Cannabaceae

(Nettle Trees and Hackberries)

 

This small group of three, quite similar looking, closely related genera are usually designated as part of the Family Ulmaceae but are now referred to the Cannabaceae. It contains Celtis, Pteroceltis and Aphananthe.  They are deciduous and vary from shrubs to chiefly medium sized trees. None of the temperate climate species are particularly ornamental in the conventional sense but all are capable of maintaining full green foliage cover while withstanding reasonably hostile conditions without marginal leaf scorch, especially the arid summers which are experienced in the hotter areas of Mediterranean climates and many continental climate areas.

 

Celtis

 

The temperate species of the genus Celtis divide broadly into the Hackberries of North and Central America - these are not particularly decorative - and the Nettle trees of Eurasia – the oriental Asiatic species of which, especially, can develop as reasonably tall, deciduous, well shaped, upright trees with good foliage cover and they can make ornamental specimens in otherwise dry, hostile summers of Mediterranean climates – maintaining good green foliage.

 

The unisexual flowers are produced in the early summer in the leaf axils and are insignificant. The fruit, which is produced in late summer or autumn, is a one seeded drupe which has an edible flesh and a coloured skin – red, yellow, orange and black in various species.

 

The oriental Nettle Trees; which include C. biondii, C. bungeana, C. cerasifera, C. jessoensis, C. julianae, C. koreana, C. labilis and C. sinensis; are all hardy enough to survive suitable niches in the UK and most can develop as regularly shaped specimens suitable for street tree use in relevant climatic areas. The Eurasian species of Nettle Trees - of which C. australis is the best known and is used extensively as a street tree in many cities of Southern Europe – also include C. caucasica, C. glabrata and C. tournefortii.

 

The Hackberries from the New World include a number of hardy species - C. laevigata, C. occidentalis (inc C. pumila), C. pallida, C. reticulata (inc C. douglasii) and C. tenuifolia but none of them exhibit significant ornamental characteristics.

 

The fruits should be collected as soon as they are ripe and the seeds extracted by gentle maceration (the case of the stone is brittle and will be damaged with vigorous action) and then fermentation for three days or so when the stones will readily rub free and sink to the bottom of the container, decant the dross and then wash the seeds – the stones are usually whitish with a rough patterned, rugose surface (the pattern of which is a species diagnostic).

 

Commercial seed is often available as dried berries but the fruits should be soaked and treated as above - as practical experience shows that sowing intact causes an erratic and attenuated germination – however there is no indication that the flesh contains a germination inhibitor. Stored seed also tends to perform poorly.

 

Thus the seed should be treated reasonably quickly without drying. Although the temperate species will eventually germinate, without chilling, after a considerable time and then erratically – the best results are achieved by a period of chilling (56 days wt 3˚C) followed by germination at 18 to 20˚C.

 

Pteroceltis

 

A monotypic genus of Nettle Tree, from Northern China and Mongolia is designated - Pteroceltis tataronowii. It can grow into a deciduous tree to 20m tall.

 

The trees are monoecious and the tiny green insignificant flowers are produced in spring. The fruits mature in the autumn and consist of a single samara which develops in the axils of the leaves. The samaras are not dissimilar to those of elm – the central nut is globose, about 1cm in diameter and the two encompassing wings can spread to 2cm.

 

The seeds exhibit an endogenous dormancy which requires a chilling treatment of 63 days at 3˚C. Germination will be most rapid and uniform at 20˚+C.

 

Aphananthe

 

The genus Aphananthe has a distribution from Madagascar through South East and East Asia to Australia and Mexico and is mostly tropical in distribution and evergreen in habit. The only relevant species with a temperate provenance and significant hardiness is the deciduous, A. aspera from Eastern China, Japan and Korea. It is an upright, fast growing tree to 20m tall with good shape and foliage cover.

 

The species is monoecious and the insignificant flowers are produced in racemes at bud break. The fruit is a single-seeded drupe (about 6 to 8mm in diameter) produced in the autumn. It is blue-black skinned and although initially fleshy, it soon dries to a hardish condition.

 

The dry drupes should be soaked and macerated to extract the seeds and then chilled for 56 days at 3˚C before germination at 20˚C.