Yucca Yucca Yucca

Some plant names are just fun to say. The genus Yucca falls very much into this category, hence the title. Go ahead, try it. 

I decided to devote a post to our local Yucca plants as I kept coming across them in both nature and a series of alternately fascinating and bizarre writings on biology. Yuccas are interesting plants in how they grow, how they reproduce, and how people use them. 

Distribution and Description

The yuccas are a prevalent local genus which are relatively easy to identify in the Denver area despite there being considerable confusion among botanists about the subtleties of hybridized varieties(1). This is because in the front range there is but one species (discounting others which may have escaped cultivation) which is Yucca glauca. Y. glauca are the familiar sprays of sword-like leaves that populate the pastures along I-70 outside of Denver, and may be found as far east as Kansas and Nebraska(2). It is hardly endangered, and is even considered a rangeland weed by some ranchers. Two other Yucca varieties, Y. baccata and Y. harrimanieae, may be found farther to the south and west on this side of the Continental Divide but seldom, if ever, in the Denver area(1). 

This photo could be of pretty much anywhere in the front range as far as the presence of yuccas is concerned. In this case, Yucca glauca (the spiny green plants about 1/3 of the way up the photo) are growing in the Deer Creek Canyon area near Littleton. Yucca are almost as emblematic of the landscape of the high planes as Saguaro cacti are of the Sonoran Desert. 

Morphologically, Yucca glauca presents as spiky lumps which gradually develop a caudex (stem) as successive generations of tough, fibrous, ensiform (sword-shaped) evergreen leaves rise from the apical meristem. This distinctive habit is characteristic of the family Agavaceae, a primarily tropical family which helps explain Y. glauca's almost exotic appearance(1). Once the leafy rosette reaches a sufficient size of about a 3-6cm diameter at the lowest green leaves, it blooms once and then dies(2). The flowers are three-sepaled and three-petaled with six anthers and a prominent three-sectioned style*. These flowers are abundant and pendant on tall rarely branched racemes that rise high above their parent plants. The fruits are dehiscent(3), beginning as fleshy, three-chambered green capsules before becoming desiccated and splitting open. 

*If you are noticing a lot of threes here, it is because monocot plants such as Yucca will typically have flowering parts in multiples of three, while dicots are in fours or fives. This is a handy trick in the field, as it can help to narrow down families and genera fairly quickly. 

Larger plants (caudex >5cm diameter at lowest green leaves) which do not flower in a given year seem to have a greater chance of reproducing by growing ramets ("pups") around their bases(2). This accounts for Y. glauca's tendency to appear as dense groups as well as individual plants. When going about the important business of reproduction, it apparently doesn't hurt to "hedge" one's bets.   

A clonal colony of Y. glauca growing near Littleton, CO. The dry spikes are from previous flowerings 

by now deceased individuals, and the clumping habit is due to clonal reproduction.  Note the tendency of older plants to form shaggy stalks as they grow in an effort to reach flowering and death. Valar morghulis. 

The border between green, living leaves and those which die to contribute to the caudex. You can get a feel for the intense toughness of the fibrous leaves (click to enlarge) which lend themselves to a wide variety of human uses (see below). 

The flowering racemes of Y. glauca. Compared to the austere parent plant, the flowers are luxuriant and fleshy. I used to think that the color of the sepals was a useful field character, but as far as I can tell it is actually not indicative of anything: we only have one front range species, but I have run into flowers with sepals ranging from purple to white. 

Yucca Ecology: Animals Using Yuccas

The inflorescences of Y. glauca are fascinating as they represent evanescent but robust ecological communities. All of our native Yucca species are obligate symbiotes with the moth Tegeticula yuccasella(1). The moth bores into the ovary of the Yucca flower and deposits an egg inside. Then it (the moth) gathers pollen from the yucca flower and stuffs it deep inside the ovary, ensuring both food for its young and pollination for hundreds of yucca ovules. This relationship is critical as Yucca flowers cannot be pollinated by simple contact of pollen with the stigma, and must have the pollen placed inside their ovaries directly to ensure fertilization. 

Yucca flowers are also frequented by parasitic aphids, which in turn are tended and protected by diligent armies of ants(4). The ants "farm" the aphids for their sugary secretions in a fascinating process that you can read more about here. While they are on the flowers, the ants can in turn have a limiting effect on populations of parasitic moths which bore into Yucca stems or which prey upon their seeds(5). In this way, aphids can ironically improve the reproductive success of the flowers they are parasitising! 

This image gives a good idea of Y glauca's flowering structure, as well as showing the presence of the aforementioned ants and aphids in the top center (click to enlarge). 

