I'm planning to make some self-watering planters, using a bed of water-
absorbent granules (vermuculite would work). I will then cover the bed of
granules with a synthetic filtering membrane and drill some overflow holes
just below the top surface of the granule bed. I will then add a filler
tube going down into the bed of granules, before filling the container with
compost, and planting.

The granules could be corse vermiculite. I wondered if anyone can suggest
an alternative material that is less expensive? Broken up building blocks
(the aerated lightweight type) might be one option. Anything absorbent that
doesn't cantain harmful minerals or chemicals and is non-perishable might
work.

Al