Yucca is not the name of a species of plant; it is the name of a plant genus that contains about 40 species. Yuccas are popularly associated with the desert climates of Mexico and the southwestern United States. The many species of yucca have adapted in slightly different ways, but they are similar in one respect: they all depend on a specific genus of moth for pollination.

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The Joshua tree is an exotic species of yucca.

Identification

Species of yucca have evolved both as shrubs and trees, both of which produce clusters of fragrant white flowers. Spanish bayonet (Yucca baccata), chaparral (Y. whipplei) and other yucca shrubs grow rosettes of stiff, sword-shaped leaves on the ground. The Joshua tree (Y. brevifolia) grows in the deserts of Arizona and California, and the yucca tree (Y. vallida) grows on Baja California. The creole word "yuca" is used for the unrelated cassava plant (Manihot esculenta).

Climate Adaptations

Species of yucca have adapted to a wide variety of climates in mountains, coastal sand, grasslands and prairies as well as the rocky badlands and deserts with which they are usually associated. Soapweed yucca (Y. glauca) grows as far north as the provinces of Alberta and Saskatchewan in Canada. The U.S. Department of Agriculture has maps showing the range of 30 different species of yucca in the United States.

Yucca Moth

All species of yucca have a symbiotic relationship with a specific species of moth in the genus Tegeticula. The yucca depends on the moth for survival; the moth depends on the yucca for survival. The female of a yucca-specific moth is able to force pollen through a narrow orifice leading to the female stigma in the yucca flower. T. yuccasella, a white moth, pollinates the Mojave yucca (Y. schidigera) and Adam's needle (Y. filamentosa), found in Missouri, and 17 other species of yucca. The moth T. paradoxa pollinates the Joshua tree, and T. maculata pollinates chaparral. There are no yucca moths in Europe, and yucca species grown in gardens there have to be pollinated by hand.

Other Adaptations

Most species of yucca have thick, waxy skins to prevent loss of water through evaporation, called transpiration in plants, and they frequently store water in thick roots. Some yuccas store water in thick, fleshy leaves; other species drop their leaves during drought to prevent the loss of water through transpiration. Dead leaves collecting against the trunk of the Joshua tree help protect it from the sun. The channeled leaves of a Mojave yucca direct dew and rainfall water to their roots. The Spanish bayonet and chaparral are said to be "fire adapted;" that is, they grow and spread vigorously after wildfires.