The second key to this design is the readily available fabric shopping bag. These common bags can be found at most supermarkets and are very inexpensive or free. The bags are made from spun polypropylene and are very porous. To test a bag simply fill it with water. If it runs out freely then it will work. Similar “Grow Bags” are available from nursery and garden suppliers at a much higher cost. Canvas or burlap bags may also work – but they will eventually rot and fall apart. The porous fabric allows excess water drainage and aeration of the root zone which is necessary for optimal growing conditions. The system is also based on the principle of “air root pruning”. As roots grow out to the porous fabric they become exposed to air, dry out and die. This causes the plant to produce dense fine feeder roots and prevents root circling. The increase in fine feeder roots leads to better nutrient and water absorption and promotes accelerated plant growth.