English oak

Identification and origin

Quercus robur belongs to the Fagaceae, the beech and chestnut family, and to the white oak group, whose acorns ripen in a single season. Its native range covers most of Europe, from the British Isles and the Iberian Peninsula to Scandinavia, Russia and Ukraine, along with Türkiye, the Caucasus and Iran. It has naturalized elsewhere, notably in North America and New Zealand.

It is a large tree with a short, sturdy trunk and a broad, rounded crown. In cultivation it usually reaches 39.5–69 ft tall and about as wide; in good conditions it can exceed 65.5 ft and grow as tall as 131 ft. It takes twenty to fifty years to reach full size, and the oldest trees live up to eight centuries, with trunks as much as 4 ft across.

• Leaves: alternate and simple, dark green above and bluish green beneath, 3–5 in long, which is small for an oak. Each has three to seven pairs of deep, rounded lobes, and the base forms two small earlobes; the leafstalk is very short. In fall they turn brown or yellow, sometimes with coppery or reddish-brown tones, and often hang on the tree for part of the winter.
• Flowers: male and female flowers grow on the same tree. The long, yellow-green male catkins appear in spring and go almost unnoticed.
• Acorns: oval, about 1–1 in long, borne singly or in clusters of two to five at the end of a long stalk of 1–3 in, which gives the species its name of pedunculate oak. The cap covers a quarter to a third of the nut. They start green and turn brown as they ripen. A seedling tree waits twenty to thirty years for its first acorns.
• Bark: grayish brown to nearly black, deeply furrowed with age.

Among the cultivars, ‘Fastigiata’, narrow and columnar, is the most common but is prone to powdery mildew; ‘Concordia’ has bright yellow young leaves. The tree feeds and shelters many mammals, birds and insects.

Light

English oak grows in full sun, with at least six hours of direct sun a day, and tolerates partial shade. Good air movement matters as much as light: in a damp, enclosed spot, powdery mildew takes hold more easily. As a bonsai, branches kept in shade tend to die back; an airy, sunny spot throughout the growing season suits it best.

Watering

In the ground, the tree prefers fertile, well-drained soil that stays moist without standing water. Rather than following a schedule, check soil moisture deep down during dry spells, especially around a young tree.

In a container, water thoroughly as soon as the mix starts to dry, without keeping it constantly soggy. In winter the leafless tree uses little water: water less, but never let the root ball dry out completely.

Potting mix

English oak adapts to most soils, whether clay, loam, sand or chalk, and tolerates lime. It does best in deep, fertile, moist but well-drained soil, ideally loam. In a container, use a free-draining mix that still holds some moisture; for a bonsai, a deciduous mix based on akadama, pumice and lava rock works well.

Temperature

The species is hardy in USDA zones 5a to 8b and withstands winters of -4–5 °F in the ground. Very severe winters can still kill back some twig tips. Once a container tree breaks dormancy, protect it from late frosts.

In a container, the roots are far more exposed than in the ground: protect the root ball from hard frost below 23 °F by moving the pot into a cold greenhouse, a garage or a frost-free shelter, or by burying it in the garden. The tree needs its cold winter rest and cannot be grown indoors.

Repotting

In the ground, English oak needs plenty of room: plant it as a specimen, lawn or shade tree, and keep it out of small yards.

In a container or as a bonsai, repot in spring: every two years for a young tree, every three to five years for an older one. Remove no more than a third of the roots. For this species, repotting can also be done just after the first leaves open, or in early fall. The English oak bonsai guide covers the soil mix and root care in detail.

Pruning

Grown as a tree, English oak needs very little pruning.

As a bonsai, structural pruning is done in early spring, before bud break; this is also when to remove overly strong terminal buds. During the season, cut new shoots back to two leaves. Avoid full defoliation, but remove the largest leaves from time to time. Keep an eye on the top, which grows more strongly than the lower branches.

Propagation

• Seed: the simplest and most common method. Sow acorns as soon as they ripen in fall, in a cold frame or outdoors; protect them from rodents and birds, which eat a great many.
• Grafting: used to reproduce cultivars.
• Cuttings and layering: cuttings usually fail to root; air layering is possible but difficult.

Common problems

One symptom can have several causes: yellowing leaves can come from waterlogged soil, drought or a pest. Look at the tree and its conditions before treating.

• Powdery mildew: very common in humid climates or poorly ventilated spots; a white coating covers the leaves. High-nitrogen fertilizer, which produces large, soft leaves, makes the tree more susceptible. Improve air movement; a severe attack can call for a specific fungicide.
• Other diseases: honey fungus, anthracnose, oak leaf blister, cankers, leaf spots and oak wilt.
• Pests: aphids, caterpillars, leaf miners, scale, oak lace bugs, borers, acorn weevils and gall wasps.
• Oak processionary moth: its caterpillars carry irritating hairs that are dangerous to people. Do not touch them, and call a professional for an infestation.

Toxicity

English oak is toxic. Its leaves, especially young ones, and its raw acorns contain gallotannins, along with quercitrin and quercetin. Eating them can cause abdominal pain, constipation followed by diarrhea that is sometimes bloody, depression, thirst and excessive urination; in horses, discolored urine and jaundice are also described. Acorns swallowed in quantity can also block the digestive tract. The tree is harmful to dogs that eat it and toxic to horses, and the poisoning is rated low in severity. It does not cause contact dermatitis. If any is eaten, contact a veterinarian or a poison control center.