This is one of the cooler plants in the Adelphi Garden. It is Yucca rostrata, an interesting Yucca endemic to one county, Brewster, in SW Texas and a few isolated areas in adjoining Mexico. This is a plant on the fringes of fanatical horticulture. I got it from a wild traveling succulent vendor/Lawyer, Rick Levengood, about 25 years ago and it has been in the ground since the late 1980s. Clearly hardiness is not an issue, though it does have the advantage of growing in almost pure sand. It was about three feet tall when I bought it for, if I remember correctly $125.00; that includes the pot so it wasn't very tall! Now it is somewhere over12' tall, probably closer to 15'. The leaves are, as in the picture, beautiful glaucous blue/gray approaching .5" wide and ~4' long. It is a spectacular plant.
After returning from North Carolina and the Shortia Expedition, I noticed it was preparing to flower. If you click on this picture you can barely see the spike, but I did climb up a ladder and got a bit better shot that is to the right. It is exciting to see a plant flower after 25 years. I know theses Yuccas do occasionally branch and I suppose that flowering could provide an impetus to branch, or not. I expect that all the rain we have had has been a factor in its flowering, but again, this is just an assumption.
For the past 5-10 years, it has been available in catalogs, on-line in small sizes, and from an occasional nursery. I can't come up with the name but north of Rehobeth, Delaware on the west side of Route 1 there is a Garden Center that look interesting from the road. It is; they have cool pottery, nice tropicals, and a variety of odd plants. I saw maybe 20 of these Yuccas there several years ago, but whoever was doing the watering didn't get it; these are desert dryland plants. They were soaking wet and the foliage was yellowing from root rot. Still they cost from 200-300$? It made me unhappy to see the plants so unhappy. Anyway, this one is happy and I will put up a picture of the flowers when they open.