Garden Life: Zone maps promote good garden decisions




These long, last summer days are beginning to pass by quickly. To make your garden minutes count, get in the habit of harvesting vegetables and herbs as a part of your evening meal preparation. A patch of garlic chives will season family meals with a heady bite and continue in the garden for many years. A few freshly picked sugar snap peas, a sprig of parsley and a handful of strawberries will not only enhance your meal but give you one more chance to putter around in the garden.

Climate zone maps have been devised to show where various permanent landscape plants can adapt. If you want a shrub, perennial or tree to survive and grow year after year, the plant must tolerate year-round conditions in your garden. This includes the lowest and highest temperatures, as well as the amount and distribution of rainfall. Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well. Zone maps provide this critical climate information.

The U.S. Department of Agriculture Hardiness Zone Map divides North America into 11 separate zones. Each zone is 10 F warmer or colder in an average winter than the adjacent zone. (In some versions of the map, each zone is further divided into “a” and “b” regions). The USDA map is the one on which most gardeners in the eastern United States rely. Many gardeners in the Pacific Northwest prefer the zone map in the Sunset Western Garden Book, since the book also includes encyclopedic information about the plants that do best in our area.

In the West, many factors beside winter lows, such as elevation and precipitation, determine growing climates. Weather comes in from the Pacific Ocean and gradually becomes less marine (humid) and more continental (dry) as it moves over and around the Cascade mountain range. While cities in similar zones in the East can have similar climates and grow similar plants, in the West it varies greatly. For example, the weather and plants in low-elevation Clark County are much different than in high-elevation, inland Tucson, Ariz., even though they are both in USDA zone 8.

Despite the heat wave of the last couple of months, we typically deal with a cool growing season in the Pacific Northwest. The first frost of the growing season arrives about mid-November. The last frost hits about mid-April, giving us at least 200 days of gardening weather. Because of these conditions, rhododendrons and conifers, as well as cool-season annual flowers and vegetables, grow well here. Warm-season crops, depending on the summer weather and your location in the region, may need season-extending techniques such as growing them in red plastic to get a sizable harvest.

The best information you can have before buying a plant is to know the individual conditions of your own garden. The USDA and Sunset Western Garden zones will help you focus in on plants that will do well in our general area. After that, my best advice is to check out the plantings in local parks, public plantings and in your own neighborhood. If you are adventurous and you want to try something that you haven’t seen in your area before, you might have to be the pioneer. Fortify that decision with as much information as possible, beginning with knowledge of your own growing zone.

If you are out visiting nurseries or garden centers, don’t forget to look for off-season plant bargains. If you love the perennial bleeding hearts and astilbes, you may be able to buy these at a greatly reduced price once they are no longer in bloom. As a marketable commodity, dormant plants with spent flowers just take up space at the nursery. Despite their fall from seasonal grace, these perennial plants are still viable and will come back as good as new next year.

If your nursery is reliable and plants in pots are always well-marked, you can buy the same plant now for half the price it cost in June. A reputable nursery will guarantee this plant as long as you plant it well in your own garden. Buying at a bargain price now, makes it worth the one year wait for the return of foliage and flowers. If you want a specific variety of daisy or if you want to add a mass planting of a specific perennial, this is a great method of plant collecting at a bargain price.

Robb Rosser is a WSU-certified master gardener. Reach him at