Posted: 23/02/2014 at 23:24
If you want to reduce the amount of compost in the pot, for economy or weight reasons, then adding coarse material in the bottom of pots will work, but what it does not do is improve drainage. Wisley, and many others, have been trying to get rid of this myth for some time. Google "perched water table" and you will see lots of discussions on it. The alpine group at Wisley used to demonstrate to visitors pouring water into identical pots, 1 with crocks and 1 without. The former holds onto more water than the latter, because it sits at the change in growing medium. What it does do is prevent water-logging by takeup from below, which can be useful, and means things like hanging baskets need watering less often, but it actually impedes drainage.