Repotting Container Plants
This information comes from an article by Dr. John Fitzpatrick, current President of the Horticulture Society of Maryland. He was responding to a question about the need to layer gravel at the bottom of a pot. We have all placed those gravel pieces under the soil to aid in the drainage of water, or so we thought...
Dr. Fitzpatrick explains there are several drawbacks to this practice. First, adding a layer of gravel decreases the amount of growing area for your plant. Now, depending on the plant, this smaller growth area might be fine but it is something to consider. Second, since the potting soil has a smaller particle size than the gravel, whenever water is added the soil filters down in between the gravel pieces , packing in tightly between the stones. As a result, the soil is packed even more tightly in this gravel layer than in the potting soil layer above resulting in slower water movement and a wet layer of soil, defeating the initial aim of the gardener.
If you would still like to have a gravel layer at the bottom of your container pots, Dr. Fitzpatrick suggests placing a landscape fabric or any rot resistant barrier over the gravel layer to prevent any soil movement. This barrier fabric needs to be secured to the inside of the pot and some gardeners use duck tape.
For his own pots, Dr. Fitzpatrick favors cutting a piece of fiberglass window screen to cover the pot's drainage holes to retain soil and take advantage of a larger growing area for his plants!