Asked November 15, 2014, 5:42 PM EST
Cook County Illinois
Thank you for your question to eXtension.
Most of the university bulletins I've seen don't go into too much detail on the size and number of holes...all they say is "have adequate drainage." I suppose you can drill a few small holes, fill the container with soil, water it, and examine how well the excess water penetrates. This is fine if you hit it correctly the first time, but having to empty the container to drill more holes would be a pain.
In Purdue's bulletin on container gardening (http://www.hort.purdue.edu/ext/HO-200.pdf ), they say: "To provide drainage, drill three or four small (1/4 inch) holes in the bottom of the container. Holes larger than 1/4 inch in diameter will allow too much soil to escape. Placing a layer of gravel or broken pottery pieces on the bottom of the container, below the soil, can help stop the flow of soil through larger holes. "
However, using Google, I find numerous self-proclaimed experts using drill bits of between 3/8 and 5/8 inch. I might lean toward that myself, since it's easy for a piece of perlite or bark to block a tiny 1/4 inch hole (I found that myself, while drilling those size holes in the bottom of a bird feeder for my wife...the millet seed plugged them up immediately).
If you're drilling ceramic, the internet experts suggest using masonry bits, avoid putting too much pressure on the drill and bit (to avoid cracking the pot), and having water available to cool the ceramic and wash away the grit as you drill.
I hope this helps. Best of luck!