yucca, any plant of the genus Yucca, stiff-leaved stemless or treelike succulents of the family Liliaceae (lily family), native chiefly to the tablelands of Mexico and the American Southwest but found also in the E United States and the West Indies. Yuccas in flower produce a large stalk of white or purplish blossoms. They are pollinated by the yucca moth, and in its absence they rarely fruit—a striking example of interdependence, since the moth, which lays its eggs during pollination and whose larvae feed on some of the developing seeds, cannot reproduce without the yucca. The leaves are usually stiff and spearlike, often with marginal threads. Several species are known as Adam's-needle, particularly those that are hardy and are cultivated in the North, most common of which is Y. filamentosa. The Joshua tree (Y. brevifolia) is a picturesque treelike species of desert regions. Mormons crossing the California deserts are said to have so named it because the grotesquely angular branches looked like the outstretched arms of a Joshua leading them out of the wilderness. The Spanish bayonet (Y. aloifolia) is another that is treelike in form, and the Spanish dagger (Y. gloriosa) is stemless or has a short trunk. The fruits and sometimes the flowers of several species of yucca were used as food by Native Americans. Certain species, particularly Y. baccata and Y. glauca, are called soap plant because of the use of their roots for soap. The fibers of some kinds have been utilized. A yucca is the state flower of New Mexico. Yuccas are classified in the division Magnoliophyta, class Liliopsida, order Liliales, family Liliaceae.

Yucca Mountain, mountain in the SW Nevada desert about 100 mi (161 km) northwest of Las Vegas. It is the proposed site of a Dept. of Energy (DOE) repository for up to 77,000 metric tons of nuclear waste (including commercial and defense spent fuel and high-level radioactive material) presently held nationwide at commercial reactors and DOE sites. The project arose from the 1982 Nuclear Waste Policy Act requiring the DOE to construct a permanent underground nuclear-waste storage facility. Proponents of the use of Yucca Mt. as a repository claim that the area some 1,000 ft (300 m) beneath the mountain is the most viable site available, arid and remote with a deep water table, and that gathering the radioactive material in one location would allow for safer and more efficient and cost-effective protection. Opponents, including the state of Nevada, cite the potential for seepage into area groundwater, the danger of transporting waste to the facility, and the likelihood of the degradation of the storage containers and the occurrence of earthquakes and climate change over thousands of years. In 2002 President George W. Bush officially designated Yucca Mt. as the site for the nuclear waste repository, but regulatory hurdles and certain legal challenges must be surmounted before the facility can be constructed and opened.

Yucca House National Monument: see National Parks and Monuments (table).

Any of about 40 species of succulent plants (genus Yucca) of the lily family, native to southern North America. Most species lack a stem and have a rosette of stiff, sword-shaped leaves at the base and clusters of waxy white flowers. The Joshua tree (Y. brevifolia) has a stem more than 33 ft (10 m) high. Commonly cultivated as ornamentals for their unusual appearance and attractive flower clusters are the aptly named Spanish bayonet (Y. aloifolia), Spanish dagger (Y. gloriosa), and Adam's needle, or bear grass (Y. filamentosa). Yucca moths (genus Tegeticula) inhabit yucca bushes, each moth species adapted to a particular yucca species. The yucca can be fertilized by no other insect, and the moth can use no other plant to raise its larvae.

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