Blue Oak

Blue Oak © SBernhard

Quercus douglasii

Description (Jepson,

  • Eudicotyledon
    • Eudicots are a major lineage of flowering plants; see family for general characteristics
  • Oak Family (Fagaceae)
  • Small or medium-sized, winter-deciduous tree
    • Round and compact canopy with many crooked branches
  • Bark is light gray with linear, shallow fissures
  • Leaves
    • Distinctly dull blue-green upper surface
    • Pale green underside with minute hairs
    • Waxy and thick
    • Lobes shallow and wavy
  • Flowers
    • Separate male and female flowers on same plant (monoecious)
    • Male flowers on yellow-green catkins (long hanging clusters of small, petalless, unisexual flowers)
    • Female flowers are inconspicuous and often solitary, growing at leaf junctions of new branches
      • Ovary inferior (below the attachment of other flower parts)
  • Wind pollinated – see Oak family to learn more
  • Fruit is an acorn, variable in shape, with a shallow warty cap; matures in one year
  • Grows to a height of 60 ft. with a diameter up to 2 ft.
  • Average lifespan of 300 years
    • Slow-growing, with annual growth on mature trees less than 1 ft. in height (Wyly 2019)
Young Tree © SBernhard


  • Native and endemic (limited) to California
    • Grows on dry, rocky slopes in woodlands and interior foothills and, commonly, as the only tree species in oak savannas (Wyly 2019)
      • “Hallmark species” of California foothills, covering nearly half of the state’s oak-covered lands (Pavlik 2014)
    • See Calflora for statewide observations of this plant
      • Core area of distribution almost completely encircles the Central Valley
  • Grows at elevations to 5,215 ft.

Uses (San Mateo County Parks prohibits removal of any natural material)

  • Wildlife
    • Oaks are a keystone species, supporting a great number and variety of wildlife and other plants
    • Provide food (acorns, leaves, roots) and habitat for many mammals, birds, butterflies, amphibians, and insects
    • Large mature trees, dead trees, and those with broken limbs are especially important for cavity-seeking mammals and birds
    • Larval food source (host) for the mournful duskywing butterfly (Erynnis tristis)
    • Pollen source for bees
      • Bees do not help pollinate oaks, which are wind-pollinated, because female oak flowers offer no nectar, so there is no incentive for the bee to visit and thus transfer pollen
    • Provide a substrate for lichens, mosses, and parasitic plants, like mistletoe
    • Oaks, particularly those in the white oak lineage, host more gall insects than any other native tree or shrub in the western United States (Pavlik 2014 and Russo 2006)
      • Galls are abnormal, tumorlike growths induced by parasites (e.g. insects, mites, or bacteria)
      • Many species of gall (cynipid) wasps co-opt an oak’s DNA to create a unique home and food for their larvae
      • According to Ron Russo, author of Plant Galls of California and Other Western States, “Blue oaks are the real stars of this gallant parade of gall wasps. They support the largest number of known species (41), as well as the most bizarre designs and color combinations of any oak galls” (Russo 2009)
      • Watch this short video on the amazing worlds of oak galls (KQED 2014)
  • Native people
    • Oak acorns were an important food for Native people, who gathered them each fall, leached out the tannins, and ground them for making mush or bread
    • Leaves chewed for sore throats
    • Poultice of ground oak galls and salt applied to burns, sores, and cuts
    • Acorn leachate used for dying baskets
    • Wood was used to make bowls
  • CAUTION – blue oak pollen is a severe allergen
David Douglas

Name Derivation

  • Quercus (KWER-kus) – from the Latin for “oaks” from classical times; possibly from the Celtic quer, “fine,” and cuez, “tree”
  • douglasii (DUG-las-ee-eye) – named for David Douglas (1798-1834), Scottish botanist and collector
    • Over 80 scientific names of plants and animals honor Douglas, more than any other person
    • At Edgewood, 9 scientific plant names honor Douglas, e.g. California mugwort (Artemisia douglasiana) and purple mouse-ears (Diplacus douglasii), as well as several common names, e.g. Douglas-fir (Pseudotsuga menziesii var. menziesii) and Douglas’ microseris (Microseris douglasii ssp. douglasii)
  • Blue oak – for the distinctive color of the leaves


  • Most drought tolerant of California’s deciduous oaks with many adaptations for water conservation (Pavlik 2014 and Wyly 2019)
    • Thick, waxy leaves, which gives the characteristic blue color, and underside hairs help prevent water loss
    • Leaves make adjustments to survive even a 30% water loss
      • Increase salt levels in the photosynthetic cells to prevent wilting
      • Build more cellulose and lignins to reinforce structure
    • Leaf canopy is relatively thin compared to other oak species
    • Acorns germinate quickly, within 1 month of maturing, with rapid development of root system
    • Roots are often extensive, growing through fractured and jointed rock to a depth of 80 ft. or more to tap groundwater reserves
    • May go dormant to conserve resources during periods of prolonged drought or extreme heat, shedding all leaves, aborting flower buds or dropping all developing acorns
      • Drought-deciduous tree species are rare, but include the California buckeye
  • Resprouts from the root crown, an adaptation to wildfires
  • Produces an especially abundant acorn crop (a mast) in highly irregular intervals; see Oak family to learn more


  • In the white oak evolutionary lineage (Section Quercus), commonly called the white oak group; see Oak family to learn more about these lineages
  • Catkins have 25 to 100 individual male flowers, and each tree bears thousands of catkins in any given year (Pavlik 2014)
  • The Ohlone Regional Wilderness (Alameda County) has what is believed to be the largest (~21 ft. circumference) and oldest (> 400 years) blue oak in the Bay Area

ID Tips

  • See Oak family for comparative chart of oak lineages
  • May be confused with valley oak (Q. lobata)
    • Blue oak
      • Leaves are a bluish-green color with much shallower lobing (sometimes just wavy-edged)
      • Bark is light gray with more narrow, linear, shallow fissures
    • Valley oak
      • Leaves are deeply-lobed
      • Bark is darker and has wider, deeper fissures
Acorn (L), Leaves (M), Bark (R)
© AFengler (L), DSchiel (M,R)

At Edgewood

  • Found on dry, rocky woodland slopes
    • Several blue oaks grow at the top of the Sylvan trail near its junction with the Serpentine trail
      • A young blue oak grows downslope and to the left of an unusual blue-leather hybrid
    • See iNaturalist for observations of this plant
  • Flowers March – May

See General References

Specific References

David Douglas [Frontispiece illustration]. 1836.  W.J. Hooker. Companion to the Botanical Magazine (Vol. 2). Public Domain.

Freyer, J.L. 2018. Quercus douglasii. Fire Effects Information System. United States Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.

KQED San Francisco. 2014, Nov. 18. What gall! The crazy cribs of parasitic wasps [Video]. Deep Look. YouTube.

Pavlik, B., et al. 2014. Oaks of California. Cachuma Press, Los Olivos, California, and the California Oak Foundation.

Russo, R. 2006. Field Guide to Plant Galls of California and Other Western States. University of California Press, Berkeley and Los Angeles, California.

Russo, R. 2009, Jul. 1. Call of the galls. Bay Nature.

Standiford, R.B. 2019. University of California Oak Woodland Management.

Theodore Payne Foundation for Wildflowers and Native Plants: California Native Plant Database. 2009. Quercus douglasii.

U.S. Forest Service. Wind and water pollination. Forest Service. United States Department of Agriculture.

Wyly, Z. 2019. Feb. 8. Species spotlight: Quercus douglasii Hook. & Arn. International Oak Society.