Can I plant a small tree in a container that is sunken into the ground, so as to contain the roots?
Planting a Tree
Certified GKH Gardening Expert
While you could attempt this feat, it may not be a good idea, as the tree may eventually outgrow the container and require digging up to repot--quite a task for in-ground potted plants. The following articles should be of some help to you: https://www.gardeningknowhow.com/special/containers/how-to-grow-container-trees.htm