There are a number of events which have to "line up" in order for a Joshua Tree to flower. Usually, a joshua tree only flowers after heavy rainfall, provided it rains at the right time of the year.
Prior to the heavy rainfall in spring, the tree requires a winter freeze not unlike some of the spring bulbs we plant in our gardens.
Once the joshua tree is exposed to these elements and the flower emerges on its panicle, the only thing left is for the yucca moth to lay her eggs and in turn pollinate the tree.
Joshua Tree, Yucca brevifolia, Joshua Tree National Park, Mojave desert, California, USA
Technical Information:
I photographed this photo with the digital SLR camera model Canon EOS-1Ds Mark III, aperture of f/8.0, exposure time of 1/100 sec. on ISO 100, as always I used a original Canon Lens, the focus lenght for this picture was 200mm.