Ask gardeners in the Napa Valley what climate zone they live in and most will say that they are in Sunset Zone 14. They would be correct if they lived on the valley floor. Zone 14 is the smallest of four zones in Napa County, as determined by Sunset’s “Western Garden Book.” The other three are Zone 7, called “Foothill-Digger Pine”; Zone 14, “Coastal Warm”; Zone 15, “Coastal Cool”; and Zone 17, “Marine.”. The margins of all climate zones are inexact. Each zone contains microclimates created by elevation, slope and exposure.
Although Sunset has identified four distinct zones, Napa County is considered to have a Mediterranean climate, with wet winters and dry summers. A major influence is the summer marine air which flows up and down, from the ocean east to the Sacramento Valley.
Winter winds tend to flow up and down over and through the hills and the valleys; the summer winds flow across this terrain. The fog in the summer moves through the Russian River Gap toward Mount St. Helena, down Napa and Pope valleys. The summer fog from the Petaluma Gap and the Golden Gate cools the southern portion of Napa County and also appears along the western hilltops.
Rainfall varies by elevation. On the valley floor, it averages 20 inches; on Mount St. Helena, 50 inches. Annual chilling hours (the number of hours each year below 45 degrees F), which affect the type of fruit trees that can thrive in the area, vary from 700 to more than 1,600. That’s partly why some species trees are difficult to grow on the valley floor but do well on the foothills. Some require more chilling hours than the valley provides. Understanding the characteristics of your climate zone can improve your chances of choosing the right plant for the right place.
Zone 7 owes its Foothill-Digger Pine name to the abundance of California foothill pines in the area. The zone comprises the eastern part of the county and includes Pope Valley and Lake Berryessa. These areas have a later spring and earlier frosts than the rest of the county, creating a growing season that may be as much as a month shorter. Summers are hotter and dryer. This is a good area for late- or long-blooming fruit and nut trees. This zone also defines most of Lake County.
Zone 14, the so-called Coastal Warm area, includes the major valley floors in the county. The coastal influence keeps this zone from being as hot as the Central Valley. It is protected from the summer fog by its distance from the ocean and the mountain ranges; thus, summers in this zone are very dry. Between April 1 and Oct. 31, the area clocks more than 1,100 hours between 70 and 90 degrees. Most deciduous fruit trees and wine grapes do well here. Sonoma County’s valley floors, including the town of Sonoma, are in this zone, too.
Zone 15, nicknamed Coastal Cool, is affected by marine air 85 percent of the time and inland air 15 percent of the time. It covers the foggy hilltops above the valleys and the southern part of the county east of Napa and includes Mount Veeder, Spring Mountain and Howell Mountain. It logs from 800 to 1,100 hours between 70 and 90 degrees during the growing season. Cane berries, some fruits and certain grape varieties prosper here. Many of the plants recommended for Zone 15 will not prosper in Zone 14 because they need moister air, cooler summers or both. The Coastal Cool area receives more chilling hours than Zone 14.
Zone 17, the Marine area, is the coolest zone in Napa County. It is also the largest, encircling the floor of the Napa Valley. It includes the Carneros District, American Canyon and the Soscol Ridge up to Napa. The heavy marine fog brings cooler temperatures and less intense sunlight here compared to other Napa County zones. The growing season may see as few as 800 hours between 70 and 90 degrees. Consequently, growing sub-tropical plants here is a challenge.
The numerous climate zones in Napa County allow for a diversity of crops and landscape plants and trees. Consult Sunset’s “Western Garden Book” for more information. The book’s maps make the zones easier to grasp. Also take a look at the Napa County Master Gardener website, under Healthy Garden Tips, for yet more information about the climate zones of Napa County.
Napa County Master Gardeners will host a free workshop on “Cool Season Vegetable Garden Planning” on Saturday, July 16, from 9:30 a.m. to noon, at Napa Valley College’s Upper Valley Campus in St. Helena. The workshop will be repeated on Saturday, July 23, from 9:30 a.m. to noon, at the UC Cooperative Extension office in Napa. Register for either session online at ucanr.org/sites/ucmgnapa/?calitem=130191&g=9191.
Open Garden Days
Napa County Master Gardeners welcome the public to their demonstration garden at Connolly Ranch on the first Thursday of every month, from April through October, from 10:30 a.m. to noon. Master Gardeners will be on hand to answer questions. Connolly Ranch is at 3141 Browns Valley Road in Napa.
Contact Master Gardeners at cenapa.ucdavis.edu or the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, 253-4221, or 877-279-3065.