Posted: 05/03/2014 at 09:35
I normally put a large stone over pot holes then grit then compost. That helps drainage and stops slugs getting in via the hole at the bottom and is a favoured place for them to lay eggs.
Polystyrene repels water and I have only used it to bulk up tall and large containers. Terracotta and cement is porous, so also no good at retaining moisture. So not sure what the RHS means by the use of such materials. If they said Biochar or hydroponic clay pebbles then yes.
Why would you put Blackcurrants in a pot? They are well behaved in the ground.