Horticulture relies on plant hardiness zoning, a fundamental system used to classify climates for agricultural planning, determining which plants can survive the winter in a given location. Understanding this designation is a primary step for anyone looking to cultivate a garden or maintain landscaping. The standard reference used globally by growers and gardeners is the USDA Plant Hardiness Zone Map, which serves as a climate guide for selecting appropriate plant species.
Spokane’s Official USDA Plant Hardiness Zone Designation
Spokane, Washington, is designated across two primary zones on the most recent USDA Plant Hardiness Zone Map: Zone 6b and Zone 7a. The majority of the city falls into the slightly warmer Zone 7a. This zoning is based on the average annual minimum winter temperature, not the average temperature. Zone 7a indicates the lowest temperature expected on average each year is between 0°F and 5°F.
A smaller portion of the city and its higher elevation suburbs are classified as Zone 6b, where the average annual extreme minimum temperature ranges from -5°F to 0°F. Plants rated for Zone 7 can reliably survive the typical coldest night in the city center. Plants rated for Zone 6 are considered hardy for the region.
Understanding the Regional Climate Characteristics
While the hardiness zone focuses strictly on winter minimums, Spokane’s overall climate is continental, resulting in distinct seasons and wide temperature variations. The climate is classified as Humid Continental with a dry, cool summer. This is a result of the city’s position east of the Cascade Mountain Range, which creates a significant rain shadow effect.
The rain shadow blocks much of the moisture moving inland from the Pacific Ocean, leading to an annual precipitation average of approximately 16.5 inches, which is considerably drier than Western Washington. This climate contributes to significant diurnal temperature variation, meaning there is a large difference between daytime highs and nighttime lows, especially during the summer. These factors mean that successful gardening requires attention not only to winter cold tolerance but also to summer drought conditions and the overall growing season length.
Practical Gardening Guidance for Spokane’s Zone
Gardeners must select plants suited for the 6b/7a range while planning around the regional dry summer and late spring frost risk. The accepted “safe” date for the last spring frost is typically around May 15th, which serves as the benchmark for planting sensitive annuals and vegetables. The average date of the last freeze, however, occurs earlier, around the first or second week of May.
Planting should begin in waves, starting with very hardy root crops like potatoes and asparagus when the soil temperature reaches 40°F, often in mid-March. Cool-season crops such as lettuce, cabbage, and broccoli can follow in early April, as they tolerate cooler conditions. Once the danger of frost has passed in mid-May and the soil has warmed to at least 60°F, heat-loving plants like tomatoes, peppers, squash, and beans can be safely introduced. For perennial plantings, choosing varieties rated for Zone 6 will ensure maximum cold tolerance across the entire metropolitan area.