James W. Cornett

It doesn’t happen every year.

Joshua trees don’t bloom every year. But when they do bloom it is usually universal, synchronized and spectacular.

Flower clusters (inflorescences), with many dozens of blossoms each in each cluster, are abundant on all three of my study sites in Joshua Tree National Park. They are also abundant on a site in Saddleback Butte State Park a hundred miles to the west. Multiple inflorescences adorn most adult trees on a site near Wickenburg, Arizona, as well on one in southwestern Utah. A two- and one-half-acre quadrat in southern Nevada is in full bloom as is another site two hundred miles to the west at Red Rock Canyon State Park. It seems everywhere I go in the Mojave Desert there are Joshua trees in bloom.

The bloom is spectacular. Counts of inflorescences on a site in Lost Horse Valley in our national park are typical. Last year 45 adult trees produced just 5 inflorescences, a degree of blossoming easily missed by Park visitors. This year those same trees produced 144 inflorescences. That’s an increase of 2,780%! Not surprisingly, all 8 park visitors that I interviewed had noticed this year’s spectacular Joshua tree bloom.

Most interesting about this phenomenon is that a dramatic increase in blooming frequency is not confined to one location or even one park. The bloom is synchronized and happens in the same year, at about the same time throughout the range of the tree. How do the trees accomplish this feat? Is there some sort of evolutionary advantage in synchronous blooming?

I will go out on a limb (no pun intended) and state for the record that I don’t think Joshua trees are communicating with each other on a master plan for blooming in any given year. One Joshua tree never says to another “Hey guys, let’s get together and all bloom at once this year. It will make visitors to Joshua Tree National Park really happy.” Joshua trees don’t have human senses so they can’t tell, see, hear or touch another tree to find out what neighbors are doing. Perhaps they have some kind of olfactory sense? But even if they did, prevailing winds from the west would push such communication in a single direction, not in all compass directions. So this is probably a negative as well.

The most likely cues for mass Joshua tree blooms are environmental. Some condition such as time, type and amount of precipitation, is responsible. Whether or not the winter, or a previous winter, was warm or cold may also play a role. Joshua trees are dependent upon particular species of moth for pollination. The cycle of Joshua trees blooming (if there is a cycle) may be tied to an increase or decrease in the moth population. There is little point of blooming if your primary pollinator is not around. Of course, this begs the question as to what affects the size of the moth population. No one yet knows the answer to the apparently irregular mass blooming of Joshua trees. The subject remains a mystery.

You can witness the spectacular bloom right now in Joshua Tree National Park. If you can’t make it this weekend you will likely miss it.

Cornett is a desert ecologist, valley resident and author of Coachella Valley Wildflowers.