Gardening With Allen: Climate zones aid plant selection
By Allen Wilson
Published: March 25, 2023, 6:03am
How can I make sure landscape plants I choose are adapted to our climate?
The U.S. Department of Agriculture has developed a climate zone map to help with plant selection. The “Sunset Western Garden Book” has also developed a different climate zone map for Western states. Most nurseries and garden stores in our area use the USDA zones.
We live in a time of warming climate, although you might feel like the opposite is true for this winter and early spring. We are probably two or three weeks behind normal in plant development this year.
The last USDA climate zone map was published in 2012. Plant hardiness zones are based upon the average annual extreme minimum temperature at a specific location. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.
The 2012 map includes 13 zones. Each zone is a 10-degree band, further divided in 5-degree zones. Compared to the 1990 map, zone boundaries have shifted in many areas. This is the result of using temperature data from a longer and more recent time period. The new map uses data measured at weather stations during the 30-year period from 1976 to 2005. The new map is generally one 5-degree half-zone warmer than the previous map through most of the United States.
Before the change western Washington and Oregon were rated as 8a. We are now rated 8b. The minimum temperature expected with 8a is 10 to 15 degrees. Zone 8b is 15 to 20 degrees. The minimum this winter in Vancouver was 18 degrees, which fits the pattern for Zone 8b. Higher elevations are more likely to be in Zone 8a or even 7b which is 5 degrees colder than 8a. That means we can plant anything that has an 8 or higher rating. We may be able to even plant Zone 9 plants in protected locations such as near a building facing east. Peruvian lily (Alstroemeria) is one plant I have noticed that is hardy in Zone 8b, but apparently not hardy in 8a.
The latest map can be accessed at planthardiness.ars.usda.gov. Since this map is 11 years old, we might be in Zone 9a in a typical year.
If you use the Sunset garden book in selecting plants, be sure to use its climate zone maps.
Allen Wilson is a Vancouver gardening specialist. email@example.com