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U.S. Forest Service

Plant of the Week

Map of the United States showing states. States are colored green where the species may be found. Range map of blue oak. States are colored green where the species may be found.

Blue oak leaves and acorns. Blue oak leaves and acorns. Photo by Beatrice Howitt, California Academy of Science.

Blue oaks. Blue oaks. Photo by J.E. and Bonnie McClellan, California Academy of Science.

Blue Oak (Quercus douglasii Hook. & Arn.)

By Forest Gauna

Blue oak is named for the bluish cast of the leaves; a stand of blue oak trees from a distance has a “cool,” relaxing blue-grey tinge, which is a welcome sight in the hot summer sun of the California foothills.

Blue oak is endemic to California. It inhabits the foothills surrounding the Great Central Valley, essentially encircling it. It is often sold as an ornamental tree, and will reward the patient homeowner who is willing to wait for it to grow. In slightly higher foothill elevations, blue oak often appears with grey pine (Pinus sabiniana), another California endemic named for its grey-green leaves. Like grey pine, blue oak provided the local Native Americans with a wealth of resources, the greatest of which were the acorns. Acorn flour was a very important food source for the natives of California, and continues to be made and used to this day. Other oaks (esp. Q. kelloggii, black oak) are used as a food source as well. Blue oak acorns were among the most commonly gathered, for indigenous Californians thought them to have a good flavour. Acorn meal must first be treated in special ways, for it contains bitter-tasting compounds called tannins that must be removed before cooking the flour. Besides acorns, blue oak also provided wood for building, for making utensils, for fuel, and produced an extract used as a dye.

The several species of oaks in California are easy to recognize, but because of their number and adaptation to similar habitats, are a bit more difficult to identify from each other. Blue oak leaves are relatively bluer than other oaks and are lobed, though not as sharply nor as deeply as other oaks. The leaf is not shiny, but rather dull and papery; instead of bending, it will break in half if bent. The bark is smooth and light grey. Often a blue oak tree will have galls; called “oak apples” The oak apples are a greyish-red when growing and greyish-black when dead. Immature wasps, inserted into the tree by their mother, cause them. Another kind of gall grows on the leaves, but these are smaller, usually just a spot on the leaf. Like most of the Fagaceae, or oak family, the flowers are yellow and inconspicuous, and borne upon catkins. Unlike some other California oaks, blue oak is deciduous.

For More Information

PLANTS Profile - Quercus douglasii, blue oak