Germinating Yucca elata and Yucca faxoniana.

Last year I sprouted several Yucca baccata (Banana Yucca) and Yucca elata (Soaptree Yucca), and while I grew out many Yucca baccata, I only managed to grow out a single Yucca elata. Despite our brutally frigid temperatures, this single Soaptree Yucca handled the Colorado winter seemingly with ease. This is encouraging - the tree yuccas fascinate me, which naturally means I plan on growing some more.

Today I began cold-stratifying eight Yucca elata seeds and eight Yucca faxoniana (Texas Tree Yucca) seeds. Yucca elata is probably the hardiest of the tree yuccas, but success with yuccas, agaves, and their kin is determined by many factors, and USDA hardiness zone is only one of these. Siting (soil structure, exposure, microclimate, etc.) seems to be as critical in determining success with members of the family Agavaceae, particularly when planting them in areas pushing the limits of their cold-tolerance. My Yucca elata is growing on an exposed bed of decomposed granite, granite chunks, pumice, and loam-amended clay (the native soil). Since our regionally native Yucca glauca (Soapweed or Great Plains Yucca, a short colony-forming species) thrives in our local soil, I'm not surprised that its relatives have done well either. Still, since many reputable sources cite Yucca elata as hardy to "about zone 6a", I am very pleased that our less-than-a-year-old seedling survived the winter. Yucca elata has a reliable branching habit, adding to its "tree" image.

Yucca elata blooming. Photo by Stan Shebs.

Yucca faxoniana - the Texas Tree Yucca - is the other tree yucca that I am attempting to propagate. This hardy tree yucca has one of the thickest, stoutest trunks (a foot in diameter) that supposedly forms a single stem reaching a mature height of up to 15 feet. I am following a similar protocol for cold-stratifying the seeds (about a month), since it seems that yuccas benefit from cold-stratification even if they are native to a milder clime. Some sources claim that Yucca faxoniana is hardy to USDA Zone 5, while others are far more conservative (USDA Zone 8), so I will just have to grow them out and see. My source claimed it was hardy to USDA Zone 5. As I wrote earlier, it seems that Yucca faxoniana is similar to Agave parryi in that differing populations of the same species can have quite different hardiness limits depending upon the conditions of their native environment. Perhaps my seeds come from a hardier population. We'll see. I plan on growing these near my Chilopsis linearis (Desert Willow) in a protected, sunny location that warms to daytime temperatures quickly in the winter.

Update 04-23-2014: I was able to read the section in Mary and Gary Irish's excellent book Agaves, Yuccas, and Related Plants on Yucca faxoniana and they indicate that it can tolerate severe cold - a relative term here in Colorado - down to approximately 0 degrees Fahrenheit. We frequently get a tad colder, but I still think siting will play a factor in its success or failure.

Yucca elata with Yucca baccata in the upper right.
Photo by Bill Jury.

I also managed to get a photo of my Yucca elata yesterday. It is putting on new growth and has very little winter damage. Not too bad for a year-old seedling.