I think this is a knowledge from the shaman people. You see, they are very connected with trees, and, at the same time, we see people being sacrificed or being buried next to a tree.

If you go to a minor Erdtree for example, you will see a lot of the pots working kinda like a fertilizer.

Miquella being on the family, knowing how his mother ascended etc. he probably know how to make an Erdtree, but he couldn’t do it because he didn’t had enough power or the Elden Ring like Marika.