Oaks, or Quercus

November 2017

Valley oakThe ground around mature oak trees is littered this time of year with the fruits of oak trees, which are known as acorns. Oak trees provide important habitat, and the acorns provide highly nutritious autumn and winter food for birds, reptiles, insects, mammals, and even humans. We are blessed to have a few oak species native to this area which we often use in restoration and bank stabilization plantings, including Quercus lobata, the mighty valley oak and the largest of North American oaks; Quercus douglasii, known commonly as blue oak; and Quercus wislizeni, or interior live oak. Oak trees are in the Fagaceae, or Beech family.

The valley oak tree is endemic (native and restricted) to California, and is a common restoration tree along the creek, as it prefers to be closer to an ample water supply on the valley floor at elevations below 3,000 feet. Many awesome examples of old valley oaks can be seen along Putah Creek, including in the UC Davis Arboretum, where some trees are estimated to be over 300 years old, pre-dating both the original town of Davisville (called Davis today) and the UC Davis campus. These mighty trees grow quickly, and can reach 20 feet tall in 5 years, and 40 feet in 10 years. They can reach ages of over 500 years, and can grow to heights of close to 100 feet. As they age, their branches develop a spreading, arching habit which is quite beautiful in open or creekside settings, but due to its height and spread it is not a good tree for urban residential yards. Leaves turn yellow to orange in the fall and they drop their acorns in October. Valley oak trees provide habitat (food, forage, nesting, and resting space) to over 50 species of birds, mammals, reptiles, amphibians and insects. Mature trees can tolerate wild fires and most will resprout from root crowns.

Blue oak trees are endemic to California, and are found at higher elevations, in the Sierra and Coast foothills. They are a major part of the oak woodlands habitat that forms a bathtub-shaped bowl up to 3,500 feet in elevation surrounding the Central Valley. They are smaller than the valley oaks, growing to 50 to 80 feet tall. They get their name from the dark blue-green tint of their leaves, which is easier to see from a distance or in contrast to other nearby oak species. They are deciduous, losing their leaves in the winter. These beautiful trees are slow growing, usually adding only a few inches in height each year. They do well on dry slopes, preferring full sun, and do not need much water, and as a result are a good restoration and backyard landscape tree.

Interior live oaks grow inland, and along the coast, on the valley floor and up into the foothills to 5,000 feet, and can live to be more than 500 years old. At maturity, it is a tall, broad tree (50 to 70 feet tall), with leathery evergreen leaves (it keeps its leaves year-round). This species is a major component of the foothill woodlands. The acorns of the interior live oak take 2 years to mature, unlike the blue and valley oak acorns, which mature in one year. It readily resprouts after fires. Since it is a smaller tree, it is more commonly found in gardens and yards, and grows well in dry well-drained soils.

Early Californians harvested acorns and stored them in granaries for next season, or ground them, leached out the tannins, and cooked the mash for immediate consumption. Poultices were made from oak galls (the casing the tree develops around insect eggs), to treat burns and cuts. Today, different species of oak acorns continue to be consumed by many peoples across the globe. It is highly nutritious, containing levels of proteins equivalent to some types of nuts as well as healthy omega-3 fatty acids. This author’s favorite acorn for baking is from the blue oak, as it imparts a rich smokey flavor to baked goods. Propagation is from the acorns, which sprout fairly easily after a couple of months in the refrigerator.