Get It Growing: Climate zones

Planning a garden has numerous variables — location, sun, and water to name a few. However, the most important question may be, “What plants will grow well in my area, or climate?”

Climate can be described as an aggregate of weather conditions for a specific region, averaged over a relatively long period of time. Climate is what the weather is statistically likely to be during any certain time of the year. Weather is what you think about when deciding how to dress for the day.

Zones: There are the USDA Hardiness Zones and the AHS Heat Zones. In the West there is the “Sunset Western Garden Book” and unique system of Climate Zones. So when you find a plant with USDA Hardiness Zone 8b, AHS Heat Zone 1, and Sunset Zone 4, what does that mean?

USDA Hardiness Zones: Revised in 2012, the U.S. Department of Agriculture has defined 13 zones based on the average annual minimum temperature using 10-degree increments; these zones are sub-divided into separate 5-degree increments for better accuracy. Hardiness zones indicate a plant’s cold tolerance or ability to “winter over” with 1 indicating the coldest temperatures and 13 the most tropical.

The northernmost portion of the Olympic Peninsula is in USDA Zone 8b (15- to 20-degree average minimum temperature). For comparison, Zone 8b also is the hardiness zone for Austin, Texas, Baton Rouge, La., and Gainesville, Fla. Does that mean we can grow the same plants here that are grown in those southern cities? Not necessarily: we can grow plants with the same cold hardiness but, without the heat, they may not thrive.

AHS Heat Zones: The American Horticultural Society has divided the United States into 12 zones based on how many days per year the temperature typically goes above 86° F, termed heat day. The AHS only takes one factor into consideration: a plant’s heat tolerance or its ability to withstand heat, with Zone 1 indicating less than one heat day and Zone 12 indicating over 200 heat days.

Most of the Olympic Peninsula is in AHS Heat Zone 2 (1-7 heat days). For comparison Austin, Texas; Baton Rouge, La. and Gainesville, Fla., are in AHS Heat Zone 9 (121-150 heat days). Clearly a dramatic climactic difference.

Sunset Climate Zones: The “Sunset Western Garden Book” uses a wide range of factors in defining climate zones including winter minimums, summer highs, elevation, rainfall, humidity, aridity and growing season — a more comprehensive view encompassing both the USDA and AHS data.

The Sunset Zones are probably the best general reference for the home gardener. All the plants described in Sunset include their range of zones. Most of the Olympic Peninsula is in Sunset Climate Zones 4 and 5, excepting the higher elevations.

It is important to know what zonal guide is being used on plant tags. Most nurseries and retail stores will have USDA Zones identified unless specifically noted otherwise. Fortunately, many local nurseries will also have a copy of the “Sunset Western Garden Book” available for reference. Take a minute to see what the Sunset Zone might tell you about the likelihood a plant will thrive in your area.

While understanding your climate is essential, remember, none of these zonal guides tells the whole story. Your garden with its unique microclimate is a world of its own: perhaps a column for another day.

For more information and maps of these climate zone guides go to: planthardiness.ars.usda.gov; www.ahsgardening.org and select “Garden Maps” in the lower “Gardening Resources” list, and/or www.sunset.com/garden/climate-zones.