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Tree of the Month - Febrary 2002: Hornbeam
Korean Hornbeam, Carpinus coreana.
Carpinus sp. are deciduous trees native to North America and Asia. Local common names for them are Hornbeam, Ironwood, Blue Beech, and Water Beach. They are Most have folded or wrinkled foliage that makes them nice for bonsai as it gives the leaves a pleated look. They make good ornamental trees and can be found in more complete garden centers. They generally grow as an understory tree and may require protection from afternoon sun in hot dry display areas. Hornbeams make good group plantings as well as specimen bonsai due to the fine branch ramification and small leaf size in bonsai culture. It grows with an attractive open habit in total shade, but will form a dense canopy in full sun. The muscle-like bark is smooth, gray and fluted. Ironwood has a slow growth rate and is reportedly difficult to transplant from a field nursery but is easy from containers. The fall color is red and orange to yellowy bronze and stands out in the in the fall. Brown leaves occasionally hang on the tree into the winter. As usual, the Asian species are the traditional bonsai species, but the American hornbeam, C. caroliniana, will make equally good bonsai.
According to Otis, in Michigan Trees, the C. caroliniana habitat is a rich deep moist deep soil along the borders of streams and swamps. It is also found in dryer soil as an understory. This indicates moderate watering and never letting the soil dry out completely. It is hardy from zone 3 through 9A.
The growth rate of the hornbeam species varies greatly. This is of particular importance if you are growing them out for larger trunked bonsai. From fastest to slowest they rank: C. betulus, C. turczaninovii, C. japonica, C. laxiflora, C. caroliniana, and C. coreana. Carpinus coreana is very popular because of its small leaves and very ramified growth pattern. However, it takes a significantly longer time than any other to thicken and add visual mass. Mature hornbeam specimens will produce hop-like flowers and seed pods. These pods are an attractive addition to the fall and winter display. Cutting produced plants from mature wood are capable of flowering in a year or two as opposed to seven years or more for seedlings.
Feeding: Feeding instructions vary greatly. Simon and Schuster's Guide recommends feeding Every 20-30 days, stopping for a month-long break in midsummer. Tomlinson recommends a weekly feeding for the first month after bud-burst, switching to every two weeks until late summer. Ordinary plant food at half strength is fine, as is bonsai fertilizer. Of course, the whole controversy can be avoided with time-released pellets. (i.e. constant feeding). However, everyone agrees, do not feed for two months after repotting. Pick what works for you.
Pruning and wiring: Can be wired from spring to autumn - some bark protection may be needed, but the hornbeam is fairly sturdy for a deciduous tree. Accepts repeated hard pruning. Prune back to the first pair of leaves on new shoots to force back budding. The best times for minor pruning are early spring and after flowering. Major developmental pruning should be done in late winter, before bud burst. There is strong apical growth in the upper part of the tree, so cut back radically at the apex, but prune the lower portions conservatively to check rapid apical growth. As it ages branches die, complicating management as a bonsai. On older trees branches may die back for no discernable reason.
Repotting: Every 2-3 years in early spring. Use basic bonsai soil. Prefers a deep pot.
Pests and diseases: Relatively few insects attack hornbeam. Several fungi cause leaf spots on Carpinus. Canker, caused by several fungi, causes infected branches to dieback and entire trees die if the trunk is infected and girdled. Severely infected trees can not be saved and any infected branches should be pruned out.