Gardeners in Sonoma County have long been grateful for living in a climate mild enough to grow food gardens year-round. Weather conditions vary from spring through summer, fall, and winter, but over long periods—years and centuries—the climate remains constant, with the exception of periods of drought or intense rainfall.
Yet within the bounds of Sonoma County, geographical features—distances from the Pacific Coast, hilly and mountainous areas that block the flow of air, valleys that collect both cool and warm air—alter both weather and climate patterns with predictable regularity. The result is a variation in seasonal, even daily, conditions from north to south, west to east.
Major and Micro Zones
There are three major microclimates within the county, all influenced by marine air from the Pacific. As cool ocean air moves inland, it is deflected by mountainous terrain and settles into low-lying valleys. The resulting airflows, along with sun exposure, have created a vast number of even smaller microclimates. There may be several within one city, town, rural area, or even a single property with surprising variations in temperature and rainfall from one area to another.
Some of these isolated microclimates occur predictably on hillsides or on valley floors; others pop up in unexpected locations to the delight of Sonoma County’s numerous amateur and professional horticulturists. These three major climate zones—coastal marine, coastal cool, and coastal warm—dictate what food crops and ornamental plants thrive within each area.
Along the north coast, the coastal marine climate is a narrow ribbon bounded by the ridges above The Sea Ranch and Fort Ross. In the central and southern parts of the county, the zone is pushed eastward as marine air rushes through gaps in the coastal hills. It extends a few miles up the Russian river then swirls southward nearly to Occidental and Freestone. Further south, marine air pours through Valley Ford and Tomales toward Petaluma.
Limited sunshine in most of the coastal microclimates favors shade-loving plants. The mild winters allow tender plants to flourish here year-round, but heat-requiring annuals and perennials do well only in protected sites.
The coastal cool climate runs across ridge tops in the north, then reaches down onto the Santa Rosa Plain and into the southern inland areas of Sonoma County. This climate region is heavily influenced by marine air, but with greater seasonal temperature fluctuations. Days and nights are warmer in summer and colder in winter. And a predictable wind comes up most summer afternoons.
Some microclimates near Sebastopol lie in thermal bands, called banana belts, on hillsides. Many gardens bask in sunshine above fogged-in, cold-air basins and enjoy some of the finest weather in the county. Stands of trees deflect intruding afternoons winds, and plants that prefer a moist atmosphere, cool summers, and mild winters do well here.
The coastal warm inland areas are the driest, hottest, and coldest in the county. The marine influence fades somewhat in this climate zone, but it continues to have a moderating influence, especially in winter, lifting the average lows above freezing. Here, vineyards now abound where orchards once flourished.
Cloverdale, Healdsburg, the Sonoma Valley, Windsor, and most of Santa Rosa feel the effects of this climate. Fruit and nut trees flourish in adequate winter chill and summer heat. The dry heat of summer, however, limits the success of many perennials; whereas, in winter, a few microclimates in protective sites allow subtropical plants to survive the cold winters.