Japanese zelkova bonsai

Placement

Zelkova is an outdoor tree. During the growing season, give it full sun; during heat waves, a little light shade keeps it from suffering. In a pot its leaves stay much smaller than in the ground, which helps with miniaturization. In fall it turns yellow, orange, red and purple, then drops its leaves: it is deciduous, and its bare ramification is the great attraction of winter.

In the ground it tolerates frost, but a bonsai in a shallow pot is far more exposed. Below 23 °F, fine twigs can die back and roots can suffer: move it then to a cold but frost-free place, such as a cold greenhouse, garage or shed.

Some zelkovas are sold as indoor bonsai. In that case, keep them over winter on a very bright, cool windowsill with good humidity, and move them outside once the frosts are over. A tree that spent the previous winter indoors should not be exposed to severe cold during its first winter outdoors.

Zelkova has few pest and disease problems, and it resists Dutch elm disease. Aphids, leafhoppers, gall mites and spider mites sometimes appear, as do leaf spots. If an infestation occurs, treat it with a suitable product, and also reassess the growing conditions, which are often to blame. The Japanese zelkova plant profile covers the tree in the ground and its hardiness.

Soil mix

A standard bonsai mix works, with an ideal pH between 5.5 and 6.5. For deciduous trees, a common blend is half akadama, a quarter pumice and a quarter lava rock. Akadama, a hard-baked Japanese clay, is sifted before use to remove dust; it breaks down in about two years, which calls for regular repotting. Pumice holds water and nutrients and encourages root growth; lava rock adds structure and aeration. The mix must hold water, drain well and let air in, all at once.

Zelkova is traditionally planted in a shallow oval or rectangular pot that shows off the even spread of its roots all around the trunk (nebari).

Watering

Water regularly: the root ball must never dry out, yet too much water rots the roots. That is why there is no fixed schedule. Check the pot every day and water only when needed, when the surface of the mix starts to dry, then soak the whole root ball thoroughly. Needs rise sharply in strong sun, in hot weather and during the spring flush of growth. In winter, during dormancy, simply keep the root ball slightly moist. Avoid very hard, lime-rich water.

Fertilizer: during the growing season, apply a solid organic fertilizer every four weeks or a liquid fertilizer every week, with a balanced nitrogen-phosphorus-potassium ratio. Another approach is to give a high-nitrogen fertilizer weekly for the first month after the buds open, then a balanced fertilizer every two weeks until late summer. Do not feed a tree that has dropped its leaves.

Pruning

Pruning builds the dense, fine ramification that makes zelkova so beautiful.

• Maintenance pruning: let new shoots grow 4 to 6 leaves, that is three or four nodes, then cut them back to one or two leaves. Repeat throughout the season.
• Structural pruning: after leaf fall, remove thick branches and shoots that have grown too long, to keep the characteristic winter silhouette.
• Defoliation: a young zelkova can be fully defoliated in early summer to encourage branching; larger-leaved zelkovas respond well. On an old tree, partial defoliation is better: remove only the largest leaves to let light into the canopy. Cut each leaf but keep its stalk. Only defoliate a strong, healthy tree, never a weak or freshly repotted one. After partial defoliation, shade the tree for about a month to protect the interior foliage that is suddenly exposed.

Styles: zelkova suits every form and size, but it is the tree of the broom style (hokidachi), which recreates its natural shape in open fields: a straight trunk that fans out into a mass of branches, then ever finer twigs.

Wiring

Wire preferably in fall, in winter or in spring before the buds open, while the tree is leafless; midsummer wiring is also possible. The smooth gray bark marks very easily: watch the wire closely and remove it as soon as it starts to bite. For a deciduous tree, use aluminum wire about one third the thickness of the branch, wound at roughly 45°. The branch sets in one to four months, depending on the tree’s vigor. Remove the wire by cutting it at every turn rather than unwinding it.

For the broom style, a traditional method is to gather and tie together the branches of a young tree so they grow upward in parallel, then remove the ties in spring.

Repotting

Repot zelkova every two or three years in spring, when the buds begin to extend; young trees, up to about ten years old, can be repotted every year, and large old specimens less often. Prune the roots carefully, aiming for an even spread at the base of the trunk. Use the mix described above.

Propagation: sow seed outdoors in fall or spring. Softwood cuttings root in summer and hardwood cuttings in winter. Air layering is done once the spring growth has hardened.