Answer: In my experience, there would be little benefit to doing this for the typical foxglove Digitalis purpurea once it has bloomed. The seeds are so prolific and, if you prefer container grown starts, so easy to start, that the work of digging and dividing becomes very unattractive -- especially when the plants seem so exhausted once they have bloomed. If you are determined to try it and have the extra garden space, there is no harm in experimenting but in my view it is not worth the effort and probable risk of failure in any case.
In my garden, the few plants that seem to have exceptionally well developed basal rosettes tend to survive for three years naturally, becoming essentially perennials. I would be leery about disturbing these plants since they are already doing so well. Most, however, do not seem to be dividable. Instead, the central flowering stalk can be cut back and sometimes additional glowering stalks will develop from the base for a second set of blooms.
Digitalis x mertonensis is more reliably perennial and could conceivably be divided after bloooming, although this too is a bit tricky since in my experience this plant is somewhat fussy.
On the other hand, it is certainly a simple matter to transplant the young seedlings so they are better distributed or more attractively spaced in the garden. This works very well if done in the late summer or early fall. Perhaps this is what the author intended to suggest.
In most gardens where foxgloves bloom every year, the trick is to allowing seedlings to develop regularly and this gives the impression of a colony of perennials when in fact there is a steady rotation of young plants replacing the old. In my opinion this time honored method is really the best.
Good luck with your foxgloves!
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