Dear Lucy, a lot of these are old sinks recovered from houses after they got bombed in the second world war! Instead of wasting them crafty gardeners used them by covering them in different mixes of cement and peat, or tufa.
Then grew Alpines and succulents in them, or annuals as well. Even the old horse troughs are used by the councils as flower beds now!
Regards.
Neil.
p,s. Irises grown in whisky, what a waste of whisky!
