Quercus glauca aka Ring Cup Oak

Quercus glauca is an evergreen tree in the beech family. Its common names include ring-cupped oak and Japanese blue oak. It grows across Asia, from the Himalayas to eastern China. Its range also includes Japan, Korea, and Taiwan. Wild trees grow in moist forests, ravines, and shaded valleys. In gardens, it forms an upright oval or rounded crown. It can also grow as a large, bushy shrub.

Garden size varies between sources and growing regions. Missouri Botanical Garden gives heights of 20–30 feet [6–9.1 metres]. RHS gives 26–39 feet [8–12 metres], with wider crowns. Its stated spread is 13–26 feet [4–8 metres]. Wild trees may reach or exceed 50 feet [15.2 metres]. RHS gives a maturity period of 20–50 years. The thick leaves have an oval to narrow, pointed shape. Their upper sides are green, with blue-green or whitish undersides. Teeth line the upper part of the leaf edge. Young leaves may show bronze or purple tones. The foliage can turn yellow-green during winter.

Small yellow-green flowers appear in spring, from April to May. Separate male and female catkins grow on the same tree. The flowers offer little visual interest. Brown acorns follow, measuring about half to three-quarters of an inch. Rings circle the outside of each acorn cup. These rings explain the name ring-cupped oak. Its lasting foliage suits a small shade tree or tall screen. Gardeners can also use it as a street tree or shrub.

Grow it in full sun and fertile, well-drained soil. Oregon State also lists partial shade as suitable. It prefers moderate moisture and soil rich in organic matter. It tolerates clay and can adapt to dry soil. RHS lists acid or neutral soil and recommends shelter. Cold winds and late frosts can cause problems. Sources give USDA zones 7–9 and UK hardiness H4. Care needs are low, but pests can still occur. These include aphids, caterpillars, leaf miners, and oak gall wasps. Powdery mildew and honey fungus are possible diseases.

Plants For A Future lists both leaves and seeds as edible. Ground seeds can thicken food or supplement flour. Bitter seeds need washing to remove tannins before use. RHS warns that eating this plant can harm dogs. Its hard wood serves for building and fuel. Galls provide tannin and dye; fresh leaf mulch can hinder growth.