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Our Lord's Candle, Spanish Bayonet, or... What Should We Call This Plant?

Hesperoyucca whipplei flowers | Photo: Gary Nach/Flickr/Creative Commons License

It's one of my favorite plants in Southern California, but I'm never sure what to call it in polite company. I'm an atheist, so the common name "our Lord's candle" just doesn't sit well with me. "Spanish bayonet" is confusing: other plants share that name. If I'm talking to avid outdoor people, they'll know exactly what I mean if I call it Yucca whipplei, but there's a problem: it's not a yucca. At least not anymore.

There are other suggested common names. In his book "Cacti, Agaves and Yuccas of California and Nevada," author-photographer Stephen Ingram calls it "chaparral yucca," Some folks combine that poetical geotag with the old churchy name and call it "chaparral candle," which I like, but no one knows what I'm talking about when I use it until I say "you know, Yucca whipplei," which brings us back to the problem of it not being a yucca.

An obvious solution is to call the plant by its currently accepted botanical name, Hesperoyucca whipplei. And that's what I usually do, but that obvious solution creates its own problems, chief among them having the conversation about why the name changed.

I don't mind that, because I'm a complete geek. But at some point even I notice that the person I'm talking to is letting his or her eyes glaze over ever so slightly, and that the conversation has drifted into monologue, and that I have just subjected a perfectly innocent attorney or auto mechanic to a lecture on more than they ever wanted to know about evolutionary biology.

Complicating things is the fact that a whole lot of field guides and other publications, in print and online, still refer to the plant as Yucca whipplei. A person can find the plant, usually by walking into it with painful results for their shins, become curious, look the plant up, and come away inadvertently misinformed as regards the plant's proper name.

Hesperoyucca whipplei is more a coastal plant than a desert plant, really. It grows from Baja through the San Diego and Los Angeles regions north to the southern fringes of the Bay Area, with a connected population in the southern Sierra as far north as Kings Canyon or so. The plant's widespread along the southwest margins of the California desert along the fringing mountains, where it gets a little extra water from runoff, and elevation provides a bit of a break from the heat. It doesn't seem quite drought-tolerant enough to make it in the deep desert, though there are exceptions. in 1931, the revered California botanist Willis Jepson found a plant (then called Yucca whipplei) growing in the vicinity of Boron in the early 1930s. (Here in Joshua Tree, in our yard in the rain shadow of Nolina Peak, a Hesperoyucca planted by our landlords has just come into full bloom.)

Each individual plant can vary from three to six feet wide, and as high. They're rosettes of long, straplike leaves that are saw-sharp on their edges and possessed of fearsome terminal spines. In its season of bloom, happening right now, those individual plants -- once they get to about six or seven years of age -- will send up flower stalks that rise 8 to 15 feet tall, covered with hundreds of delicately scented white flowers about an inch and a half across. Sometimes, as in the photo above, those flowers are bordered in red. The flower stalk is the "candle" referred to in the common names. It's not so much votive candle as Roman candle.

Hesperoyucca whipplei in bloom in 2005 | Photo: Vgrubsky/Wikimedia Commons/Creative Commons License

Like its not-so-close-after-all relatives the Yuccas, Hesperoyucca depends on a pollinating moth to reproduce. After the plant's flower stalk sets seed and dies the individual plant generally dies as well, much like its other cousins the agaves.

Hesperoyucca is actually a bit more closely related to agaves than are its former colleagues in genus Yucca. In fact, it's the species' relationship to other plants that prompted the renaming.

Or perhaps I should say "re-renaming." Nineteenth Century botanist Georg Engelmann called the plant Hesperoyucca in the first place, adding the specific epithet "whipplei" after U.S. Army surveyor, botanist, and Civil War casualty Amiel Weeks Whipple, for whom the Whipple Mountains south of Needles are also named.

Engelmann later reconsidered his take on the species, reclassifying Hesperoyucca as a "section" within the genus Yucca in the late 1800s, which formally renamed the species Yucca whipplei. In the century since, botanists have been ambivalent about that call, with several opining that Hesperoyucca really needed to be considered as a separate genus.

This isn't just an arbitrary matter of deciding which pigeon-hole to put an organism in, stamp-collector style. The goal of taxonomists these days is to align their names -- their "taxa" -- to fit the ancestry of the group they're naming, so that the overall structure of taxonomy describes the evolutionary history of life on earth. During most of the 20th century, scientists had only gross physical morphology to guide them, and they had to deduce which plants were related to which others based on which characteristics the groups shared, and which they didn't.

The problem was that members of the Hesperoyucca group were so variable that classifying them was really tricky, both in relation to their Yucca cousins and among themselves. There was the difference in flower color noted above. Some forms of the plant died after flowering, like the agaves, and some lived on after flowering, like the yuccas. Some created clumps of offsets, and some remained as solitary rosettes through their lifespan. This made it tough to determine what the proper boundary was to draw between Hesperoyuccas and other yuccas.

The discovery of methods of analyzing DNA helped sort things out. Comparing stretches of DNA from different organisms gives scientists a far more precise way to determine how closely related each is to the others.

In 2000, botanist Karen Clary published the results of DNA studies of Hesperoyucca. Clary showed not only that Hesperoyucca had a lineage quite distinct from other Yuccas, but that it was far more closely related to the genus Hesperaloe -- a group of plants from the Chihuahuan Desert in Texas and Mexico that no one had ever proposed might be yuccas.

As part of the attempt to make botanical taxonomy describe evolutionary history, botanists these days try to have each genus describe a group consisting of one (usually hypothetical) common ancestor and all its descendants. Such a group is called a clade. The Yucca clade, for instance, would include all living yuccas and all their progenitors back to that unknown first yucca.

In order for Hesperoyucca to be kept within genus Yucca, that clade would need to extend back to the hypothetical common ancestor of the two groups. Problem is, as Clary showed, that common ancestor is also the ancestor of Hesperaloe. If Hesperoyucca is to stay in Yucca, Hesperaloe would need to be renamed Yucca as well.

So far, no botanist has been willing to suggest lumping Hesperaloe into Yucca. It's a subjective call, but the two groups are just too different.

So Yucca whipplei is no more. Instead, hikers in Southern California get their shins impaled by Hesperoyucca whipplei. To be honest, the pain hasn't changed much.

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