Quercus douglasii aka Blue Oak

Quercus douglasii, or blue oak, is in the beech family. It comes from California and forms dry woods there. It grows around the Central Valley and southern inland valleys. It lives on rolling hills and lower mountain slopes. Garden trees reach about 20–60 feet [6–18 metres]. Some reach 90 feet [27 metres]. One source gives 39 feet [12 metres]. Trees grow slowly but can live for many years. In the southwest of its range, it may form shrubs.

The crown is round. Its thick branches are often crooked. Trees in open sites may have short trunks. Their leaves can reach almost to the ground. Gray bark splits into small, square plates as trees age. Thick, brittle twigs are gray or reddish-brown. The leaves are simple, each with one blade. They grow in turn along the stems, not in pairs. Their edges may be wavy or have shallow lobes. Some have smooth edges. Leaves are blue-green on top and paler beneath. Fall leaves turn pale pink, orange, or yellow. Trees on moist sites may keep their leaves longer.

Male and female flowers grow on the same tree. In spring, male flowers form hanging clusters called catkins. These are yellow-green. Small female flowers grow by leaves on new shoots. Wind carries the pollen. One source gives May as the bloom time. Acorns mature within one year and ripen in October. They range from oval to a shape that tapers slightly. Shallow cups have rough scales, and ripe nuts turn brown. A large seed crop may come before several lean years.

Grow blue oak in sun. Young plants accept some shade from the side. It withstands drought and grows in dry or moist soil. Sources differ on the best soil for this tree. They name sand or deep, fertile loam. It also grows in clay. It accepts soil from slightly acid to slightly alkaline. Reported USDA hardiness spans zones 8–11. Cool British summers can curb growth. They can leave wood at risk of winter frost damage. Sources differ on how well it withstands coastal exposure. Deep taproots make older nursery plants hard to move.

Blue oak can serve as a specimen or street tree. It can form a screen or woodland canopy. Plantings can also help stop soil loss through erosion. Insect larvae can cause growths called galls. These supply tannin and dye. People have used it for basket rims and acorn-cup buttons. They have also used it to make medicines. Some Indigenous peoples of North America ate cooked seeds as staple food. Ground seeds can thicken stews or mix into bread flour. Wash bitter seeds to remove tannins before use. One source lists no known hazards. This does not mean the tree is safe for pets.