Chinese elm

Identification and origin

Ulmus parvifolia belongs to the elm family, Ulmaceae. It is native to China, Korea, Japan, the Ryukyu Islands, Taiwan and Vietnam, and has naturalized elsewhere, notably in the United States. Its species name means “small-leaved,” and its flaking bark gives it the other common name, lacebark elm.

The alternate, elliptic to ovate leaves are usually 1–2 in long, up to 3 in. They have a lopsided base and finely toothed edges, and are dark green and glossy above, paler and slightly hairy beneath. In fall they usually turn a dull yellow, sometimes red or purple, and either drop or partly persist depending on the climate.

Unlike most elms, it blooms in late summer rather than in spring: small, reddish-green, inconspicuous flowers, followed in fall by samaras about 1 in long, each seed surrounded by a round, papery wing. The bark is its greatest asset: on older trees it flakes off in irregular patches, revealing a mosaic of gray, cream, orange, brown and green.

The tree typically reaches 39.5–49 ft tall and 24.5–49 ft wide, less often 69 ft. The crown is rounded, with long, often drooping branches, and growth is moderate to fast. Because seedlings vary a great deal, there are many cultivars:

• ‘Hokkaido’: a dwarf with tiny leaves, about 12 in tall after five years and 3 ft after thirty, with corky bark; grown in rock gardens and as bonsai.
• ‘Seiju’: a sport of ‘Hokkaido’ with slightly larger leaves that can eventually reach 10 ft.
• ‘Drake’ and ‘Sempervirens’: nearly evergreen foliage in mild climates.

Do not confuse it with Siberian elm (Ulmus pumila), which is often sold as “Chinese elm” too; Siberian elm has larger, double-toothed leaves and deeply furrowed bark. Japanese zelkova (Zelkova serrata) also looks similar, but its bark stays smooth and its leaves are narrower and more pointed.

Light

Full sun suits it best; it tolerates light shade or part shade with four to six hours of direct light a day. Choose an open spot away from buildings: the wood is weak, and branches break easily under wind, snow or ice.

Watering

Once established, Chinese elm is drought tolerant and copes with both dry sites and soils that are occasionally wet. It still looks its best in moist soil: during long dry spells, check soil moisture well below the surface and water deeply if it is dry, rather than following a set schedule.

In a container, the root ball tolerates neither prolonged drought nor constant wetness. Wait until the surface dries, then soak the whole root ball: sometimes daily in hot weather, much less often in cool weather.

Potting mix

It adapts to almost any soil, whether clay, loam, sand or chalk, acidic, neutral or alkaline, as long as it does not stay waterlogged. It prefers rich, moist, well-drained loam, but tolerates urban conditions, air pollution, road salt and, to a moderate degree, salt spray. In a container, a standard, free-draining bonsai mix works well.

Temperature

The tree is hardy in USDA zones 5b to 10a, meaning winter lows of about -15 °F. Trees from northern China withstand frost better than those from the south. Depending on how harsh the winter is, it either drops its leaves or keeps them until new growth appears. It tolerates urban heat and wind, but it is sensitive to ozone pollution.

Repotting

In the ground, it transplants well. A few large roots can spread far from the trunk close to the surface, and they can occasionally lift sidewalks or get into sewer lines. A young tree with a trunk less than about 2 in in diameter often needs staking: its heavy crown and still-unstable root system make it lean.

In a container, repot young trees every two years in spring, and less often as they age. The roots tend to grow crooked and tangled: prune them carefully to develop an attractive flare of surface roots at the base of the trunk.

Pruning

A young Chinese elm needs real structural training, which pays off in a long life; a mature tree needs only minimal pruning.

• Structure: choose a tree with branches spaced along a single trunk, even a slightly crooked one. Avoid trees topped in the nursery, whose main limbs all arise from one point: they tend to split apart as the tree ages.
• Young trees: during training, leave branches on the lower trunk to help it thicken.
• Mature trees: an occasional light thinning shows off the branch structure. Remove root suckers and low, drooping branches.
• When: avoid pruning elms from mid-spring to mid-fall; prune during dormancy instead.

Propagation

• Seed: the simplest method for the species. Samaras ripen in fall; seedlings vary widely and will not reproduce a cultivar.
• Cuttings: softwood cuttings in summer, an easy method that rarely fails.
• Grafting and chip budding: used for cultivars.

The tree self-seeds and can become weedy nearby: where it naturalizes, consider planting native species instead.

Common problems

A single symptom can have several causes: leaf drop, for example, can come from the approach of winter, a move, poor watering or a pest. Look at the tree and its site before treating.

• Dutch elm disease: Chinese elm is usually quite resistant.
• Verticillium wilt: the species is susceptible; wilts, rots and cankers show up now and then.
• Cankers and dieback: cankers on young trunks in overly wet soil, occasional twig blight, leaf spots, coral spot, honey fungus and Phytophthora.
• Pests: aphids, bark beetles, scale insects, elm gall mites, elm leafminer, borers and chewing insects. It shows good resistance to elm leaf beetle and Japanese beetle.
• In containers, in dry air: spider mites and scale. Regular sprays of water help keep them down; avoid diluted lime sulfur and systemic insecticides, which can make it drop all its leaves.
• Leaf drop on a potted tree after a move: as long as the twigs stay flexible and green under the bark, the tree will leaf out again.

Toxicity

No toxicity has been reported for dogs or cats: Chinese elm does not appear among the plants listed as toxic to these animals. Even so, keep children and pets from chewing on any part of it.