Answer: What a huge job to move large yuccas!
Unfortunately gardeners generally have better success trying to move smaller plants. You are right that it is important to preserve as much of the tap root as possible and that is very hard to do with the larger plants.
Several points come to mind here. Yuccas can tend to look ratty early in the spring, but by now I would expect some fresh growth around the outer edges. If the yucca crowns you brought home were blooming crowns last year, they could simply be dying off due to old age. It is also possible that you are overwatering. Remember that yuccas generally do not require very much water at all.
If you are patient, you may discover smaller side shoots coming up around your apparently dying yuccas. If this is intolerably ugly, you may wish to try again. If you return to the site where you dug these, eventually you should find many smaller yuccas sprouting from the old tap root. It is also a fairly simple process to sever a side shoot or two from a larger plant. (Smaller pieces from the perimeter are the easiest to take.) The best part of this method is that the wild "mother" plant is left to survive in place.
Spring is a good time to do this. Select a smaller crown from the outer edge of the plant and separate it using a sharp flat spade. Try to get as much of the tap root belonging to the side shoot as possible, but leave the main root whole. Plant the resulting small crown in a well drained, sunny spot. Be careful to plant it no deeper than it grew before. Water well at transplanting and it should survive in your garden. In a very few years you will have a nice large clump.
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