I have been on NARG conferences and US trips a few times, some with Kenton Seth. "Crevice gardens" seem to vary somewhat in N America. In Montreal for example there is a very large CG of the type seen at Wisley. I.e lots of thin rock slices packed closely together. (the toast rack principal) The plants have to work hard to get down to the water, and there is very much more rock than plants. In other gardens it was difficult for me to see that a garden was a crevice garden as the displays looked at first glance like the old "plum pudding" type but with many more rocks. I guess it depends on if thin slices are available locally when you build it; the crevice being the important thing. How easy it is for plants to get at the moisture will depend mostly on how often it rains, (naturally of otherwise) but the construction type will also play a part.
