Juniperus has traditionally been divided into two large sections: in section Juniperus all leaves are of juvenile form (scale-like leaves being found only on the cone peduncle), while in section Sabina juvenile leaves are restricted to seedlings and the lowermost branches; mature leaves are scale-like, decurrent, decussate or rarely whorled (Farjon 1992). These leaf characters are somewhat plastic, and mutants with unusual leaf morphologies are not uncommon, both in cultivation and in nature. Several members of section Sabina have only a single seed in the cone (monoseed), and in this respect they resemble Microbiota Kom. (Rushforth 1987a). Phylogenetic studies using DNA and morphological data suggest that Juniperus is most closely related to Cupressus and Xanthocyparis (Gadek et al. 2000, Little et al. 2004) (see p. 290), though these studies included only a few representatives of this large genus. As with many other genera of Cupressaceae, the name ‘cedar’ is applied to several species of juniper.

The principal reference works for the genus are Robert P. Adams’ Junipers of the World: The Genus Juniperus (2004) and Aljos Farjon’s A Monograph of Cupressaceae and Sciadopitys (2005c), both of which contain detailed descriptions and numerous illustrations of almost every species. These should be consulted for further botanical details. Very comprehensive illustration of many species and a huge range of cultivars is to be found in D.M. van Gelderen and J.R.P. van Hoey Smith’s Conifers: The Illustrated Encyclopaedia (1996). Many also feature in Richard L. Bitner’s recent Conifers for Gardens (2007). Illustrations appearing in these works are not mentioned in the species accounts that follow.

Juniperus is an extremely important genus of conifers, but has an image problem in consequence of the plethora of dwarfish cultivars, in various colours and shapes, used only too frequently in boring ‘landscaping’. As so often the fault lies not really in the plants, but in a lack of imagination on the part of planters or designers. To compound the problem, many of these cultivars are susceptible to fungal diseases which can cause them to die off in large patches, necessitating their removal. In consequence it seems that junipers are seldom treated with great respect – which is a shame since many, both species and cultivars, have really valuable garden qualities when well sited.

One particularly attractive species, given a short description by Bean (B487), is J. formosana. This has been cultivated for a long time. Dallimore & Jackson (1966) suggest that it may even have been introduced by Fortune in 1844, and it was probably also collected by Wilson (Sargent 1916). A few old trees exist – for example, at Bedgebury, where the oldest specimen was planted in 1935. In 2007 this was 6 m tall, with three trunks, the largest of which has a dbh of 15 cm (D. Luscombe, pers. comm. 2007). Juniperus formosana has been reintroduced in recent years, from both mainland China (for example, SICH 616) and Taiwan (ETOT 89), and is now much more frequent in arboreta than before. As a young tree it makes a beautiful column of pendulous light green foliage, given a special ‘edge’ by the broad white stomatal bands on both leaf surfaces. It seems to be widely tolerant, good specimens having been seen at the JC Raulston Arboretum, Quarryhill and Kew. One species that could certainly be attempted more frequently, especially in sunny, drier gardens, is J. deppeana Steud., the Alligator Juniper, so-named for its beautiful bark, but although popular in North America this is very seldom seen in Europe. It can have excellent blue foliage, and is the most glaucous of all Juniperus species. A superior clone, ‘McFetters’, from Arizona, has done well at the JC Raulston Arboretum (Dirr 1998) and is obviously worth seeking out. It is curious that with the exception of J. scopulorum the western American junipers have received little horticultural attention. Among these is J. osteosperma (Torr.) Little, a familiar, common species of the Great Basin and Colorado Plateau, which would surely make a useful water-wise plant for gardens in the southwestern United States and in the drier parts of Europe. Both of these species can be very slow-growing when young, especially in arid conditions (Nold 2008). From central Asia (Xinjiang, China), usually forming a spreading shrub but occasionally a small tree, is J. pseudosabina Fisch. & C.A. Mey. This has rather glaucous foliage and is considered by Daniel Luscombe of the National Pinetum, Bedgebury, Kent, to be an unusually attractive species (pers. comm. 2008) and well worth growing.

The species described below are mostly collectors’ items, but several have unexpected distributions in the wild that give them an added interest. It would seem that (as can often be the case with tropical members of usually temperate genera) the genes for hardiness have not altogether been lost. Planting sites should however be chosen with regard to origins!

Propagation of Juniperus is by seed, or by cuttings, but these can result in poor root systems that lead to wind rock, especially of columnar forms (Dirr 1998). Several major pests and diseases afflict the genus, especially fungal diseases caused by Phomopsis, Phytophthora and Kabatina which result in dieback of the shoots and often lead to the death of the plant. Bagworm caterpillar infestations are a problem in North America (Huxley et al. 1992, Dirr 1998). There are many others! Planting in a lean, dry, sunny site may help reduce the risk.