potted tree grown into soil
I recently moved into a new home that has a lovely Japanese maple in seemingly good health on a sloped yard. When I removed the groundcover (non-native English ivy), I discovered that the tree was still in the original pot (mostly) and had developed a significant root system in the soil on the slope. It cannot be moved and seems to be healthy, but I'm concerned about the tree's future health. What can be done? We are considering building a sort of cedar planter box around it, but don't see a way to have a closed bottom that would prevent soil/water from just flowing through and down the slope. Any creative ideas out there?