There are many aspects to consider in order to be successful at growing grapes in the maritime climate areas of the Pacific Northwest (PNW). Typified by cool summers, mild winters, and variable precipitation throughout the growing season, these climate conditions resemble those of classic winegrape regions of Europe such as Champagne and Burgundy in France, the Ahr and Franconia areas of Germany, Vinho Verde in Portugal, and the areas on Lake Geneva and Lake Zürich in Switzerland. This suggests that the maritime PNW has potential for producing grapes and wine types similar to those areas.
Quality winegrapes can be grown in western Washington, provided careful consideration is given to choosing the appropriate site, variety, rootstock, and cultural practices. There are several limiting climatic factors, however, that impact the success of growing winegrapes in maritime areas of western Washington, including temperature, growing season length, and amounts of precipitation.
Since temperature is a major limiting factor in cool-climate viticulture, accurate measurement of heat accumulation at a potential vineyard site is important. Heat units in the maritime PNW region can vary from 1400 to 2300 growing degree days (GDD; base 50ºF). Once the GDD of a site is determined, the process of selecting varieties that are best suited for that site will allow you to concentrate on producing high quality grapes.
Length of the growing season is another major determinant in successfully establishing vineyards. Generally, winegrape cultivars need a minimum of 160 frost-free days. Except at some high elevations in the foothills of the Cascade and Olympic mountains and in some low-lying areas, the PNW offers a long enough growing season for winegrapes.
Precipitation can be another limiting factor in quality grape production. Rainfall in the coastal region can vary from approximately 12 to 50 inches per year. The Washington State University Northwestern Washington Research and Extension Center (WSU NWREC) in Mount Vernon has recorded a 40-year annual precipitation average of 32 inches. Most of the precipitation in western Washington falls as rain from late autumn to early spring, with occasional brief intervals of snow. Summers are relatively dry, particularly from early July to early September.
When preparing to establish a new vineyard in western Washington, due diligence is necessary for success. The two largest preparation decisions are site selection and choosing which varieties to grow. Each of those decisions entails numerous considerations and choices.
Site Selection and Preparation
Site selection is one of the most critical decisions you will make. Factors that influence the suitability of a site include: temperature and microclimate, slope and aspect, soil drainage, and soil chemical and physical properties. The availability of water for irrigation also must be considered for sites that are likely to experience long dry periods during late summer.
Temperature and Microclimate. Temperature is critical during grapevine development and fruit production. It can determine the type of cultivars and species that can be grown successfully in a given area, because some cultivars require more heat than others or ripen later in the season, or both. (See Appendix A for an annotated list of grape cultivars.) Slight adjustments in row orientation and trellising can be made to minimize or maximize the amount of heat available at a site.
In addition to in-season temperature, winter temperatures significantly influence vine growth. Most areas in western Washington are not subject to damaging low winter temperatures; however, insufficient cooling in some areas can exacerbate erratic vine development in the spring.
To accurately measure temperatures at a site, a data logger that records temperature is necessary. From the data gathered, you can calculate the accumulated growing degree days for your site. (See “Calculating Growing Degree Day.”) Data loggers range both in price and in types of data they record. In general, daily recording of minimum and maximum temperature is all you need; but hourly temperature information is better.