Jooni

An exemplary photo of a Zelkova bonsai

Zelkova

1 Species

Species

Zelkova serrata

Japanese zelkova (Zelkova serrata), known in Japan as keyaki, is a deciduous member of the elm family and the classic tree for the broom style. A zelkova bonsai is an outdoor tree: it is very hardy and needs a cold winter dormancy.

It suits both beginners and more experienced growers: it grows vigorously, tolerates hard pruning and buds back readily, although building a fine, even broom takes years of patient work. Its standout traits are the fan-shaped crown of dense, fine twigs, small serrated leaves that turn yellow, orange or red in autumn, and smooth grey bark that flakes on old trunks to reveal orange patches.

At a glance Zelkova serrata

Difficulty
Intermediate
Placement
Outdoors
Light
Sun to partial shade
Watering
High
Min. temperature
-10°C (14°F)
Foliage
Deciduous

Fertilizing

Feed from spring until early autumn. Use liquid fertilizer every 2 weeks at full strength or weekly at half strength, or solid organic fertilizer once a month.

  • Organic fertilizers such as rapeseed cake or fish emulsion work well and release nutrients gradually.
  • Feed refined trees lightly in spring so the new shoots and leaves stay small.
  • Stop feeding in winter.

Placement

Keep your zelkova bonsai outdoors all year in full sun or partial shade. In hot summers, give light shade in the afternoon, as the thin leaves of trees in shallow pots scorch easily.

Zelkova is frost-hardy and needs a cold dormancy, but fine twigs and roots in a shallow pot are vulnerable. Protect the tree below -10°C (14°F) in a cold frame or unheated shed, out of drying winter wind. Do not keep it indoors.

Pruning

The broom-style crown is built by repeated pruning rather than wiring. Let shoots grow to 4-6 leaves, then cut them back to 1-2 leaves, and repeat throughout the growing season.

  • Do structural pruning in late winter or early spring, before the buds open.
  • To start a broom, cut the trunk and let several shoots grow from the cut, then shorten them repeatedly so they fork into a fan.
  • Remove unwanted sprouts on the trunk and in branch forks as they appear in spring, and thin crowded twigs so the inner ones don't die back.

Partial defoliation of a healthy tree in early summer reduces leaf size. Wire sparingly, as the bark marks easily.

Repotting

Repot young trees every 1-2 years and mature trees every 2-4 years, in early spring as the buds start to swell. Prune the roots moderately and spread the surface roots out radially to build a good root base.

  • Use a free-draining mix, such as akadama with pumice or fine grit; zelkova grows well in slightly acidic to neutral soil.
  • On young trees, keep the developing surface roots covered with soil so they thicken.
  • Water thoroughly after repotting and keep the tree out of wind and strong sun for 2-3 weeks.

Watering

Zelkova likes moist soil, and shallow pots dry out quickly. In hot summers check twice a day, as shallow pots may need water morning and evening; in spring and autumn a daily check is usually enough.

  • Water whenever the surface starts to dry, until water runs from the drainage holes.
  • In winter the leafless tree uses little water, so water only when the soil begins to dry.
  • Never let the soil dry out completely, but avoid waterlogging.

Seasonal care

  • Spring: Repot as the buds swell and do structural pruning before they open. Remove unwanted sprouts as they appear and start feeding once the leaves have opened.
  • Summer: Water once or twice a day and give light afternoon shade in heat waves. Keep cutting shoots back to 1-2 leaves and partially defoliate strong trees in early summer if needed.
  • Autumn: Stop feeding and enjoy the autumn colour. After leaf fall, check the broom structure and remove crossing or crowded twigs.
  • Winter: Keep the tree outdoors for its dormancy and protect it below -10°C (14°F) and from drying wind. Water only when the soil begins to dry.

Common problems

Zelkova is a robust tree; most problems come from dense growth, dry soil or sap-sucking pests.

  • Inner twig die-back: Fine twigs inside the crown die when dense outer growth shades them. Thin the crown regularly and keep the tree in good light.
  • Aphids and scale: Sap-sucking insects on young shoots and twigs cause sticky, distorted leaves. Rinse aphids off, scrape off scale insects and use an insecticide on heavy infestations.
  • Spider mites: Speckled, dull leaves and fine webbing appear in hot, dry weather. Spray the foliage with water regularly and use a miticide if needed.
  • Leaf scorch: Brown, crisp leaf edges come from strong sun or a pot that dried out in summer. Provide afternoon shade and check the water twice a day in hot weather.