Field elm bonsai

Placement

Ulmus minor is a fully cold-hardy deciduous tree: in the ground it handles the harshest continental winters, below -4 °F, and an elm bonsai should live outdoors all year. Give it sun or partial shade. In winter, protect it above all from wind: fully exposed to icy air currents, it can lose its finest twigs, which are the main asset of an elm bonsai. It does not, however, need to be brought indoors.

Soil mix

A basic bonsai mix suits it. A common recipe uses equal parts akadama, pumice and lava rock; add more akadama if you cannot water often, and more lava rock in a humid climate. Field elm tolerates acidic, neutral or alkaline soil. What matters most is perfect drainage, in a pot with drainage holes.

Watering

Keep the soil evenly moist, never letting it dry out completely or stay soggy. Check it every day and water when the surface starts to dry. In strong sun, high heat or during the vigorous spring flush, you might need to water far more often. On the other hand, watering every day out of habit, without checking, waterlogs the soil and eventually rots the roots.

Fertilizer: as soon as the buds open in spring, give a high-nitrogen fertilizer every week for the first month, then a balanced fertilizer every two weeks until late summer. Do not feed a tree that is out of leaf.

Pruning

Field elm is a vigorous, fast-growing species that takes frequent pruning well, and pruning makes the branching denser. Let shoots extend three or four nodes, then cut them back to one or two leaves. Repeat as often as needed through the season.

Save structural pruning of thick branches for late summer or fall: large wounds made in spring or summer form bulky, unsightly calluses. Once the leaves have dropped, carry out the yearly pruning of the branch structure. A healthy tree can be defoliated in midsummer, which makes wiring easier.

Styles: elm suits every form and size, but the broom style fits it especially well because it echoes the natural shape of the genus. Grown from an air layer, a field elm can become a well-branched bonsai in about ten years.

Wiring

The bark marks easily, so wire with care and keep an eye on the wire. The easiest times are when the tree is bare, in late winter, when the structure is clearly visible, or in midsummer right after defoliation. Use aluminum wire about one third the thickness of the branch, wound at roughly 45°, and wire every branch you plan to shape before bending any of them. On a thick branch, protect the bark first with damp raffia; bend gradually, applying pressure on the inside of the curve, and avoid bending the same branch repeatedly. During growth, branches thicken fast and the bark can swallow the wire within one to four months: check it often and remove it as soon as it starts to mark, cutting it at every turn. Once the structure is set, a well-pruned elm might no longer need wire.

Repotting

Elms have a powerful root system and do not like being pot-bound for more than a year: a tree that is too cramped grows poorly and declines. Repot young trees in training every year, and more refined specimens every two to three years. Do it in spring, as the buds lengthen. Remove the old soil from the sides and bottom of the root ball, prune the roots without taking off more than about a third, and work soil into the gaps around the roots before watering thoroughly. New shoots often appear within a few weeks, but full recovery can take a season. After repotting, keep the tree out of wind and harsh sun, and wait for it to recover before any heavy pruning or feeding.

Propagation: seed sown outdoors in fall or spring, softwood cuttings in summer, hardwood cuttings in winter, and air layering once the spring growth has firmed up.

Pests and diseases: watch for aphids, leafhoppers, scale and gall mites, tiny mites that cause galls on the leaves; a winter spray of lime sulfur on the bare wood kills those that overwinter. Elm leaf beetles chew the foliage. Dutch elm disease, spread by bark beetles that mainly seek out elms more than about twenty years old, affects bonsai far less. Phytophthora root rot can develop in soil that stays too wet.