Genus of 35-40 species of deciduous trees from woodland areas in Europe, Asia and North America. They have alternate, prominently veined, entire or toothed leaves 5-7cm long. The leaves are a mid-green through the growing season before turning a bright yellow or red in Autumn, the leaves remain hanging on the tree throughout winter. Carpinus species have silvery-purple fluting of the bark which helps identify it from Fagus species/Beech. Though slow starters they become very vigorous growers, Hornbeams can reach heights of 25 metres. Hornbeams prefer long growing seasons with hot summers however their leaves burn if not given some protection from the sun at the height of summer.
Species suitable for Bonsai include:
Carpinus betulus /European hornbeam; Bright green foliage turning clear yellow in Autumn. Fully hardy though sensitive to early frost in October when still warm trunks and frozen roots can interrupt the flow of sap.
Carpinus turczaninowii /Korean hornbeam; Less vigorous form reaching 10metres when mature native to Japan, Korea and China, very small leaves and delicate, branching growth pattern has orange-red Autumn colour. Needs some protection from winter cold and loses lower branches when weakened.
Carpinus caroliniana /American hornbeam; Spreading shrubby tree, needs protection from cold in winter and when weak can lose lower branches.
Bonsai cultivation notes
Position Slight shade particularly from mid-day sun. Provide some frost protection in winter. The ideal for Hornbeam is to give them maximum sunlight in Spring until temperatures rise in late Spring. Then they should be provided with some light shade to avoid leaf-scorch in the heat of the Summer.
Feeding For trees in training, feed every week for a month after leafbreak in Spring, every two weeks thereafter. For refined bonsai, it is better to with-hold fertiliser until after the first flush of growth has hardened in late Spring. This encourages finer growth with shorter internodes. From late Spring onwards, feed with an organic slow-release fertiliser every 5 to 6 weeks.
Repotting In spring (as buds extend) every two years, when tree is young and in development. Hornbeams have a tendency to have a small number of straight, vigorous roots that emerge at angles from the trunkbase. These need to be pruned hard back to encourage more numerous branching roots. For Hornbeam bonsai, allow to become rootbound to encourage more refined growth. This may take 5+ years depending on the size of the container. If a mature Hornbeam fails to extend new shoots, then it requires some fresh soil for roots to grow into and can be repotted.
Pruning Pinch out apical buds as soon as possible as they emerge to encourage short internodes. Allow the initial Spring flush of growth to fully extend and harden off, then prune back the new growth very hard. This hard pruning will encourage backbudding and a second flush of shorter, more compact growth for the remainder of the year while ensuring the tree remains vigorous. Remove large leaves as and when necessary.
Defoliation At midsummer remove all leaves leaving just one at the tip of each branch.
Propagation Sow seed outside in Autumn. Softwood cuttings in mid-summer.
Pests and diseases Coral spot, caterpillars and aphids, sun scorch to leaves.
Styles Suitable for all styles in medium to large sizes.