I am a new greenhouse owner and have grown tomatoes, melons and cucumbers in 12" pots. I have used Bowers Peat Free compost, and as it dries out quickly, I have installed timed irrigation, gravity fed from rainwater butts. Using a variety of methods from mini-drippers to micro-soaker hose this has had limited success. The most successful is a ring of micro-soaker hose round each pot, but even so, the Bowers compost drains so freely that water is running out the bottom of the pots before the compost is full soaked across its breadth. The water just seems to take the path of least resistance straight downwards.
I have potted vines in John Innes compost watered by the same method without any problems.
Has anyone any experience of other peat-free compost? If the Bowers was mixed with loam based (J Innes) compost, would that help?