tree whose branches are placed on Buddhist graves
樒 refers to a specific tree, the Japanese star anise (Illicium anisatum), whose aromatic branches are traditionally placed on Buddhist graves as an offering.
樒 combines the tree radical 木 with 密, which likely contributes the sound ミツ. The character was created to name this specific tree.
A tree (木) with dense, secret-like (密) foliage: the 樒 tree, whose branches are used in secretive Buddhist grave rituals.
For ミツ, imagine the dense (密, みつ) branches of the 樒 tree being placed on a grave: 密 -> ミツ, and the tree's reading follows.