What is a Blue Oak?

The blue oak, also known as Quercus douglasii, mountain oak and iron oak, is a perennial, deciduous tree native to California. Blue oaks can be found in the central and coastal portions of the state, with a habitat covering 39 of California’s 58 counties. Its densest populations can be found in the foothills of the Sierra Nevada mountain range and coastal mountains.

Blue oak trees vary in height, depending on environmental factors such as soil quality and climate. They grow slowly, as little as 1–3 inches (2.5–7.6 cm) annually, though growth spurts of up to 1 foot (30 cm) might occur. A mature blue oak typically reaches a height of 15 feet (4.5 m), but under the right conditions, the trees can reach up to 60 feet (18 m) or more. The tallest blue oak on record is a tree in Alameda County with a height of 94 feet (28.7 m).

Scottish botanist David Douglas is credited with naming the blue oak for the bluish tint to its waxy leaves. These leaves are about 1–3 inches (2.5–7.6 cm) long with wavy margins that create seven shallow lobes. Its leaves form a broad, irregular open canopy, with a diameter roughly equal to the tree’s height. Autumnal colors range from yellow to pastel pinks and oranges, and the tree typically sheds its leaves in winter, though an especially hot or dry period will cause the leaves to fall early.

These trees are well adapted to the warm and arid Californian climate. The waxy coating on the leaves, for instance, helps to retain moisture. They thrive in areas receiving between 15–30 inches (38–76 cm) of rain annually. Excessive moisture can encourage fungal growth in the soil, and these fungi can attack tree roots. Over-watering can seriously threaten the health of the tree, making it a poor choice for landscaped areas that receive regular irrigation.

Like all oak trees, the blue oak produces hard, nutty fruits called acorns. The blue oak’s acorns are long and thin, and they are gently tapered at the end. Acorns are an important source of food for wildlife, including songbirds, squirrels and deer.

People also have used the blue oak’s acorns for food, grinding them into flour. Native Americans of the region ground them to make a flour that could be used to make bread. Other parts of the tree, such as the bark and roots, have been used for traditional remedies and weaving. Though no longer popular as a regular part of the diet, acorn flour is still used in some Native American rituals and celebrations.