Gardening in Washington: zones, frost dates and when to plant

Two climates divided by the Cascades

NOAA’s state climate summary describes Washington as two states in one. Pacific storms bring frequent rain west of the Cascades, and rainfall rises with elevation on the windward slopes. Parts of the Olympic Peninsula get more than 150 inches a year. East of the mountains, much less ocean moisture arrives, and areas along the Columbia River in the interior average less than 10 inches. The ocean also moderates temperatures in the west. Central and eastern Washington swing much further between summer and winter.

For gardeners, that means:

• Western Washington (Puget Sound, the coast, the Olympic Peninsula and the lowland valleys): cool summers, mild winters and wet falls and winters. WSU Extension in Snohomish County notes that below about 1,000 feet, frost in the soil is rare and a light mulch usually prevents it.
• Central and eastern Washington (Wenatchee, the Columbia Basin and the Inland Northwest around Spokane): far drier than the west, with a wider swing between summer and winter. WSU’s Gardening in Central Washington page describes low humidity, drying wind in spring and summer, and summer highs that can top 100°F around Wenatchee. Conditions change quickly with elevation: Leavenworth, up the valley, has much cooler summer nights and a shorter season.

What limits Washington gardens

West side: not enough heat. WSU Extension’s Home Vegetable Gardening in Washington (EM057E) says many warm-season crops need warmer temperatures than western Washington provides. Cool springs there also make it hard to get good germination of crops that like warm soil. The guide suggests row covers and plastic tunnels where heat is the limit.

West side: drainage. Because falls and winters are wet, WSU stresses well-drained soil, especially for anything left in the ground over winter. EM057E recommends raised beds to improve drainage where rainfall is heavy.

East side: heat, wind and dry soil. In north central Washington, drying summer winds and strong sun can scorch leaves. Soil there tends to be neutral to alkaline and, around Wenatchee, low in organic matter and slow to hold water. The same summer heat is an advantage for melons, squash, tomatoes and beans, which WSU lists among the crops that benefit.

Statewide: soil-borne disease. EM057E warns that Verticillium wilt and Phytophthora root rot are common in the Pacific Northwest. It recommends not planting the same family in the same bed for 5 to 7 years where possible.

What WSU Extension recommends

EM057E covers site, crop choice, planting, irrigation and pest management, and includes a suggested planting calendar for the Pacific Northwest. It notes that exact dates have to be worked out by trial in your own area, so treat the frost dates for your city above as a guide rather than a rule. Other key points from the guide:

• Give vegetables at least six hours of direct sun a day.
• Most crops need about 1 inch of water a week. If rain doesn’t supply it, irrigate with sprinklers, soaker hoses, drip tape or by hand. That covers most summers on both sides of the state.
• Many cool-season crops can grow through the winter west of the Cascades, depending on the microclimate.

The western Washington winter garden

WSU Extension in Snohomish County describes how to plant for fall and winter harvest. Some examples:

• Broccoli: direct-seed until mid-July or transplant until mid-August. A fall crop often keeps producing past Thanksgiving.
• Brussels sprouts and ballhead cabbage: seed by July 1 or transplant by August 1.
• Carrots: sow by mid-July. They hold in the ground until you dig them.
• Spinach: sow in mid-August for fall, or in September to overwinter for an early spring crop.
• Garlic: plant from late October to about November 10 for an early summer harvest.
• Fava beans and peas: sow in November for a June crop.
• Overwintering crops: leeks, rutabagas and winter radishes can stay in the garden and be harvested through the winter.

East of the Cascades

On the dry side, irrigation is the main job. The central Washington page advises long, deep watering that reaches the full root zone rather than several short sessions, and grouping plants by water need. It also recommends adding organic matter, because many soils there are sandy or silty and hold little water.

Soil testing

WSU no longer runs soil tests. WSU Extension in Whatcom County suggests a test when you start a new vegetable garden, to set a baseline, and repeating it three to four years later if fertility was adequate. It names the University of Idaho Analytical Sciences Laboratory and the University of Massachusetts lab as university options suited to western Washington soils. Commercial labs serve other areas. Whichever lab you choose, keep using the same one so results compare from year to year. County Master Gardener clinics can help you interpret the report. If you garden near old orchards or heavy industry, EM057E advises asking your Extension office or health department about testing for heavy metals.