The Celtis (Hackberry) originates from China and Taiwan and has a strong yet graceful trunk with neat oval leaves. In southern Europe, bonsai amateurs often grow the Celtis australis, but nurseries tend to import the Celtis sinensis, so we shall describe that variety here. This deciduous tree may reach 20 metres in height, with a soft grey bark, and green oval, serrated, leaves that are sometimes slightly velvety underneath. In autumn these turn a very pleasant shade of yellow. It produces little black-red berries — strictly for the birds only!
Propagation: It’s very easy to propagate this variety from the seeds. Keep them in sand in a cold place for 3 months, and then in spring sow them in well-draining soil. Cuttings from lignified twigs are also easily taken in June.
Repotting / soils: As this tree grows vigorously you need to repot young plants every year in March-April, and mature specimens every 2-3 years. Use a well draining soil. Pumice seems to be better than Akadama (though you can still use that, of course). Do not use organic soils. It can grow without any problems in acid or basic soils.
Styles / wiring: Formal or informal upright styles are the most common, but you can create other styles, too, like broom style, half cascade, or forest planting. The wood is flexible. You can wire with aluminium wire from April to end of June. You can also wire in September. Check the wires regularly because they can easily injure the bark. Do not wire during the winter because the wood is brittle at that time.
Watering: Water every day during the growing season, but make sure you have draining soil because roots do not like water stagnation. During periods of heat, you can also mist the foliage in the evening.
Diseases / pests: Too much water without draining soil may generate root rot. Red spiders can attack if there is no air circulation and the atmosphere is insufficiently damp (but the soil itself must NOT be damp!). Caterpillars also like its leaves very much. Check for them
Pruning / pinching: During the growing season you can pinch the new buds to slow the growth. In June you can defoliate the lignified twigs and cut their tips. Instead of the leaves you will have new shoots and the tree will become more dense. In southern Europe you can defoliate until mid-August. When the tree is at rest you can cut off the large unnecessary branches. But do not cut them too short. Leave a stump.
Fertilisation: Apply organic fertilisers, like Hanagokoro, from April to June, Biogold until the end of August; then until it loses it leaves, give it a 0% nitrogen fertiliser with only P and K available. The secret is not to give nitrogen in the autumn so that the tree stores starch for next spring.
Placement: This tree loves sun. So as soon as the temperatures are sweet outdoors, place it in the full sun. During the winter keep indoors, in a conservatory or greenhouse, and make sure it lives in a well-ventilated, brightly lit atmosphere.
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