It's called Kokedama. Instead of a pot or a hanging basket, I wrap the roots and soil in a thick layer of long fiber sphagnum moss, then I bind the whole thing together with twine. According to a book I read, the thick compressed sphagnum actually retains water better than a terra-cotta pot, which is known to sort of wick moisture away from the roots and soil. (I take that factoid with a grain of salt, but here I am having made 20 of them) Whenever the ball feels dry and sort of light, I soak it in a bucket of water, either until the ball sinks, or about 10 minutes, whichever comes first. If you want to learn more, I was given this book for my birthday and I highly recommend it. It taught me in one day.