Tim, Your plants looks great. Don't you have any of my hybrids with Yucca angustissima ssp. kanabensis?
Don, the roots of the hybrids are usually a mix of those on the parents. The roots of second generation hybrids seem to lean towards what ever genes is most off.
I always find it very interesting to see the roots of the hybrids, when we are transplanting seedlings. The look on the roots will give you a little hint of (depending on the age of the seedling) how the plants probably will look like when it's mature. And it will also give you a hint on the hardiness of the hybrid.
In hybrids involving any of the species from the rupicolae serie, the typical relatively thick roots is, as with the species of this serie, visible at a very early stage, and the forming of rhizomes is earlier than usual for rupicolae's
First time I noticed this, with the roots of Yuccas, were in seedlings from a batch of seed I got from Mesa Garden in 1997, they were sold as seed no: 1984.3. Yucca faxoniana X glauca and were collected by Dave J. Ferguson. Ferguson told me, that this hybrid was found growing in a street median in the city of Albuquerque New Mexico. The "mother" plant was growing in a planting of Yucca glauca (or Yucca elata without trunk!) and Yucca faxoniana.
Ferguson later told me in a personal letter, that the original "mother" plant is not anymore, since some one has bulldozed it....
The roots on the seedlings were very variable in form, and those with "faxoniana roots" had leaves resembling a narrow leafed form of faxoniana and those with roots like glauca had leaves very much like those of Yucca glauca.
During the years that past by, I have lost those the faxoniana type forms, and only a few of the glauca type forms remain alive. The plants grow extremely slowly and is often loosing the rosettes during the winter, so they have to start from the rhizomes again. A few years ago we planted one inside the unheated greenhouse, and it's doing just fine in there.
Well I got carried away, so I better stop here
Have a nice day