That's a really helpful reply, thanks so much.
I'm going to install Irrigatia system in big greenhouse for my daughter. She cannot carry/ lift watering cans anymore after an accident.
Why I was wanting a substrate/ capillary layer was in case of ant irregularities in level in the trays. From your experience of no substrate...I am probable better putting a wee something under trays to level them?
Be lighter too.
I was thinking of thin layer of compost. I have seen capillary matting but some seem to look really urgh after a bit and wonder re harbouring bacteria/ nasties. Probably overthinking it.
The Irrigatia company have really been fab at looking at solutions. It was yours and Redalders posts that got me onto that :)