Blue Oak

Bonnie’s drawing on this cover of the Obispoensis includes an acorn, a couple of leaves and a two individual blue oak (Quercus douglasii) trees from Shell Creek.

This species of oak is extremely common in a vertical band through the center of our Chapter area. It is most common east of the Santa Lucia crest and west to the San Juan River drainage. It occurs only occasionally near the coast where it is replaced by the coast live oak (Q. agrifolia). In the Carrizo Plain area the Tucker oak (Q. john-tuckeri) replaces it.

Unique coloring

I suspect all of us who know the tree know it as blue oak. Its common name refers to its bluish green deciduous leaves and/or its pale gray bark. Other names I’ve found include iron oak, mountain white oak, or mountain oak. The light blue/gray color is particularly evident when compared to evergreen oaks such as liveoaks (Quercus agrifolia, interior live oak (Q. wislizeni) and gold cup oak (Q. chrysolepsis), all of which live within or near the blue oak range. But remember, both leaves and bark are quite variable in color based on where the tree grows.

Leaves and bark are lighter (i.e., more gray or blue when the tree grows in open groves on sunny south and west facing slopes and darker and greener where moisture is present such as north and east facing slopes).

Blue oaks prefer well drained soils so they tend to be found on foothill slopes surrounding California’s Central Valley. Yes, blue oak is endemic to California, which means that it is found naturally only within the political boundaries of California.

A great deal is known about the ecology of the blue oak. So much that it is difficult to chose what to emphasize in a general piece such as this. When I did a web search of Quercus douglasii a fantastic tell-all forest service website headed the list. The web address of this site is One thing I will mention about the web site is that the small amount of stuff I knew already I noted was correct. This leads me to conclude that the vast amount of detail I didn’t know is also true.

Ground cover

One item worth mentioning is the ground cover of herbs Bonnie has drawn around the base of the oak trees. The species found in this area within the drip line of the tree’s canopy are quite different in composition and abundance from the species outside the drip line. Several hypotheses have been proposed for this phenomenon. First, the deep roots of the oak bring up nutrients from deep in the soil where they are below the reach of the shallower-rooted herbs. Because the leaves are a “leaky” system, some of the water soluble nutrients get deposited on the surface of the leaves where they are washed off and drop to the soil under the tree. It has also been noted that during the hot parts of the day, cattle seek shade under the trees. While there, they deposit undigested or unabsorbed nutrients under the tree. Either way, it is hypothesized that there is higher nutrient availability under the tree’s canopy than outside it.

Blue Oak and California native ethno-botany

I will make just a quick note on native California peoples use of the blue oak acorns. All writers discussing California native ethno-botany acknowledge that acorns of this species and most other oak species were gathered and used. In a list of acorns used by the Native Californians that I found on the internet, blue oak tops the list. Essentially all references refer to it as producing the “sweetest” acorn. I assume that means it has the best flavor, which should mean it has the lowest tannin content. Tannins are complex chemicals that are not only bitter tasting, but also interfere with digestion by creating blockages in the digestive tract.

Since tannins are water soluble, they are removed by leaching. Native Californians usually leached acorn meal by placing it in a basket and then placing the basket in running water. I’ve heard people ask where they found the water for all the required leaching. Today, if one wants to eat acorns, one must use treated tap water. That would prove to be quite expensive. One must remember that pre-European Native California populations were relatively small and scattered. There was no Mexican- or European-style field agriculture (except within the Colorado River Valley) in California.

There was habitat manipulation as was discussed by our recent banquet speaker, Kat Anderson, but the smaller population and low impact vegetation manipulation would mean that most streams would flow longer into the dry season and be less polluted than we no find them today. They could simply have been able to put their acorn meal filled baskets into any nearby water course with no ill effect.

by Dirk Walters, illustrations by Bonnie Walters | Dirk and Bonnie Walters are long-time CNPS-SLO members, contributors, and board/committee participants. In addition to his work at Cal Poly, Dirk is the current CNPS-SLO Historian.