Japanese zelkova

Identification and origin

Zelkova serrata belongs to the Ulmaceae, the elm family. It is native to northern, central and southeastern China, Manchuria, Korea, Japan, Taiwan and the Kuril Islands, and has become locally naturalized in Germany, Czechia and New York State.

It is a large deciduous tree. In cultivation it usually reaches 49–78.5 ft tall with a similar spread, after twenty to fifty years; in the wild it can exceed 98.5 ft. Growth is moderate to fast. When young it takes on a graceful vase shape: a straight trunk that opens into many upward-reaching branches, then into a mass of fine twigs. This fan-like branching is what inspired the broom style of bonsai.

The leaves are simple and alternate, oval to oblong, pointed at the tip, with sharp, regular teeth along the margin. They are 1–3 in long. Dark green in summer, they turn yellow, orange, burgundy or purple in fall, depending on the tree. Young shoots are reddish.

Small, inconspicuous greenish flowers appear in spring, followed by tiny drupes with no ornamental value. The bark is smooth and gray, dotted with lenticels; with age it flakes slightly in small patches.

Several cultivars are available: ‘Green Vase’, vigorous and narrowly vase-shaped, with bark that peels to reveal orange patches; ‘Musashino’, narrow and columnar; ‘Village Green’, with reddish fall color; ‘City Sprite’, more compact, yellow in fall; and ‘Wireless’, broad and red in fall.

Light

Full sun suits it best, with at least six hours of direct light a day, though it also accepts partial shade. Any exposure works, but it appreciates a spot sheltered from cold, drying winds. Grown as bonsai, it stays in full sun through the growing season, with a little shade during heat waves. In a container its leaves stay much smaller than in the ground.

Watering

In the ground, once well established, zelkova tolerates drought and copes with soil that is sometimes dry. It dislikes waterlogged ground, however: avoid spots where water stands.

In a container, things are different: the root ball must never dry out, yet too much water rots the roots. Check the pot every day, but water only when the surface of the mix starts to dry. Needs rise sharply in strong sun, in hot weather and during the spring flush of growth. In winter, keep the root ball slightly moist. Avoid very hard, lime-rich water.

Potting mix

It adapts to clay, loam, sand or chalk, whether acidic, neutral or alkaline, as long as the soil drains well. It tolerates alkaline soils, heavy clay and dry sites, but not ground that stays wet. Deep, fertile, moist soil lets it perform at its best. In a container, use a free-draining bonsai mix; a pH between 5.5 and 6.5 is ideal.

Temperature

Japanese zelkova is hardy in USDA zones 5a to 8b. In the ground it withstands northern European winters, down to -4–5 °F. It stands up well to wind and adapts to urban conditions, which makes it a popular street tree. It is, however, highly susceptible to ice damage.

In a container it is far more vulnerable: in a shallow bonsai pot, fine twigs can die back and roots can suffer below 23 °F. Move it then to a cold but frost-free place, such as a cold greenhouse, garage or shed.

Repotting

In a container or as bonsai, repot every two or three years in spring, when the buds begin to extend; young trees can be repotted every year, and large old specimens less often. Prune the roots carefully to build an even spread at the base of the trunk. Choose a free-draining mix: for deciduous bonsai, a common blend is half akadama, a quarter pumice and a quarter lava rock. A shallow oval or rectangular pot shows off the spreading roots.

Pruning

In the ground it needs very little pruning: a light touch is enough to keep its handsome natural outline.

As bonsai, by contrast, it takes repeated pruning well: new shoots are allowed to grow four to six leaves before being cut back to two, and thick branches are removed after leaf fall. Defoliation is possible on a young, vigorous tree. A dedicated bonsai guide is available on this site.

Propagation

Seed is the most common method. Sow in fall, outdoors or in containers in a cold frame; spring sowing is also possible. Softwood cuttings, preferably taken from young plants, root in summer, and hardwood cuttings in winter. Grafting is done in winter, and air layering also works.

Common problems

Japanese zelkova has few pest and disease problems. It resists Dutch elm disease and elm leaf beetle, two scourges of its elm cousins, although cases of Dutch elm disease remain possible, as does honey fungus. It can be affected by cankers and phloem necrosis. Japanese beetles, leafrollers, aphids, leafhoppers and mites turn up occasionally, as do leaf spots.

In cities, along streets and in parking lots, it shows signs of stress: crown dieback and bark cracking. Remember that one symptom can have several causes: drying foliage can come from lack of water, airless soil, damaged roots or a canker. Look at the whole tree and its growing conditions before acting.

Toxicity

No toxicity has been reported for Japanese zelkova: it does not appear among the plants listed as toxic to dogs and cats.