English oak bonsai

Placement

Quercus robur is a large deciduous tree from Europe and southwest Asia that can live for centuries. As a bonsai it stays outdoors year-round, in full sun and in a well-ventilated spot throughout the growing season. Branches left in shade die back, and still, humid air encourages powdery mildew, the species’ main weakness.

In the ground, English oak is very cold-hardy. In a container, its roots are exposed to frost and need protection as soon as temperatures drop below 23 °F. Wait for the first frosts before sheltering the tree: it needs cold to go dormant. A cold greenhouse, a garage or a frost-free shed all work; you can also bury the pot in the garden with soil just over the roots, out of the wind. Avoid long warm spells that wake the tree too early, and once it breaks dormancy, protect the young shoots from late frosts. Never bring it into a heated room.

Oak suits upright and slanting styles, multi-trunk designs and forest plantings, in medium to large sizes. Subtle deadwood works well; on a deciduous tree, a hollow in the trunk, or uro, looks more natural than long dead branches, because the wood tends to rot. Choose a glazed pot in light, earthy tones. In nature its leaves measure 2–6 in: the whole art lies in reducing them, as the following sections explain.

Soil mix

English oak tolerates a wide range of soils, lime included, but in a pot it needs a free-draining mix that still holds some water. A classic deciduous mix combines half akadama, a quarter pumice and a quarter lava rock. Sift the akadama, and the lava rock if it is dusty, so the mix stays airy. In a rainy climate, raise the share of lava rock; if you cannot check on watering often, raise the akadama.

Watering

Water thoroughly as soon as the mix starts to dry, then wait for it to dry again: oak does not like to stay constantly wet. Needs rise sharply in summer, in full sun and full leaf. In winter the leafless tree drinks little: water less, but never let the root ball dry out completely.

During the growing season, apply solid organic fertilizer once a month or liquid fertilizer every week; another approach is half-strength fertilizer every two weeks, starting once the first spring flush has hardened off. Avoid very high-nitrogen fertilizers: they produce large leaves and long internodes and make the tree more susceptible to insects and powdery mildew.

Pruning

Structural pruning and major cuts are done in early spring, before the buds open; this is also the time to remove the strongest terminal buds. During the season, cut new shoots back to two leaves and pinch out apical buds, except on a branch you want to extend. The top grows more strongly than the lower branches, so trim it carefully.

Leaf reduction takes method. Regularly remove all large leaves, except on the weakest branches, and thin the foliage at the shoot tips to let light in. Tips often form a whorl of four, five or more leaves: keep only the smallest, and a new flush of smaller leaves will follow. Avoid full defoliation, which can actually enlarge the leaves and, on a weak tree, kill off lower branches. Infrequent repotting and removing apical buds before they extend in spring also help reduce leaf size.

Watch for powdery mildew, which is very common when air movement is poor. Less often, oak leaf blister, bacterial leaf scorch and gall rusts appear. Pick off aphids, scale, leaf miners and caterpillars by hand or hose them off before reaching for any treatment. Do not touch oak processionary caterpillars, whose hairs are dangerous to people.

Wiring

Oak can be wired, but wire marks stay visible on its bark for a long time. Wire preferably in late winter, when the tree is leafless and its whole structure is easy to read. Aluminum wire suits deciduous trees: choose wire about a third as thick as the branch, wrap it at a 45° angle with at least one turn around the trunk or a neighboring branch as an anchor, then bend gradually while supporting the inside of the curve.

Check the wire as soon as growth resumes: a branch can swallow it within one to four months. Remove it before it marks the bark by cutting it at every turn rather than unwinding it. For thick branches, or to avoid marks altogether, use guy wires anchored to a strong root, a sturdy branch or the pot. Do not wire a weak or freshly repotted tree.

Repotting

Repot a young oak every two years and an older tree every three to five years, in spring, as the buds start to move. Two other windows also work for Quercus robur: just after the first leaves open, or in early fall. Never remove more than a third of the roots. After repotting, keep the tree out of wind, scorching sun and frost, do not fertilize too soon, and avoid any heavy pruning during recovery, which can take a full season.

Oak is very easy to grow from seed: sow acorns outdoors as soon as they ripen. Cuttings usually fail to root and air layers rarely succeed. Its garden care, hardiness and toxicity are covered in the English oak plant profile.