Plant Hardiness Zones
Plant hardiness and climate zones are designed to assist gardeners in identifying plant species suitable for their climate. The organizations that publish plant hardiness zone information rely heavily on historical data when preparing their maps and documentation.
The various hardiness and climate zones are a good place to start when selecting new plants, but the final decision should always be based on your individual garden environment.
USDA Plant Hardiness Zones
The 1990 version of the USDA Plant Hardiness Zone Map shows in detail the lowest temperatures that can be expected each year in the United States, Canada, and Mexico. These temperatures are referred to as “average annual minimum temperatures” and are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico. The map shows ten different zones, each of which represents an area of winter hardiness for the plants of agriculture and our natural landscape. The 1990 version also introduces zone 11 to represent areas that have average annual minimum temperatures above 40 F (4.4 C) and that are therefore essentially frost free.
The majority of Grays Harbor and Pacific Counties, Washington, fall into USDA Plant Hardiness Zones 8a and 8b, which have an average annual minimum temperature range of 10 to 15 F and 15 to 20 F, respectively.
The concern with the USDA zones is that they are based only on average annual minimum temperatures, and do not take into consideration other factors which significantly impact horticulture, such as day-length, rainfall, frost, minimum and maximum temperatures, sunshine, and soils. The result is that large geographic areas of the US may be grouped together into a single USDA zone when, in fact, they actually have significantly different growing conditions. For example, while few would consider Aberdeen, Washington, and Dallas, Texas, as having similar climates and gardening conditions, both are in USDA Hardiness Zone 8a.
National Arbor Day Foundation Hardiness Zones
In 2006, the National Arbor Day Foundation published a Hardiness Zone Map, which takes into account the general warming trend observed since the last USDA map was published in 1990. The new NADF zones are based on the most recent fifteen years of data available from the National Oceanic and Atmospheric Administration’s 5,000 National Climatic Data Center cooperative stations across the United States. Compared to the 1990 USDA map, on the 2006 NADF map, many areas of the US have shifted at least one full hardiness zone in a warmer direction.
The most significant difference between the USDA and NADF maps in our region is that the area immediately around Grays Harbor is in NADF Zone 9 (minimum low temperature of 30 to 20 degrees F), much warmer than USDA Zone 8a (minimum low temperature of 15 to 10 degrees F). The rest of Grays Harbor and Pacific Counties are in NADF Zone 8 (minimum low temperature of 20 to 10 degrees F), roughly the equivalent of USDA Zones 8a and 8b.
Like the USDA Plant Hardiness Zones, the National Arbor Day Foundation Hardiness Zones are primarily based on a single factor (minimum temperature), and thus are subject to similar concerns.
Sunset Climate Zones
The Sunset Climate Zones are yet another plant hardiness system. These are the climate zones used in Sunset’s popular Western Garden Book. Sunset’s Climate Zones take a wider range of factors into account than the USDA or NADF: growing season, timing and amount of rainfall, winter lows, summer highs, and humidity. The other systems tell you only where a plant may survive the winter; Sunset’s Climate Zones attempt to give you a better indication of how your plant will fare year-round. Most of Grays Harbor and Pacific Counties are in Sunset Climate Zones 4 and 5.
Sunset Zone 4
Cold-winter Western Washington and British Columbia. Growing season: early May to early Oct. Summers are cool, thanks to ocean influence; chilly winters (19 degrees to -7 degrees F/-7 degrees to -22 degrees C) result from elevation, influence of continental air mass, or both. Coolness, ample rain suit many perennials and bulbs.
Many people know Zone 4 for the miles of tulips in the Skagit Valley. In fact, this area has more spring bulbs under cultivation than all of the Netherlands; the slightly colder winters of Zone 4 help induce dormancy in the bulbs. Zone 4 extends into the greater Seattle area.
Sunset Zone 5
Ocean-influenced Northwest Coast and Puget Sound. Growing season: mid-April to Nov., typically with cool temperatures throughout. Less rain falls here than in Zone 4; winter lows range from 28 degrees to 1 degree F/-2 degrees to -17 degrees C. This “English Garden” climate is ideal for rhododendrons and many rock garden plants
Zone 5 includes the coastline areas of Washington and Oregon that are famous for lush vegetation. While not particularly warm in the summer (it’s hard to grow tomatoes in some areas), the long growing season favors flowering plants, such as fuchsias. Native plants of all types, including salal and Oregon grape (Mahonia aquifolium), thrive in this zone.
USDA Plant Plant Hardiness Zone Map. The United States National Arboretum, USDA. A hyperlinked map of the USDA Plant Hardiness Zones for the United States. Includes brief descriptions for each zone, representative plants for each zone, and additional information on the map.
National Arbor Day Foundation Hardiness Zones. A US plant hardiness zone system that reflects recent warming trends.