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A genus of about thirty species in the New World, ranging from Central America and the West Indies northward through Mexico to the USA, as far North as California and N. Dakota in the west, and New Jersey in the east. The leaves are borne in rosettes at the apex of a woody stem which in the acaulescent species does not rise much above ground-level, in others attaining a height of 30 ft or even more. Leaves long and relatively narrow, pointed, crowded in a spherical or hemispherical head. Flowers white, creamy white or greenish, sometimes stained with pink or purple on the outside, drooping or sometimes horizontal, produced in erect panicles or racemes. They are composed of three outer segments and three inner ones, free except at the base. Stamens six, with short filaments. Ovary sessile or shortly stalked, three-chambered. Style short and thick, with three erect lobes, the stigmatic surfaces facing inward (but see Y. whipplei). Fruits dry or sometimes fleshy, ovoid or oblong, up to 2 or 3 in. long. Seeds dark brown or black.
We owe the generic name to John Gerard, who believed that his plant (see Y. gloriosa) was the 'yuca' of the Caribbean. In fact the plant so called was the common manihot or cassava. Gerard's error was pointed out by Parkinson in the Paradisus (1629), but the name persisted and was legitimised by Linnaeus in its present sense.
In their general aspect the yuccas are quite distinct from any other group of hardy shrubs. Their foliage is essentially of a tropical or subtropical character, which, combined with a peculiar stateliness and beauty of flower, gives the genus a unique value in gardens. They are especially suitable in formal arrangements, either isolated or in groups, and their effectiveness in flower is enhanced if a dark background can be given them.
Considering the regions of which the species described below are native, it is remarkable that they are so hardy and adaptable to our climate. The commonest species come from the coast regions of the south-eastern United States, yet they withstand 30° or 32° F of frost uninjured. Compared with wild plants, our garden ones have longer, larger leaves, but smaller inflorescences. They appear to thrive in any soil, but prefer a sandy loam in a position fully exposed to the south. In such a position they never suffer from drought, nor do they, except for a diminished crop of blossom, appear to be affected by cold, wet seasons. As the stems lengthen, they ultimately decay at the older part, and fall over by their own weight. The tops can be made to strike root by trimming off half the leaves, and placing the stem in a pot of sandy soil, giving it a place in a greenhouse until rooted. The dwarf species like Y. flaccida can be increased by division, and most of the species produce rhizomatous underground stems which make plants when cut off and potted.
In the wild, most species of yucca are dependent for pollination and seed-setting on the activities of yucca moths, allies of the gipsy moths. The moths emerge from the pupae some days before the flowering of the species with which they are associated. With the aid of special tentacles on its maxillae, the female gathers pollen from one flower and rolls it into a ball. It then lays its eggs in the ovary of another flower and with an extraordinary deliberateness rams the pollen-ball into the stigmatic chamber of the ovary. The grubs when they emerge feed on the ovules, but leave enough to provide an adequate supply of seeds. The yucca is entirely dependent on the services of the moth, and equally the moth could not exist without the yucca. But two species, Y. aloifolia and Y. whipplei, do not need the moth for pollinàtion, and some species supposed to do so have set seed in European gardens. In any case, the yuccas hybridise readily when artificially cross-pollinated. It should be added that, according to Trelease, Y. gloriosa and Y. recurvifolia flower too late even in the wild to receive the services of any yucca moth, and are consequently sterile. He suspected that both might be the result of past hybridisation between fertile species, presumably Y. aloifolia and Y. filamentosa (in which Trelease included the species now known as Y. smalliana), and owe their perpetuation to vegetative means.
Bowles, E. A. – 'Yuccas for English Gardens', Journ. R.H.S., Vol. 47 (1922), pp. 105-9.
McKelvey, Susan. – Yuccas of the Southwestern USA. part 1 (1938), part 2 (1947).
Molon, G. – Le Yucche, 1914.
Russell, J. – 'Yuccas in Britain', Journ. R.H.S., Vol. 96 (1971), pp. 491-5.
Trelease, W. – 'The Yucceae', Missouri Bot. Gard. 13th Report (1902), pp. 27-133.
Webber, J. M. – Yuccas of the Southwest. Agricultural Monograph No. 17 (1953).