I do wonder about plum slate, whether it sometimes has something nasty in it. I planted a row of fruit trees alongside a wall of my house, that I then protected from weeds by covering with weed excluding fabric and a mulch of plum slate. All the fruit trees died. Of course it is possible that the problem was something in the soil, as there was a layer of coal dust in the soil along that wall. But sometimes i wonder if it was the slate. But other people grow shrubs in just the fashion I describe with a slate mulch, so it must normally be OK.