Yucca Brevifolia - Growth and Development

Growth and Development

Joshua trees are fast growers for the desert; new seedlings may grow at an average rate of 7.6 cm (3.0 in) per year in their first ten years, then only grow about 3.8 cm (1.5 in) per year thereafter. The trunk of a Joshua tree is made of thousands of small fibers and lacks annual growth rings, making it difficult to determine the tree's age. This tree has a top-heavy branch system, but also has what has been described as a "deep and extensive" root system, with roots possibly reaching up to 11 m (36 ft) away. If it survives the rigors of the desert it can live for hundreds of years with some specimens surviving up to a thousand years. The tallest trees reach about 15 m (49 ft). New plants can grow from seed, but in some populations, new stems grow from underground rhizomes that spread out around the Joshua tree.

The evergreen leaves are dark green, linear, bayonet-shaped, 15–35 cm long and 7–15 mm broad at the base, tapering to a sharp point; they are borne in a dense spiral arrangement at the apex of the stems. The leaf margins are white and serrate.

The flowers are produced in spring from February to late April, in panicles 30–55 cm tall and 30–38 cm broad, the individual flowers erect, 4–7 cm tall, with six creamy white to green tepals. The tepals are lanceolate and are fused to the middle. The fused pistils are 3 cm tall and the stigma cavity is surrounded by lobes. The semi-fleshy fruit that is produced is green-brown, elliptical, and contains many flat seeds. Joshua trees usually do not branch until after they bloom (though branching may also occur if the growing tip is destroyed by the yucca-boring weevil), and they do not bloom every year. Like most desert plants, their blooming is dependent on rainfall at the proper time. They also need a winter freeze before they will bloom.

Once they bloom, the trees are pollinated by the yucca moth, which spreads pollen while laying her eggs inside the flower. The moth larvae feed on the seeds of the tree, but enough seeds are left behind to produce more trees. The Joshua tree is also able to actively abort ovaries in which too many eggs have been laid.