Air pruning pots work well. When a root tip reaches air near the edges of the container/pot the root tip dies and it splits off in two going in different directions. It stops roots from hitting the edge of the pot and then circling the pot, which causes plant stress when it happens.
Superoots Air-Pot is one manufacturer and another is RootMaker. There is also a chemical way of doing the same thing called Griffin's Spin Out. Copper hydroxide suspended in a carrier is painted on the inside of pots and when root tips hit the copper hydroxide the root tips die the same as when in contact with air and then split off. Some do not like the idea of the copper hydroxide and others do not consider it to be a problem. Myself I prefer the air root pruning types of pots but that is only based on opinion and not on test results of copper hydroxide having been found in harvested herb or anything.
