Pulsatilla vulgaris var.alba

Sorry for my ignorance, Pcw, but what is tufa?

It's the rock.It's a limestone that forms with organic matter inside.As that rots away it leaves the rock porous so roots can work there way through and it's easy to make planting holes.

Is that the same stuff florists use?

If you mean that foam stuff then no.It is used in aquariums though

What did they use before the foam mounds?
Aquariums, hmm, not my cup of tea, but that's not to say I didn't hear of it in that context, though, so you may have hit the nail on the head.
The stuff that's rattling around in the back of my head amazes me at times. Lol.