What it is

Meet the Joshua tree

To care whether it survives, it helps to know what it actually is — which is stranger and older than it looks.

It isn’t really a tree

The Joshua tree — Yucca brevifolia — is a giant yucca, a monocot in the asparagus family, more closely related to grasses and agaves than to any oak or pine. It has no true wood and no annual growth rings, which is part of why its age is so hard to pin down. What looks like a gnarled tree is really an oversized desert lily that learned to stand up.

Actually, it’s two trees

What most people call “the Joshua tree” is two species. The western Joshua tree (Yucca brevifolia) grows taller with a single main trunk; the eastern Joshua tree (Yucca jaegeriana) is generally shorter and branches nearer the ground. Their ranges meet in a narrow zone in southern Nevada. Each is pollinated by its own species of yucca moth — one of the clues that led scientists to split them. Most casual guides still treat them as one; a serious source shouldn’t.

The moth it can’t live without

This is the best story the desert has. The Joshua tree is pollinated by nothing but the yucca moth (Tegeticula). A female moth gathers a ball of pollen, deliberately packs it onto a flower’s stigma, and lays her eggs inside the flower’s ovary. Her larvae eat some of the developing seeds; the rest become the next generation of trees. Neither partner can reproduce without the other — a mutual dependence so tight that biologists study it as a textbook case of coevolution. It also means a Joshua tree’s future is bound to an insect’s: threaten one, and you threaten both.

Slow, and very old

Joshua trees grow only a few inches a year — commonly cited at roughly half an inch to three inches, faster when young. Because they lay down no growth rings, ages are estimates: the Park Service suggests an average around 150 years, with some individuals likely far older. A Joshua tree usually grows as a single stalk until its tip flowers or is damaged (by frost, or a boring weevil); only then does it branch. Every crook in a big one’s silhouette is a record of something that happened to it.

How it makes more of itself

Two ways. From seed — which requires the moth, a good rain year, and usually the shelter of a “nurse” shrub for the seedling to survive. And clonally, by sprouting new stems from its roots, so a cluster of “trees” can be one organism. Who plants the seeds is a puzzle: by one hypothesis the seeds were once carried by the giant Shasta ground sloth, which went extinct ~13,000 years ago; today rodents that cache seeds do much of the work. That lost partnership is part of why the tree may struggle to migrate as the climate shifts.

The bloom

Joshua trees flower in spring — roughly February to April — in creamy green-white clusters. But not every year: a big bloom seems to need the right winter rains and a spell of freezing nights. That unpredictability is exactly why it’s worth tracking. See whether they’re blooming right now →

What lives in it

A Joshua tree is an apartment building for the desert. Ladder-backed woodpeckers drill nest holes in its trunk; Scott’s orioles hang nests in its leaves; the desert night lizard spends its whole life among the fallen limbs and litter beneath it. Two dozen–plus bird species, lizards, and countless insects use it for food, shelter, and hunting perches. When a grove is lost, its tenants are lost with it — which is what makes the threats to this one tree a threat to a whole community.

Where it lives

The Joshua tree is endemic to the Mojave Desert — a creature of a specific band of high desert across California, Nevada, Arizona, and Utah, mostly between about 1,300 and 5,900 feet. It defines that landscape so completely that we named a national park after it. That narrow niche is also its vulnerability: as the climate warms, the band of desert it can live in is shifting and shrinking.

Sources & further reading

• Joshua Tree National Park — “Joshua Trees”
• Godsoe et al. & Smith et al. — two species and their moths (coevolution studies)
• U.S. Geological Survey — Joshua tree & climate research
• CA Dept. of Fish & Wildlife — Western Joshua Tree

Where this page hedges (“commonly cited,” “by one hypothesis”), it’s because the science is genuinely unsettled — not everything about a 150-year-old desert lily is nailed down. When in doubt, follow the sources.