Its not only moisture, but also need pleny of room for the roots. There is a polymer that can be added to potting soils that soaks up water and turns to a gel. It will swell up and deliver a lot more moisture requiring less watering. Gardeners Supply also has a big window box planter with a water reservour at the bottom with cloth wicks. You add about half a gallon of water and it holds it several days to water the soil above. I use these outdoors for my summer flowers.