Question: How essential is a drainage hole for a pot?

Answer: It’s required. Most folks think that it’s because you would over-water a plant if you didn’t have the drain hole, and that is, of course, possible. However, a good gardener can avoid that pitfall. What you cannot avoid is the accumulation of mineral salts in the soil. Each time that you water the plant, and every time that you feed it, you’ll be introducing more salts to the soil mix. It’s exactly the same problem as the Great Salt Lake, and, unless you can leach those minerals out of the soil periodically, the plant will eventually die.