Just back from an amazing botanizing trip to coastal NC, which, among other things was yucca heaven. The two dominant yuccas in this area were Yucca aloifolia – foreground and Yucca gloriosa – back. The two species can be distinguished by Y. aloifolia having narrower, lighter, smooth leaves, with a scabrous (sandpapery) dorsal midrib and yellow leaf margin, while Yucca gloriosa has very scabrous leaves on the top and bottom, along with a red leaf margin
Some of the forms of Yucca aloifolia very quite short, maturing under 2′ tall with very narrow leaves. These grow right beside full size plants, indicating that their dwarf stature is genetic and not due to the harsh coastal conditions.
Most of the Yucca gloriosa grew as solitary stalks of green leaves with a slight glaucous cast.
while others maintained much older blue foliage
Some like this are almost certainly hybrids of Yucca aloifolia x Yucca gloriosa with characteristics of both parents. The diversity of species in nature is truly incredible, yet most people pass by without stopping to marvel.
And these green forms also appeared to be intermediate between both parents. I only saw one plant of Yucca filamentosa in the area, and no hybrid with it.