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Ilex is the only genus in the Aquifoliaceae and has well over 400 species, though Galle (1997) considered that there may be as many as 800. In addition, the monospecific mountain holly genus Nemopanthus has recently been placed in synonymy with Ilex (Powell et al. 2000). The hollies have an almost worldwide distribution in temperate, tropical and subtropical climatic zones, and are particularly diverse in eastern and southeastern Asia and South America. The genus is best known for its evergreens but there are about 30 deciduous species. In growth form they range from small, creeping shrubs to large trees; there are also several epiphytic species and a few climbers (for example, I. clemensiae Heine, I. scandens Cuatrec.). Hollies are generally slow-growing. Their branches are covered in lenticels, which are highly conspicuous in the deciduous species. The leaves are extremely variable in shape and size, and by no means always spiny. The leaves are alternate (rarely opposite) and typically thick and leathery, though they may also be thin and papery, and the veins may be prominent or obscure. Most hollies have serrate, crenulate or entire leaf margins, and marginal spines – as found in I. aquifolium – are actually rather rare; the degree of armature on the leaves can change with age, and even in I. aquifolium the mature leaves are sometimes entire. Stipules are present in most species, though they are often very small (< 1 mm). Ilex is dioecious, with unisexual flowers on separate plants. The flowers can be either solitary or in groups (cymes) and the cymes can in turn be either solitary or fasciculate. Solitary inflorescences are produced by species that flower on new growth, fasciculate inflorescences by species that flower on stems from previous seasons. Holly inflorescences are often inconspicuous, and the pistillate inflorescences usually have fewer flowers than those of staminate plants. The flowers are typically white, cream, pale yellow or greenish, though pink, red and brown flowers occur. They range in size from 0.3–0.4 to 1–1.3 cm diameter and may be fragrant. The fruit is fleshy with a thin, papery skin and multiple seeds. Most species have red fruits, though white, yellow and orange forms occur in some species; black fruits are particularly common in the South American species. The fruits are produced on pedicels that vary in length between 0.2 and 4.5 cm. The ‘seeds’ are actually pyrenes, with a woody, bony or leathery endocarp enclosing the true seed (Hu 1949a, 1949b, 1950, Galle 1997).
Valued for both its presence in the landscape and its decorative fruits, Ilex is an important genus of trees and shrubs, and is one of those for which there is a specialist society. The Holly Society of America, founded in 1947 and based in the United States, promotes interest in all members of the genus and produces the useful Holly Society Journal, published twice a year. It is the International Cultivar Registration Authority for Ilex and accredits good holly collections as Holly Arboreta and Experimental Test Centres for the education of the public in the use of holly. Most horticultural attention is rightly paid to the abundant selections of the popular species and hybrids. These are described in great detail in Fred C. Galle’s Hollies, The Genus Ilex (1997), which should be consulted for information on the vast array of cultivars available. More recently, Hollies for Gardeners by Chris Bailes (2006) has been published, providing a useful adjunct to Galle.
Galle’s invaluable book also describes a very large number of wild species of Ilex, most of which are not in cultivation. A surprising number of them are tropical or subtropical in origin and therefore unlikely ever to be useful subjects in our area, but there are also many horticulturally unknown ones from China and elsewhere in temperate eastern Asia to provide a challenge to collectors. One tropical species that could occasionally appear in temperate gardens as a pot plant is I. paraguariensis A. St. Hil., from Paraguay, Argentina and Brazil. In South America it is used to make a popular stimulating ‘tea’, mate (the plant being known there as Yerba Mate), and it is sometimes cultivated as a curiosity. Mate is an acquired taste, especially for palates accustomed to infusions of Camellia sinensis. It is likely that many other hollies are being tested in cultivation. For example, the following are currently being grown under glass at Arboretum Bokrijk, Genk, Belgium, and some might well be hardy in milder parts of maritime Europe: I. canariensis Poir., I. formosana Maxim., I. lonicerifolia Hayata, I. maximowicziana Loes. (J. Van Meulder, pers. comm. 2007).
Many gardens and arboreta have fine collections of hollies, often dominated by the popular species and cultivars, but usually containing a fair representation of the ‘others’. In Europe the largest collection is at Arboretum Bokrijk, with over 600 taxa, including several of those described here. In France a large collection forms the Conservatoire National d’Ilex, at Arboretum des Près de Coulands, Orléans. In the United Kingdom the long-established collection at Kew is comprehensive, and there is a National Plant Collection of Ilex at RHS Rosemoor, Devon. The Valley Gardens at Windsor Great Park also hold a major collection of Ilex. In the United States Ilex is well represented in many arboreta, including the important and developing collection at the Washington Park Arboretum; also the US National Arboretum, the Morris Arboretum, the Bernheim Arboretum and Research Forest, Kentucky, and the Polly Hill Arboretum, Martha’s Vineyard, Massachusetts. In general hollies are quite tolerant of soil type, although they thrive best where they do not become too dry in summer. The species described below are of varying degrees of hardiness, and planting sites should be chosen accordingly. Propagation is by cuttings or seed, but germination can be erratic and hybridisation is frequent. The dioecious nature of hollies requires that both sexes are present for viable seed to be set, but the pollinator can be another related species. This is unimportant if all that is required is a decent crop of berries, but for conservation purposes, especially of the rarer species, it is vital that groups of mixed male and female clones are maintained, to avoid the problem of having a cultivated population of plants of only one sex.
A very large genus of deciduous and evergreen trees and shrubs found in almost all parts of the habitable globe except western N. America and Australasia. In gardens they are best known by the evergreen group, especially by I. aquifolium and its numerous forms and hybrids. They have very frequently angular young shoots; leaves alternate, stalked. Flowers of little or no beauty, small, often dull white, produced in the leaf-axils, the males and females usually on separate plants. Petals and stamens four to six. Fruit although commonly called a berry, really a drupe, usually red or black, with a thin, fleshy outer layer, surrounding one of several nutlets – generally termed seeds.
The most valuable hollies are undoubtedly those with evergreen foliage, but the deciduous ones, especially those earlier known under the generic name of Prinos, are sometimes handsome in fruit. Owing to the frequently unisexual character of the plants, these often fail to appear if both sexes are not grown. All the species like a moist, loamy soil. (For propagation, see under I. aquifolium.)
The standard work on Ilex is: T. Loesener, ‘Monographia Aquifoliacearum’, Nov. Act. Acad. Caes. Leop.-Carol. Nat. Cur., Vol. 78 (1901), pp. 8-500, and Vol. 89(1908), pp. 20-312. For the species of China, this is superseded by: S.-Y. Hu, ‘The Genus Ilex in China’, Journ. Arn. Arb., Vol. 30 (1949), pp. 283-344 and pp. 348-387, and Vol. 31 (1950), pp. 39-80, 214-40 and 242-63.