When using Rainwater Terrace planters there a few things that can be done during the wetter months to help stop the planters becoming too wet or water logged.
First open up the holes in the bottom of the side planters with a sharp instrument such as a screwdriver.
Then put a layer of horticultural grit on top of the capillary mats.
Add more horticultural grit to the compost.
Adding vermiculite might also help to retain some moisture.