The Blue Oak (Quercus douglasii) is an oak in the white oak section of the genus, Quercus sect. Quercus. It is native to the foothills surrounding the Central Valley of California, USA. It is also sometimes known as the Mountain Oak, and occasionally the Iron Oak.
Blue Oaks are medium-sized trees growing up to 15–25 m tall, usually with a somewhat irregularly-shaped crown, and a trunk 0.5–1 m in diameter. The bark is light gray with many medium-sized dark cracks; from a distance, it can appear almost white. The name Blue Oak derives from the dark blue-green tint of its leaves, which are deciduous, 4–10 cm long, and entire or shallowly lobed. The blue color can be subtle but becomes much more evident when viewed next to one of the live oaks with whom it shares its range, which tend to have much greener leaves. The acorns are 2–3 cm long, with a moderately sweet kernel, and mature in 6–7 months from pollination. It prefers dry soil and plenty of sunlight.
Natural hybrids between Blue Oak and the related Shrub Live Oak (Quercus turbinella), Valley Oak (Q. lobata) and Oregon White Oak (Q. garryana) often occur where the species grow together in the same area. Other species sympatric with the Blue oak include Canyon live oak and Pacific madrone.