The blooming cycle of the Joshua Tree is totally dependent on climatic conditions. Depending on the timing and intensity of winter rains, blossoming can occur any time from March to May, and can vary from very sparse to a rare abundance of blossoms in relative wet years. We were fortunate to be there in a good blooming year.
The flowers begin as greenish buds clustered near the end of the branches. The six tepals (3 petals and 3 similar sepals) then open up, to reveal a pod-like ovary, surrounded by six stamens, with one pistil. The flowers are waxy, white-to-greenish-white, and 2-3 inches long (5-7 cm). The flowers emit a sweet fragrance reminiscent of coconut.
The Joshua Tree pollen is so sticky that it cannot be carried by the wind. Pollination is accomplished only with the help of the tiny Yucca moth, with which this species has evolved a symbiotic relationship. This moth (Pronuba yuccasella) collects the pollen from a number of flowers and then chooses one flower as a home for its offspring. It deposits the pollen on the flower’s stigma in the process of laying eggs in the flower’s ovary. The newly hatched larvae will feed on the seeds (consuming only about 20 out of 200 in each ovary).
After the Joshua Tree flower is pollinated, the ovary swells with seeds and the flower falls away. The ripened fruit then falls to the ground where the seeds are scattered, often taken up by the wind. Very few will be able to germinate in the harsh desert conditions, but it is by this process of seed propagation that the new plant colonies are established.
The Joshua Tree can also propagate by shoots from its roots or crown, and this is particularly important when the upper part of the tree is destroyed by fire.
Information for this post was taken from a variety of online sources: Flower Essence Society, Joshua Tree NP Info Page, Desert Tortoise Preserve Committee, Graeme Somerville: The Biogeography of The Joshua Tree (Yucca brevifolia).