Potted and container gardening is a wonderful way to enjoy plants indoors and in limited spaced areas. The only problem is when the dirt falls through the necessary drainage holes.
You can always use the old tried and true method of adding a layer of gravel on the bottom but not only can some of that slip through the holes it can interfere with the draining. What we do is to simply cover the holes with something that will stop the dirt from escaping but still allows the water to get through.
Two choices worth using are, unbleached coffee filters and used dryer sheets. If you choose to use the dryer sheets, use ones that have been used a couple of times so that all of the softener and scent is gone. It is actually a good way to recycled them in a productive way.