Blue oak (Quercus douglasii) is the most drought-tolerant among California’s native oak species that are deciduous or lose their leaves. There are 21 California native oak species, 10 that grow into trees and 11 that are shrubs. Of the 10 that grow into trees, white or valley oak (Quercus lobata) and black oak (Quercus kelloggii) are fall-winter deciduous, while Engelman oak (Quercus engelmannii) is summer deciduous in scorching heat — dropping its leaves to conserve moisture — but evergreen where summers are more moderate. Blue oak is named for its attractive bluish-green foliage, which turns pastel autumn colors before dropping off. Blue oak grows to a height of around 50 feet with a spread of 70 feet, so you will need a good-size backyard to accommodate it. However, it is a slow grower that may live for half a millennium. It also has a strong immune system and does not succumb to sudden oak death, an invariably fatal disease caused by a soil-dwelling, funguslike organism.<div></div><br>