European hornbeam bonsai

Placement

Carpinus betulus is an outdoor tree. During the growing season, give it sun or part shade in a spot with good airflow; if summers are very hot, some afternoon shade keeps it from suffering. Do not grow it indoors: like all temperate trees, it needs the cycle of the seasons and a true cold winter rest to stay vigorous.

In the ground, hornbeam does not mind frost, because its roots reach deep into the soil. In a shallow bonsai pot, however, the root ball freezes easily. The tree can stay outdoors with light protection and tolerates nights of roughly 14 °F to -4 °F, but its roots need protection whenever it freezes hard. Wait for the first frosts before sheltering it: that cold helps it go dormant.

Where nights often drop below 14 °F, keep it in an unheated greenhouse or cold frame, which evens out temperature swings and guards against drying wind and excess rain. Otherwise, wrap the pot in foam insulation or bury it in the garden up over the roots, out of the wind. In milder climates, with nights around 25 °F, simply set the pot on the ground out of the wind, perhaps covered with a sheet. Vent the shelter on sunny days: a long mild spell would wake the tree too early, and young buds cannot survive a freeze. In spring, protect new shoots from late frosts.

Soil mix

The mix must hold enough water between waterings, let the excess drain immediately and stay airy. For a deciduous tree like hornbeam, a blend of half akadama, one quarter pumice and one quarter lava rock works well. Sift it to remove the dust so the mix stays airy and drains well. Akadama breaks down after about two years and then loses its aeration.

Watering

Water as soon as the surface of the mix starts to dry: the root ball must never dry out completely. Avoid the opposite extreme as well, since permanently soggy mix causes root rot. Do not water on a fixed schedule: look at and feel the mix, because needs vary a great deal between a windy summer day and a cool one. In winter, during dormancy, keep the root ball slightly moist and water only when it dries.

Do not use very hard, lime-rich water: watered with it all the time, hornbeam eventually yellows from chlorosis. During the growing season, apply a solid organic fertilizer every four weeks or a liquid fertilizer every week, making sure it also gets micronutrients.

Pruning

Hornbeam responds well to pruning, even hard pruning.

• Branches: remove heavy branches in spring, before the buds open.
• Buds: large terminal buds can be taken off.
• Shoots: let new shoots extend and harden, then cut them back to two leaves.
• Fall and winter: if you prune at this time, always leave buds on the twigs, or they will die back.

After repotting, wait for the tree to recover before any major pruning. The dry leaves that sometimes persist until spring are part of the tree’s winter appeal.

Wiring

Wiring is possible, but hornbeam’s smooth bark marks quickly: remove the wire in time, before it cuts into the bark, because the scars would stay visible for decades. On a deciduous tree, wiring is easiest in late winter, when the tree is bare. Choose aluminum wire about one third the thickness of the branch and wrap it at about 45°. In the growing season, branches thicken fast and wire can bite in within one to four months: check often. To remove it, cut the wire at every turn instead of unwinding it.

Guy wires and strings are a good alternative to coiled wire: anchored to a strong root, another branch or the pot, they let you lower a branch that is too thick or stiff to bend with wire.

Repotting

Repot every two years in spring, before the buds open; older trees can be repotted less often. First check the roots by sliding the tree out of its pot: if they circle around the root ball, it is time. Untangle them with a root rake or chopstick and remove no more than about 30% of the roots. Water thoroughly, then keep the tree out of scorching sun, intense heat and frost until it recovers, with no fertilizer or hard pruning.

Hornbeam is easily raised from seed; its cuttings and air layers root somewhat better than those of beech. Watch for mealybugs, scale and powdery mildew: treat with a suitable product and improve light and airflow. Root rot occurs when the tree is overwatered.