Tough, hardy yucca shrubs are evergreen members of the agave (Agavaceae) family. A very common garden yucca is "Adam's Needle" (Yucca filamentosa), hardy in U.S. Department of Agriculture plant hardiness zones 5 through 10. It is noted for its stiff, sword-like leaves and tall stalks of pendulous white flowers, which are borne in June or July, depending on the zone. Yucca, including "Adam's Needle," attracts butterflies, but it is pollinated by moths.
Yucca pollination is performed by yucca moths (Tegeticula spp.), with different yucca moth species performing the task east or west of the Rocky Mountains. In general, a female yucca moth visits a flower, collecting pollen from the anthers, which are the male reproductive parts. Moving on to anther flower, the moth first lays an egg among the unfertilized ovules; then leaves the pollen in a cavity within the flower's stigma, or the female reproductive part. After hatching, the yucca moth larva eat some of the seeds contained in the plant's ovary; uneaten seeds are distributed and produce new plants.
Though yucca is adaptable to light shade, best flowering (and pollination) takes place if the plants are grown in full sun. The plants thrive in dry, sandy soil. Drainage is important, so clay soil should be generously amended with organic material with some fine gravel added. Once established, yucca is drought-tolerant. Yuccas produce fertile seed, but they also multiply by offsets and can be divided if the clumps get too big. Remove spent flower stalks once the blooms fade for a neater appearance.
All yucca varieties are moth-pollinated, including "Color Guard" yucca (Yucca filamentosa "Color Guard"). It is hardy in USDA zones 4 through 10 and has the same characteristics as the species, except that the leaves have green margins and ivory white centers. A closely related species, giant or spineless yucca (Yucca elephantipes), hardy in USDA zones 9 and 10, produces multiple stems and sword-shaped leaves in spiral rosettes. The flowers are white and similar to those of "Adam's Needle" yucca. "Variegata" (Yucca elephantipes "Variegata") has leaf variegation similar to that of "Color Guard."
In addition to attracting the moths that pollinate it, yucca is unattractive to deer and rabbits. It is suitable for providing architectural interest, especially in areas where animal browsing is a problem. Yucca is also well-suited to container culture, which will not hamper pollination. Because of its drought-tolerant nature, yucca is a suitable choice for the xeriscape or dry garden, and variegated varieties can lighten up the color scheme in such situations.