Oak

Oak trees prefer an airy place in full sun during the growing season. The European Oak, the White Oak and other northern oak species are frost hardy when they are growing in the ground, but they need winter protection when they are planted in containers. A cold but frost-free greenhouse, garage or shed is a good winter place for an oak bonsai tree.

Water the oak thoroughly when the soil gets dry, but avoid constant soil wetness. Water less in winter but never let the rootball dry out completely. Continue reading about watering Bonsai trees.

Watering

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Apply solid organic fertilizer once a month or use a liquid fertilizer every week during the growing season. Don't feed with very high nitrogen which would cause large leaves, long internodes and increased susceptibility to insects and mildew.

Hard pruning is done in early spring before the buds open. Strong terminal buds can also be removed then. New shoots are cut back leaving two leaves. Don't defoliate oaks completely because it would weaken them too much, but you can remove the largest leaves now and then. Trim the upper parts of the crown attentively because they grow stronger than the lower branches. Continue reading about pruning Bonsai trees.

When oaks are wired be careful to remove the wire before it bites into the bark. Wire marks will be visible for a very long time. Guy wires can be a good choice instead.

Young oaks should be repotted every two years, older ones every three to five years in spring before the buds open. Don't cut off more than one third of the roots. Continue reading about repotting Bonsai trees.

Oaks are easily propagated from seed. Cuttings and air-layers don't root in most cases.

Oaks often suffer from powdery mildew. A mixture of 10 g baking soda, 10 ml rapeseed oil, one drop of dishwashing detergent and 1 l of water can help when sprayed repeatedly at intervals of several days. Less often oak leaf blister, bacterial leaf scorch and pine-oak gall rust occur. In severe cases ask a professional gardener for help and use a specific pesticide. Some insects and mites cause galls which do not harm the tree in most cases. Aphids, scale, leafminers and oakworms (caterpillars of diverse moths) can also bother oak trees. You can try to squeeze, scratch off or collect these or blow them away with a jet of water instead of or before using chemical pesticides. Beware of the caterpillars of the oak processionary moth whose toxic hairs are dangerous for humans. For more detailed information on these techniques, check out our Bonsai tree care section.