Climate zones - USDA map vs Sunset map

Information about climate zones (USDA or Sunset mapping systems) is a helpful guide for growing plants here in Washington.

This month I would like to talk a little bit about zones. I'm not referring to parts of a football field, but rather the type of zone that describes the climate or temperature range that a particular area of the country can expect. There is an official US Department of Agriculture (USDA) Zone map for the entire country and also a Sunset zone map that covers the same area. Sunset’s zone map is far more detailed, but not as widely used as the USDA one.

Our USDA zone description in the West most part of Western Washington is 8b. This means that for all areas of the country that share that zone, the normal lowest temps in winter are about the same; this seems fairly simple until we put it in practice. For example, Myrtle Beach, South Carolina is also in zone 8b. Indeed our winter lows are in the same range (15f to 20f) as theirs, but there is more to it than that. When calculating, they also take into account the summer highs, as well as humidity and length of season.

The Sunset zone system takes the USDA calculations and also accounts for total rainfall and a number of other factors. The USDA zone map divides the country into 26 or so different zones, while the Sunset system has over 40. Kirkland (and most of the Puget Sound lowlands) are solidly in Sunset zone 5.

Using climate zone information

Most reputable nurseries will only sell plants that will grow in the surrounding area. So in a local nursery you will most likely see plant tags that are labeled “Zone 5-9” or something like that. Our zone 8 is contained in that range, so the plant will most likely do fairly well here. In years past you really had to watch out with this, especially at big box stores. In some cases, the tag won’t have any zone information at all, but a little internet research will give you that info. Today, most places you shop for plants will be labeled in USDA zones, if they are labeled at all. If the nursery is using Sunset zones, the tag will state that clearly.

Zones are important for a number of reasons beyond whether they will freeze in the winter. If you had a certain gooseberry back home as a kid in New England, and now want them in your yard here in Washington, a little research will tell you that it will work in USDA zones 3-7, but not in zone 8. In zone 8, it might not get cold enough in the winter for that plant, so try planting one or two as a test. Looking into the Sunset description for that plant will yield even more info.

Want to live on the edge? There are boutique nurseries out there that run the ragged edge of what will grow here, usually for the warmer. Experiment a little. Trying growing plants in a controlled environment or greenhouse and see if you can grow something from a different climate zone altogether.

I hope this has shed some light on how Environmental Construction Inc. tries to determine what plants will grow well in your area. Try it yourself, and don’t be afraid to experiment!

Category: Tips for Planting