~ Carla Albright, Gardening Matters

For the last twenty years, I have been gardening on the basis that I live in a USDA Plant Hardiness Zone of 8. This is important because it affects the type of plants that will survive winter temperatures. The most recent of the USDA maps was put out in 2012 and since Oregon State University was a part of the research team, I felt pretty confident in being in a Zone 8. That was also the year they broke the zones down into “a” and “b” so they could be more specific about the temperatures and I moved up to a Zone 8b.

To go back a bit, the USDA Plant Hardiness Map is used to divide the United States into zones according to “average annual extreme minimum temperatures at a given location at a particular time period” or, basically, what the lowest typical temperatures are in an area. The map doesn’t reflect the lowest temperatures ever recorded, just the annual averages. I came from a Zone 5 in Western Pennsylvania, which meant our winter temperatures could (and often did) dip to -20 to -10 degrees. Believe me, there were winters it got colder than that… but I digress.

I was thrilled to move to coastal Oregon in 2001 where my zone jumped to an 8, meaning the winter temps would range from +10 to +20 degrees. That dramatically expanded the types of plants I could put in the garden and not have to worry about over-wintering. I could grow everything I grew in Pennsylvania like roses and rhododendron, but oh, so much more: hardy fuchsia, heather, escallonia, lupines…the list goes on and on.

Jump ahead to 2021 when I found an online nursery for some unusual succulents that I wanted to try. And when I put my zip code into their search engine, imagine my surprise to see I was now in a Zone 9. “Hmmm,” I thought to myself. “That doesn’t seem right. Especially since the USDA hasn’t recently updated those maps.” So more research was in order.

I tried other online nurseries but with pretty much the same result. The majority of the reputable ones showed my zip to be in a Zone 9a. This means instead of low winter temps of 10 to 20, I could now grow plants that can tolerate low temps of 20 to 30.

But not so fast! I tried plugging in zip codes for other cities in Tillamook County and found a wide range of Zones. Tillamook city, Cloverdale, Wheeler, Oceanside, Netarts and Garibaldi were all Zone 8. But Rockaway Beach, Manzanita and Nehalem were in Zone 9. Okay, I can see areas closer to the ocean’s warming effect would be in Zone 9, but that doesn’t explain Oceanside and Netarts in Zone 8. More exploration was needed.

I went onto the United States Department of Agriculture website to find out as much as I could as to how the maps were researched and created. And this is where it got really interesting, at least to me.

The partnership with the USDA’s Agricultural Research Service (ARS) and Oregon State’s Prism Climate Group resulted in a very comprehensive result. The previous USDA maps had been published in 1990, so the 2012 map was able to take advantage of much more sophisticated technology to divide the zones. Each of the 2012 zones is a 10-degree Fahrenheit band, with the “a” and “b” reflecting a 5-degree F band. The new map utilizes a Geographical Information System (GIS) that allows the user to plug in their zip code for an updated zone. This was what I was able to easily use to find the zones for all of Tillamook County.

The USDA’s ARS and OSU asked horticultural experts across the country to review their geographical zones and give input for the draft maps. A longer and more recent time period – 30 years instead of 13 years of the 1990 map – of weather data was also used. And, of course, the improved technology in recording and reporting the weather stations was available. This included using algorithms for tracking data between the weather stations. Algorithms were also used to account for nearness to large water bodies, elevation, and positions in the terrain such as valleys or ridge tops. There were also more weather stations around the country to gather data.

The bottom line to all this is that we need to know the USDA zones for our neighborhoods to be able to choose plants that will thrive. I expect the dramatic changes in climate we have experienced over the last ten years will result in an even more modern and updated USDA map in the near future. In the meantime, check your personal zone by going to https://planthardiness.ars.usda.gov and using their interactive GIS-based maps.

~~Note: Readers might be interested in the OSU ^TM Growing Oregon Gardeners: Level-up Series presentation ‘Adapting Your Garden for Climate Change‘. This Level-up Series is available to the public and free of charge. Recordings of previous presentations and upcoming live presentations can be found on the Tillamook Master Gardeners website’s Classes page.