Flora of Napa County, California

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St.Helena Palisades, Napa County, California. @ C.Michael Hogan

February 9, 2012, 12:00 am
February 1, 2013, 2:32 pm

Napa County is a rural county in northern California encompassing nearly 500,000 acres (200,000 hectares), about 0.5 % of the landmass of the state. It is located 25 miles (40 kilometers) north of San Francisco in the North Coast Ranges. San Pablo Bay, an arm of the San Francisco Bay, is near its southern extent. The eastern boundary is drawn across the Vaca Mountains and Blue Ridge which marks a portion of the western edge of the Central Valley (Sacramento Valley). The western boundary stands almost 30 miles (48 km) from the Pacific Ocean. At 38 degrees N, the latitude is approximately that of Lisbon, Portugal and Athens, Greece. The topography is rugged. encompassing four mountain ranges and intervening valleys with elevations ranging from sea level to 4000 feet (1200 meters).

Geology factors

Cascade onion on volcanic outcrop, Bear Valley, Napa County

The geologic history of the county is complex including tectonic accretion of oceanic sediments that generated hydrothermal metamorphism. These ancient landmasses include significant outcrops of serpentinite, Jurassic Franciscan sandstone and shale. During the Cretaceous, intense sedimentation flowing from the Sierra Nevada covered the seabed to the west with alluvium to great depths, including what is now the eastern quarter of the county. As the oceanic plate dipped below the continental plate, it formed steeply folded ridges. These are capped with sandstone and shale. During the Pliocene and early Pleistocene, volcanic flows poured rhyolite, andesite and basalt across the ridges and valley of the Napa River Basin. Marine sedimentation formed the landscape of the lower Napa and Carneros Valleys.


In addition to latitudinal similarity with Athens and Lisbon, Napa County also shares a similar climate, best described as Mediterranean. Rain falls primarily from November to April with a five to six month drought from May to October. Complicated by the topography of ridges and valleys, rainfall varies from about 20 inches (51 centimeters) annually near the south extent of the county (American Canyon) to about 58 inches (147 cm) per annum near Mt. Veeder and Mt. St. Helena along the western and north boundaries respectively. Rainfall extremes range from less than 10 inches (25 cm) in south county areas in the driest of years to over 100 inches (255 cm) in the wettest locations in wet years. Rainfall is captured by a network of seasonal and perennial streams leaving the county via three drainage basins. The Napa River drains into the San Pablo Bay, Putah Creek drains into the Central Valley and Suisun Creek drains into the Suisun Bay. Maritime influences draw summer fog into the Napa Valley while trapped cold air sometimes fosters “tule” fog in the Berryessa Valley during winter.

Flora diversity

St.Helena fawn lily, Napa County. @ C.Michael Hogan

The complexity of climate patterns, hydrology, geology and topography promotes exceptional botanical diversity. Napa County has been placed in a top tier of biological diversity by The Nature Conservancy. Approximately 20 percent of all species found in California have been recorded in the county. A number of endemic and near endemic species have been identified in the county including Calistoga popcornflower (Plagiobothrys strictus, Boraginaceae)and Napa bluegrass (Poa napensis, Poacaeae) occurring in only two locations near Calistoga. A number of species such as Hepserolinon sharsmithii, Leptosiphon jepsonii, Sidalcea hickmanii ssp. napensis, and Trichostema ruygtii, have been described from Napa County in the last 20 years. Two additional species are nearing publication.

Over 65 vegetation types have been recognized in Napa County (ICE 2004). The northwestern third of the county is dominated by a mosaic of conifer, hardwood and mixed forests. Dominant species include Douglas Fir (Pseudotsuga menziesii), Knobcone Pine (Pinus attenuata), Black Oak (Quercus kelloggii), Coast Live Oak (Quercus agrifolia ssp. agrifolia), California Bay (Umbellularia californica) and Madrone (Arbutus menziesii). Soils are primarily of volcanic origin and slightly acidic. In the center of the county from American Canyon in the south to Knoxville in the far north, serpentinite has a strong influence on the plant communities including shrublands, woodlands and grasslands. A number of narrowly endemic and rare species such as Eriogonum nervulosum, Hesperolinon bicarpellatum, Streptanthus breweri ssp. hesperidis and Navarretia rosulata occur here. The Napa Range, bordering the east side of the Napa Valley, is significant for its volcanic inspired communities. There is a mix of chaparral, rock outcrops and hardwood forests that support a number of narrow endemics including Holly-leaf Ceanothus (Ceanothus purpureus), nodding harmonia (Harmonia nutans), green coyote mint (Monardella viridis ssp. viridis) and Napa bluecurls (Trichostema ruygtii). An undescribed paintbrush (Castilleja sp.) also occurs in this district. The eastern sector of the county is subject to the influences of interior summer heat and thereby an imposing annual water deficit. This area is dominated by hardwood forest, woodland and chaparral. Common species include Interior live oak (Quercus wislizenii ssp. wislizenii), Foothill Pine (Pinus sabiniana), Blue oak (Q. douglasii), chamise (Adenostoma fasciculatum), and scrub oak (Quercus berberidifolia). The lower Napa Valley and surrounding ridges support grassland and brackish marsh communities.

Flora documentation

Astragalus claranus in bloom, Taplin Road, Napa County. @ Jake Ruygt

Nearing completion is a manuscript titled, A Flora of Napa County,that includes comprehensive treatment of native and naturalized plant species that have been recorded to occur within the natural landscape of the county in addition to many urban weeds. This will include over 470 alien species in addition to more than 1210 native taxa. About 35 of the native taxa can no longer be found in the county and were discovered during research of herbarium records. The flora is the culmination of 35 years of field study through much of the county, sometimes not far ahead of the bulldozer. Data accumulated from Ruygt's field study supplements historic documentation and is supported by over 12,000 herbarium voucher collections by numerous botanists. A Flora of Napa County will be useful to novice and professional botanists and stand as a historical record of local plant distribution, since the landscape is rapidly changing. This flora will also be useful to botanical interests beyond the border of the county, due to the broader distribution of many of the species occurring in Napa County.

Human disturbance

Napa County has become a world renowned agricultural region, with a resultant positive influence on the economy of the county, but negative influence on the distribution of many native species. The first pioneers to the Napa area arrived in the 1840’s taking advantage of the rich pasturage for large herds of cattle. Twenty years later the cultivation of grain was widespread and by the 1890’s, the county supported over 100 wineries and 18,000 acres of vineyard. Prohibition led to the temporary decline of wine production, but by the 1960’s development hastened dramatically. Much of the grassland and Valley Oak Savannah habitats have been since been converted to vineyard and large of blocks of land have been fenced, with an outcome of discouraging deer, rabbit, mountain lion and black bear mobility. The Napa Marsh has been confined by levees, some constructed over 100 years ago. To date, about 20 % of the county has been converted to urban, commercial and agricultural uses. Despite this, Napa County remains largely rural and large areas are devoid of development.


  • Weber, Lin. 1998, Old Napa Valley, the History to 1900. Wine Ventures Publishing.
  • California Native Plant Society. 2001. Inventory of Rare and Endangered Plants of California, Sixth Edition.
  • Napa County Wine Industry Growth, Master Environmental Assessment. 1990-2010 Napa County Department of Agriculture. Napa County Agricultural Crop Reports.
  • Ruygt, Jake. A Flora of Napa County. Unpublished. San Francisco Estuary Institute, 2012. Historical Atlas of Napa County.


Ruygt, J. (2013). Flora of Napa County, California. Retrieved from http://editors.eol.org/eoearth/wiki/Flora_of_Napa_County,_California