Quercus petraea aka Sessile Oak

Quercus petraea is a large oak that sheds its leaves. Its names include sessile oak, Cornish oak, and Irish oak. It belongs to the beech family, Fagaceae. Its native range runs from Europe to northern Iran. It is native to Britain and Ireland. Wild trees grow in woods and fertile upland sites. In western Britain, it can dominate woods on acid soils.

The tree grows slowly enough to need long-term space planning. NCSU gives its growth rate as medium. Garden trees reach 60–80 feet [18.3–24.4 metres] tall and wide. RHS lists heights above 39 feet [12 metres], without an upper limit. Oregon State reports heights reaching 150 feet [45 metres]. Trees on stormy, exposed sites may stay much smaller. They may reach just 33–50 feet [10–15 metres]. The crown spreads into a broad, rounded shape. The trunk extends well into the crown. Young bark is smooth and grey. Deep cracks form as the tree ages.

The leaves grow in an alternate pattern along the twigs. Their upper sides look glossy and dark green. Each leaf has four to six pairs of rounded lobes. The pale underside may carry hairs along its main veins. Leaves can reach about six inches long. Autumn tones range from yellow to orange-brown. Smooth, grey-brown twigs have small, tawny lenticels. Greenish-yellow catkins hang from branches as new leaves emerge. Chicago records flowering from March through June. Oval acorns grow in clusters with almost no stalks. Each cup covers about one-third of its nut. The acorns ripen in their first year, during autumn.

Grow this oak in full sun or partial shade. It favours deep, fertile soil with good drainage. Moist, acid soil suits it, but its soil range is broad. It also accepts poorer, drier ground and higher soil pH. RHS lists clay, loam, sand, and chalk as suitable. Its hardiness ratings are USDA zones 4–7 and RHS H7. Maturity can take more than 50 years. Its large crown provides shade where space allows. Birds and mammals eat the acorns. The tree also supports moth and butterfly larvae. Builders use its wood for cabinets and other furniture.

Possible pests include aphids, caterpillars, leaf miners, and gall wasps. Oak processionary moth may also attack it. RHS lists powdery mildew and honey fungus as possible diseases. NCSU reports some resistance to mildew and deer damage. Raw acorns and young leaves can cause poisoning when eaten. Dogs and horses are also at risk. Acorns become edible only after processing removes their tannins. Prepared seeds can supply flour for bread or thickening stews.