This water-saving ‘ready-made garden’ design is popular among urban gardeners here in Denmark: it’s comprised of a simple styrofoam planter, and a 40L soil bag.
Holes are cut in the top and bottom of the soil bag, and soil is pushed into three pillars that reach down into a water reservoir. The reservoir is filled with water and fertiliser through a channel on the side of the planter; capillary action (and eventually, plant roots) draw the water up.
The rate of evaporation is much lower with bottom-watered plants, and the styrofoam also keeps the water cool. The reservoir can hold a week or more of water.
A popular DIY version involves using plastic tubs, creating a reservoir underneath with bottles, or a permeable barrier.