There is nothing much more puzzling to the beginner in floriculture than the word " drainage" when applied to potted plants, especially as some authors of recent years have ignored the theory of drainage entirely and pronounced it wrong in theory, and a waste of money and time in practice. There are few farmers, nurserymen or market gardeners who do not believe in the practice of draining. Some land may need it more than others, but all are benefited by a system of drainage except it be a good loam lying on a gravel.
I am not going into the art of draining land, but the principle, if right with land, is right in our flower pots. Years ago it was carried out to an absurdity. A piece of crock in the bottom of a 4-inch geranium or canna is absurd and no one does such a thing now. If a plant is going to stay in a pot but a few weeks or even a few months and is necessarily a quick and strong rooter like a lily there is no need of any drainage. In the case of plants that may stay a year, or perhaps two or three years, in the same pot, if you were sure that the water was always going to pass freely away as it does in the first month or two, there would be no need of drainage, but worms get in and work the soil into a putty state, or the soil gets so packed at the bottom of the pot that water does not pass away freely.
Much as plants enjoy the watering when in need of it, terrestrial plants don't exactly feed on it. The water passes away, leaves the soil moist and full of moist air spaces, which the roots are continually absorbing till it is gone and they want more. See how easy it is to kill most any plant when the water remains in the pot and keeps the soil for a few days saturated. So plants want water to pass through the soil but not remain there, and with all plants that are going to remain any time in the same pot (azaleas are a good example) they should have- what we call drainage.
With pots not over five inches in diameter a broad crock at the bottom covered with a piece of green moss, is enough, and with larger pots in addition to the piece of crock covering the hole an inch or so of broken crocks. The green wood moss is much better for the purpose than sphagnum because the sphagnum soon rots and the compost gets down among the crocks. You will sometimes see the healthiest and strongest roots of a plant down among the crocks. I believe it is because they find there the conditions to suit them best - perfect drainage.
How particular we are that the benches of our carnations and roses are drained by simply keeping the boards one-half or three-quarters of an inch apart, so that if watered heavily it can pass quickly away. And so long as our flower pots are made with that one small hole we will have to make provision to let water escape freely. The author who thirty years ago laughed at the old fogy notion of draining a flower pot lived to alter his opinion and freely acknowledged it.