Q: I have a copper planter that I want to put a Japanese maple in. I do not want to put holes in the planter. What is the best way to prepare the planter or do I have a choice? The plant will be kept outside.
A: Your first job is to match the size of the container to the size of the tree. If the container is larger than twenty-four inches across and similarly deep, you have a chance of keeping a small-sized Japanese maple alive. If your container is smaller you need to research bonsai technique.
Your second task is to devise a way to support the soil mass (and the tree above it) on top of a gravel layer a couple of inches deep. You should have enough space for any excess water to trickle through the soil and accumulate in the gravel, not in the lower layer of soil. Covering the gravel with weed block fabric is one way to accomplish this. Watering properly will always be a problem with an undrained container. What will you do if there are several days of rain?
All this is to say…it is possible to plant a Japanese maple in an undrained container but it requires much planning beforehand.