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The six species of Zelkova occur in temperate Eurasia, with three species in the Mediterranean and western Asia, and three species in eastern Asia (Denk & Grimm 2005). Zelkovas are deciduous trees with smooth bark. Unlike the elms, the branchlets are never corky or winged. The leaves are in two distinct rows; they have pinnate venation and each vein extends to the leaf margin, where it terminates in a tooth. There are two stipules at each node, though these are caducous, leaving a pair of scars at the leaf base. Zelkova is polygamous. Staminate flowers are clustered in the lower leaf axils of young branchlets; the perianth is campanulate, with four to six (to seven) lobes, and the stamens are short. Pistillate and hermaphrodite flowers are solitary, or rarely in clusters of two to four, in the upper leaf axils of young branchlets, the perianth with four to six lobes. The fruit is a scarcely fleshy drupe with a dorsal keel; the perianth and stigma are persistent (Ainsworth 1989, Andrews 1994, Fu et al. 2003).
Zelkova is one of the minor but important genera of trees, possessing few species but among them a handful of arboretum essentials. The magnificent Z. carpinifolia and Z. serrata are long established and well known (Andrews 1994), but Z. schneideriana shows signs of becoming an equally fine tree. The most recent addition to the genus is Z. sicula G. Di Pasquale, G. Garfì & Quézel, known only from a tiny population with limited genetic diversity in southeastern Sicily, where it was discovered in 1991. There it grows as a deciduous shrub, 2–3 m tall, with small, rounded, crenate leaves that are hairy on both sides. It is in cultivation in Italian botanic gardens and also at the National Botanical Conservatory of Brest, France (IUCN 1995–2006).
Several cultivars of Z. serrata have made an appearance in recent decades, especially in the United States, including ‘Flekova’ (Green Vase) which is particularly commended by Dirr (1998), ‘Spring Grove’, ‘Urban Ruby’ and ‘Village Green’. These vary somewhat in growth habit, vigour and autumnal colouring, and ‘Flekova’ and ‘Village Green’ are considered resistant to Dutch elm disease (DED) (van den Berk 2002), which may be an important consideration. Unfortunately, as close relatives of the elms, Zelkova are susceptible to many of the same problems of disease and foliage pests as Ulmus. They can be killed by DED if infected, but there is some evidence that they are less attractive to the Scolytus beetles that carry the fungus, and that infection is by no means always fatal (Andrews 1994). Damage to the foliage from the Elm Leaf Beetle and other insects can, however, be quite severe. They seem to grow well in any reasonable soil. Propagation is from seed or as cuttings, with selected cultivars being grafted onto seedling Z. serrata.
Nearly allied to the elms, the four species of Zelkova in cultivation are amongst the most interesting and handsome of hardy trees. They have smooth, beech-like trunks with a scaling bark, and deciduous, alternate, coarsely toothed, feather-nerved leaves, usually harsh to the touch like those of elm. Flowers unisexual; both sexes produced on the same twig, the males at the base, the females solitary or few in the leaf-axils above them; both sexes small, green, and of no beauty. Seed-vessel roundish, 1⁄6 to 1⁄4 in. long, with the calyx adhering at the base, slightly horned at the top.
The zelkovas should be grown in deep, moist, loamy soil where the position is moderately sheltered. Z. carpinifolia is the best known of them, and appears to be adapted to all but the most inclement parts of Britain. Both it and Z. serrata should be raised from imported seed, although they can probably be grafted on elm, as are the other two species.
The generic name derives from the local word for Z. carpinifolia. An earlier name for the genus is Abelicea, for the origin of which see Z. abelicea, but Zelkova has been conserved.
It is disturbing that the zelkovas are now being attacked by Dutch elm disease, borne by beetles migrating from dead elms.