperatures in Zone 8 are 10 to 15 degrees F and Zone 9 ranges from 20 to 30 degrees F. (My “good old Zone 5” goes from -10 to -20 degrees, just for comparison.) The USDA map is more detailed than the NOAA map and is the standard for determining what plants will grow where. The USDA maps hadn’t changed from 1990 until they released a newer version in 2012. In that year, we moved from a Zone 8 to a Zone 8.b. Half-zones are not really given their due. Plant tags still say “Good for Zones 6 to 8” and don’t reflect those half-zones. I err on the side of caution and stay with Zone 8 plants.
(Then we also have the totally different climate maps of the Sunset publishing group. On those maps, the Northwest Coast Marine areas are considered a Zone 5. The Sunset books have a much more specific series of maps and their number of zones go from Zone 1 to Zone 24. This is important to know when referring to the “Sunset Western Garden Book” but doesn’t help in the nursery or when ordering from seed catalogs as they still use the USDA map for reference.)
While the USDA maps are not concerned nor do they reflect long-term changes in temperatures, that is precisely what the NOAA maps are for: to indicate and evaluate those climatic variations. They try to answer questions about climate more specifically. The NOAA scientists broke down every 30-year period from 1971 to 2000 and 1981 to 2010 so they could compare the older maps and look to the future 30-year period from 2011 to 2040.
The NOAA maps are showing a big change from the year 2000 to 2010 and an even bigger projected difference from 2010 to 2040. By the year 2040, NOAA is predicting the area west of the Cascades and Coast Ranges as being squarely in Zone 9. While our cool Tillamook summers mean we can’t overwinter banana trees in the county, it does open up some more desirable plants for our gardens that are now considered “tender.” Certain type of Mediterranean plants that are iffy here currently might be able to handle the warmer zone. Of course, we still have to allow for our famous summer winds coming from the north.
One of the drawbacks of the NOAA maps is that while they reflect long-term climates, they do not predict that one winter in every 10 or 20 years where our temps get down below a normal range. That kind of one-of-a-kind cold snap would wipe out those tender perennials. As gardeners, we still have to pay attention and act conservatively.
And warmer temperatures aren’t all good. Although we might be able to grow new kinds of plants along the coast, we also might see invasive plants that don’t presently grow here. The one that comes to mind is the awful kudzu that plagues Georgia, North Carolina and other parts of the Southern United States that is now being seen in Ohio and even Ontario, Canada. As if English ivy isn’t bad enough!
If you want to check out the article by Ms. Popovich – including a very cool interactive map – here is the link: https://www.nytimes.com/interactive/2019/05/23/climate/plant-hardiness-zones-shifting.html.
I promise you will start looking at your gardens in a whole new – and warmer – way.