The state of Washington falls into U.S. Department of Agriculture hardiness zones 6 through 9. The USDA hardiness zones rate average temperatures across the country, so gardeners may gauge whether a particular plant is likely to be perennial in their area. Washington's winter average temperatures fall between zero to 20 degrees F, and each zone varies by 10 degrees, with zone 6 being the coldest, but that's just the beginning of planting a Washington garden and observing gardening zones.
Average Minimum Temperature
Always choose plants that are hardy to the average minimum temperature in your region. Plants must be cold-hardy enough to withstand the lowest winter temperatures, not summer warmth. Grays Harbor County, for instance, has an annual minimum temperature of 10 to 15 degrees F, which puts it in zones 7b to 8a. Coast-side Pacific County, with its average annual minimum of 15 to 20 degrees F, is in zones 8a to 8b. Northern, inland counties get as cold as minus 20 degrees, or zone 5a. Average annual minimum temperatures are listed on the USDA hardiness zone map, though consult local resources for your specific area's average temperatures for the most accurate information.
Frost dates are an important consideration for any gardener, no matter the zone. A light freeze, which occurs at 29 to 32 degrees F, will kill very tender plants but will not harm more mature growth. A moderate freeze, which falls between 25 to 28 degrees F, is very harmful to fruit trees and semi-hardy plants. Severe freezes, 24 degrees F and lower, will damage many plants. Knowing frost dates is an important factor in planting schedules. Olympia's last spring frost usually occurs on May 5, the first fall frost on October 6. In Seattle, the last frost falls around March 10, the last frost around November 17. Spokane's last spring frost occurs around May 2, the last frost around October 3.
Look to native Washington flowers when choosing new garden plants. Wildflowers that grow with no cultivation make for very low-maintenance garden plants. Choose flowers that bloom in different months to enjoy color and life in the garden throughout the spring and summer. Western serviceberry, which produces edible fruits, flowers in April and red osier dogwood trees produce vibrant flowers in May. Rocky mountain iris and bitterroot also flower in May. Common yarrow, red columbine and blazing star all produce blooms in June. Wavy-leaf thistle flowers appear in July, and common sunflowers create their bright yellow blossoms in August.
Plants that are native to Washington are already well-adapted to the soil and weather that's found in your Washington-area garden. Vine maple, tall Oregon grape, western hazelnut, snowbrush, ocean spray, western honeysuckle, Indian plum, mock orange, western spirea, black huckleberry and red huckleberry all grow naturally in Washington, so they require very little if you want them to thrive in your garden.