Posted: 13/03/2015 at 00:01
I would very carefully research each plant before you actually plant it, partcularly any of the spreading ones or anything described as groundcover. Once you get something invasive growing in a rockery, it will be virtually impossible to eradicate it later. I think the mixture sounds about right. Personally, I think an ideal mixture would be 50% John Innes No3 (recommended for permanent plantings) and 50% grit, which avoids getting any weed seeds in with the soil. However, that might be rather expensive for a large rockery, so 25% soil, 25% multi-purpose compost and 50% grit would be a good bet.