Industrial designer Joey Roth developed a self-watering planter for use indoors or out. It is made from porous unglazed earthenware:
"Soil and plants are placed in the outer donut-shaped chamber, and the center chamber is filled with water. The unglazed terracotta's natural porosity allows the water to move from the center chamber and into the soil, based on the soil's moisture (and thus the plant's need for water). The terracotta wall both regulates and filters the water. A simple lid on the top of the water chamber prevents evaporation."
The design is based on the Olla, a terracotta pot for irrigation that has been in use for 4,000 years. See also: