The Grand Defiance of Agave Blooms

Agave deserti blossoms | Photo: Cody H./Flickr/Creative Commons License

Heading downhill yesterday toward "downtown" Joshua Tree I saw it: a gigantic pillar rising from the desert floor, slate blue-gray and covered in imbricated scales, looking like nothing so much as an improbable asparagus spear six inches in cross section. Eight or ten feet high, it wasn't there the last time I passed by -- so it'd been growing at a rate of a foot a day or so.

It was the developing stalk of a flower, growing out of one of my neighbors' Agaves.

The agave in question isn't native to this part of the desert. It's an Agave americana, originally hailing from the highlands of Central Mexico but planted worldwide over the last 250 years or so. You see them dominating coastal California gardens, breaking stucco retaining walls, and perforating the shins of the unwary along neglected sidewalks.

In places with a little bit more water than we have here in Joshua Tree, where an Agave americana can grow to be eight feet across or so, the species' flower stalk can reach heights of 30 feet. The one in my neighborhood will be lucky to reach 20 feet; it takes a lot of metabolic resources to grow a stalk that high, and local agaves struggle to put on growth even in non-blooming years. Blooming can use up resources equivalent to a full year normal growth, or more. Water and sugar and minerals that had been stored in the plants' fleshy leaves and roots go into erecting the plants' floral pillars. the agave depletes itself to build that flower stalk. After blooming there's nothing left; the plant dies.

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Biologists refer to this "all or nothing" strategy of reproduction as semelparity. It's a sensible way of maximizing the amount of resources that go into creating the next generation of your species, as long as you don't plan to get to know your kids. It's a poetic gesture: putting everything you have into reproduction and then dying, the botanical equivalent of a salmon swimming upstream to spawn and then succumbing to the ordeal.

Agave americana was imported into the California deserts, but two of its cousins got here on their own: Agave deserti, whose rosettes of spiny leaves get to about two feet tall and wide at most, and Agave utahensis, whose several California varieties are the size of soccer balls or smaller. Both are found at upper elevations in the easternmost parts of the California section of the Mojave Desert, and Agave deserti also grows from the Santa Rosa Mountains south through Anza-Borrego and into Baja.

Outside the desert there's one more California Native species of agave: Agave shawii, which just barely hangs on along the San Diego coast. It's more prevalent in Baja: urban development of chaparral coastal bluffs has depleted the plant's U.S. habitat. And that's it for the Golden State, which is relatively devoid of agaves compared to places like Arizona, which has 40 or so native species. Texas and Mexico have even more. There are about 135 species of agave native to North America, and only three are native to California, one only by the skin of its teeth. It hardly seems fair.

Agave utahensis var. nevadensis, Spring Mountains, Nevada | Chris Clarke photo

Many agaves cheat on the "bloom and die" strategy by producing clonal offsets, commonly called "pups" by succulent gardeners. Both California desert native species do so, as does Agave americana for that matter. A rosette will send up a flower stalk, bloom, set seed and die, but its offsets will live on, their precious bodily fluids undepleted. Like many other desert plants, agaves can thus live to rather astonishing ages as clonal rings -- centuries or more.

Agave deserti blooming in Anza Borrego | Photo: Cody H./Flickr/Creative Commons License

Despite the possibility of longevity through cloning, most agaves don't come anywhere close to deserving their common name of "century plant," based on folklore that the plants grow for 100 years before making that grand, defiant reproductive gesture. Agave deserti's lifespan tends to run no longer than 50 years. Agave americana's is more like 30. I don't seem to be able to find a reference that ventures an opinion on the lifespan-before-flowering of Agave utahensis, but based on the number of ten-foot flower stalks I see in most clumps of the species I'd venture a guess of less than 40 years.

How did agaves decide on the whole semelparous reproductive strategy? Most experts seem to think a coevolutionary arms race is to blame. Agaves are pollinated by flying animals, mainly bats, though hummingbirds do visit the flowers of some species, as do larger insects such as hawkmoths. When you're pollinated by animals that are flying around looking for your flowers, getting those flowers up off the ground as high as you can is a good strategy. Your visitors will see your flowers from farther away, for one thing, and they'll be safer from ambush predators than they would be if your blooms stayed down in the underbrush. And once your pollinating species gets used to the idea that you're putting your flowers out on tall spikes, they'll search for tall spikes and may well overlook the shorter ones.

Evolutionary pressure to attract those pollinating animals, in other words, likely encouraged the ancestors of our present-day agaves to grow taller and taller flower stalks to compete with their kin. At some point this internecine arms race reached a critical threshold, as arms races generally do: the ancestral agaves started building flower stalks so tall that they didn't survive the process.

As long as a few seedlings survived, though, it didn't matter. All that matters to evolution is the continuation of the species. Very few agave seeds grow to maturity; only those lucky few that land in a sheltering nurse plant in a year with the right combination of moisture and temperature will sprout, and only a very small percentage of those will make it through their first year.

With clumps of agaves sending up flower stalks each of which might scatter several thousand seeds across the landscape, the math just barely works out in favor of survival.

Seed pods of Agave deserti, Santa Rosa Mountains | Chris Clarke photo

It doesn't always work, even for agaves carefully tended by human gardeners. In the late 1980s I brought home an Agave americana in a four-inch pot from the nursery where I worked at the time. It was one of the variegated ones, with a cream-colored band down the center of each leaf, and I carted it on move after move to seven different gardens over the next 20 years, in an ever-growing series of pots at first and then bare-root in the back of my pickup truck. By 2005, planted in its last home in my front yard in Pinole, California for three years, it was the size of a 1967 VW Beetle. My days of moving it were at an end. Even after my divorce in 2008 and my move to the Mojave, I carried the hope that at some point in the following decade my ex-wife would text me, let me know that the blooming process had started. I'd trek back to the Bay Area to rest my hand on the stalk, I thought, and think back to the earlier days when it was a strapling in a four-inch pot, and so was I.

Last year she told me that the plant had just died without blooming, and she'd had to hire a crew to come out and remove it. She didn't mention whether there were any offsets left.

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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