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Josef - the bed has not used sharp sand or builder's ballast as is usually recommended. We had a large supply of fine potting grit left over from the nursery. This is actually a heat shattered flint and the smaller material left over from what is used for rendering house walls makes a good potting grit, very sharp and with a little larger particle sizes than sharp sand. However, it is not very attractive as a surface(!) so I have topdressed the bed with 6mm chippings, and generally redo this every year. The plants do take a while to get established and might well do better in sharp sand which is likely to hold more moisture. But on the whole they have prospered quite well.The bed was made simply by removing about 30cm of soil and filling in with the fine grit - no membrane was put in to separate the soil from the bed and so worms (and ants!) will have gradually begun to mix in the soil and I am sure quite a few plants will have put roots down to the soil below.I would like to make a bed with sharp sand or ballast to see how it compares, and also a crevice bed which seems especially good at growing choice plants, but we don't have access to suitable local stone so easily in Kent.