plum
楳 centers on the plum tree or its fruit, the plum. It is a specific character for the plum plant.
楳 combines 木 (tree) with 某, which likely contributes the sound. It is a phono-semantic compound for the plum tree.
The left side 木 is a tree, and the right side 某 looks like a fruit-bearing branch. Picture a plum tree heavy with ripe plums.
For バイ, imagine buying a plum tree at a market: buy -> バイ, and the tree 木 is the plum tree you take home.