I have a number of thse which I use to grow herbs.
Is it a good idea to put capillary mating in the bottom - or whatever the stuff is called that is used for hanging baskets?
Some of the troughs are quite shallow, and I thought this would help retain moisture, and perhaps help with drainage.
probably wont help much as the matting may suck the water away from the plants. It wont help drainage and may cause water logging. Stuff used for hanging baskets is to hold the compost in and does not offer much in the way of water retention. It is best if you mulch the compost at the top with gravel or bark or moss to help retain moisture.