graft
椄 centers on joining a living branch or shoot to another plant so they grow together: grafting. The meaning is narrow and technical.
椄 combines 木 (tree/wood) with 妾, which likely contributes the sound. The character is rare and its exact historical development is uncertain.
A tree 木 beside 妾: picture a gardener carefully joining a branch to a tree, like a graft taking hold.
For セツ, imagine a set of grafting tools laid out beside the tree: set -> セツ, and the graft is secured.