Japanese holly bonsai

Placement

Japanese holly is grown outdoors all year. It likes plenty of light: in spring and fall, give it full sun, which keeps growth compact. On the hottest summer days, however, it is better off in part shade.

In the ground, the shrub handles frost well; in a bonsai pot, things are different. Protect it from hard frost and shelter it from strong winds, which also damage its foliage in winter. A cold frame is the ideal shelter during the coldest months. It is sometimes sold as an indoor bonsai, but it does much better outside and needs a cool winter.

Several cultivars are used for bonsai. ‘Convexa’, with domed leaves, is even more compact than the species; ‘Stokes’ and ‘Ivory Tower’ are also grown. The species is dioecious: the small black berries appear only if a male and a female plant bloom at the same time near each other. The Japanese holly plant profile covers identification, hardiness and cultivars of the shrub in the garden.

Soil mix

What Japanese holly fears most is standing water: in poorly drained soil it is very prone to root rot, including black root rot, which thrives in cold, waterlogged soil. The mix must therefore drain perfectly. A blend of akadama and pumice with a little humus suits it well.

For a homemade mix, combine two parts peat-free potting soil, two parts coarse sand or lime-free grit and one part ericaceous potting mix, made for acid-loving plants. Avoid anything limy: alkaline conditions turn the foliage yellow.

Watering

Never let the root ball dry out completely, but do not keep it soggy either. Check the soil every day: as soon as the surface is dry, water thoroughly until water runs out of the drainage holes, then wait for the surface to dry again. In hot weather this can mean once or twice a day; in cool weather or in winter, far less often. Pot size, soil mix and exposure all change the pace a great deal. Rainwater is best, since it leaves fewer mineral deposits in the pot.

Fertilizer: during the growing season, apply a balanced liquid fertilizer every week, or a solid organic fertilizer once a month. Avoid overfeeding, which produces soft shoots that are more vulnerable to pests and disease. Nothing during winter dormancy or right after repotting.

Pruning

Japanese holly tolerates constant and even severe pruning very well, which is its great strength as a bonsai.

• Pinching: let each new shoot develop four leaves, then cut it back to two. Repeated over time, this gradually makes the branching denser.
• Flowers and fruit: stop pruning in early summer if you want flowers and then fruit.
• Structural pruning: the tree tolerates hard pruning, which makes it easier to build the framework.

Styles: its naturally dense, stiff, rounded habit and small leaves lend themselves to compact crowns made of rounded masses, much like Japanese ball-shaped pruning. That stiff habit also makes it less pliable than many species: it is shaped mainly by pruning, with wire used to refine branch placement.

Pests: watch for spider mites, leafminers and vine weevils, and treat them with a product suited to the pest you have identified. Scale insects can also settle in.

Wiring

Wire from late spring through summer. Japanese holly branches are fairly brittle, so bend them with care. The bark scars easily under wire: protect it and keep a close eye on the wire.

Aluminum wire is easier to handle and suits broadleaf trees well. Choose wire about one third as thick as the branch and wind it at roughly 45°. Depending on how vigorous the tree is, it stays on for one to four months: check it often and, as soon as it starts to bite, remove it by cutting it at every turn rather than unwinding it. For a branch that is too thick or too brittle, use a guy wire anchored to a sturdy root or to the pot instead, protecting the bark with a piece of rubber.

Repotting

Repot every two years in spring, with root pruning. The best time is just before growth begins, when the buds are about to open; do not repot in summer or winter. Older trees can wait three to five years: slide the tree out of its pot and repot only if roots are circling the root ball.

Untangle the roots, remove some of the old soil and take off no more than about 30 % of the root mass. Anchor the tree with wire, fill the gaps and water thoroughly. For the next few weeks, keep the tree away from extremes, with the soil slightly moist and no hard pruning or fertilizer, while new fine roots form.

Propagation: cuttings are the most reliable method, using semi-hardwood shoots taken at the end of summer. Seed is possible, but germination can take two years.