Last year's fruit. Note the dehiscence of two of the three lobes from each other (third lobe not visible). You can also clearly see one of the bore holes left by Tegeticula yuccasella just to the right of the image center, as well as another more obscured one on the lower lobe.

Yucca Ethnobotany: People Using Yuccas

 

Members of the genus Yucca are culturally important plants to virtually all of the indigenous peoples in their native ranges, and Y. glauca is no exception. It is used as a medicine, food, material for cordage/basketry, as soap, and as a cow fart mitigation factor. 

Medicine 

The Blackfoot and Cheyenne tribes used the roots for a number of topical applications including easing arthritis pain and as a dressing for cuts and inflammation(6). Interestingly, the Dakota and Isleta peoples used the root as an aid to "make hair grow." Presumably the latter application was more psychosomatic in effect than anything, or else the male pattern baldness drug industry has missed a serious holy grail. In modern research, Y. glauca has been investigated for the cytotoxic activities of the steroidal saponins (soap-like substances) found in its roots(7), i.e. when we throw this plant at cancer, does it kill it? The authors did find two substances which induced cell death in cancer cells, so presumably research is ongoing. 

 

Food 

Many Native Americans such as the Apache, Lakota, and Navajo tribes ate the green fruits following a number of methods of preparation including roasting, pickling, eating raw, and sun-drying(6). The flowers are reportedly edible raw as well.

Cordage/Basketry 

Virtually every tribe that lived anywhere near Y. glauca's native range used the leaf fibers to make ropes, baskets, and nets. Of particular note are the Tewa of Hano Indians, who used the leaf fibers to make whips used for beating novices in initiation ceremonies. Thank you, sir, may I have another! This blog post gives a good feel for how relatively easy it is to harvest and transform yucca leaves into ropes. 

Soap 

This is the other use for Y. glauca that virtually every tribe engaged in. Apparently all parts of the plant produce some amount of saponins, as the preparation methods for many tribes involve macerating (soaking) the whole plant and using the water for washing. The part that is richest in saponins appears to be the roots, as this was both the more popular preparation method among Native Americans and the preferred method for isolating saponins for research purposes. 

A post demonstrating how yucca roots may be used as soap to clean wool for spinning into yarn may be found here. I recommend checking this link out, as it really gives a good feel for how soapy a single yucca root can be. 

Cow fart mitigation factor 

This use falls very much under the category of things that are not known until they are sought after. While I was combing research databases for info on yucca-derived saponins, I discovered that there is a burgeoning body of literature addressing the ability of yucca-derived saponins placed in cow feed to reduce a cow's methane emissions. This is not just one odd study, but actually represents a hot area of research large enough to warrant its own meta-analysis. The meta-analysis is free here for those interested. I find it heartening that these researchers are doing their part to reduce global warming (methane is a major greenhouse gas), but I was taken off guard by this particular line of scientific inquiry. 

Thank you for reading about Yucca glauca! I didn't even know how much I didn't know about this surprisingly information-dense native plant, and it was a pleasure to study. I hope that you enjoyed this excursion into the pokey world of yuccas. 

References 

1. Weber, W. A., & Wittman, R. C. (2012). Colorado Flora: Eastern Slope Boulder, Colorado: University Press of Colorado.
2. KINGSOLVER R, W. (1986). VEGETATIVE REPRODUCTION AS A STABILIZING FEATURE OF THE POPULATION DYNAMICS OF YUCCA-GLAUCA. Oecologia (Berlin), 69(3), 380-387.
3. Colorado Plant Database (2015). Yucca glauca. Retrieved from http://jeffco.us/coopext/plantdetail.do?sna=Yucca%20glauca&image=1
4. SNELL, R. S., & ADDICOTT, J. F. (2008). Limiting the success of stem borers ( Prodoxus quinquepunctellus) in yuccas: indirect effects of ants, aphids, and fruit position. Ecological Entomology, 33(1), 119-126. doi:10.1111/j.1365-2311.2007.00946.x
5. Perry, J., Addicott, J., & Mondor, E. (2004). An indirect mutualism: ants deter seed predators from ovipositing in yucca fruit. Canadian Journal Of Zoology, 82(5), 823-827.
6. Moerman, D. E. (1998). Native American Ethnobotany. Portland, OR: Timber Press, Inc.
7. Yokosuka, A., Suzuki, T., Tatsuno, S., & Mimaki, Y. (2014). Steroidal glycosides from the underground parts of Yucca glauca and their cytotoxic activities. Phytochemistry, 101109-115. doi:10.1016/j.phytochem.2014.02.002