She uses it to refer to the divine nature. She uses it to mean vitality, fertility, and fecundity, amongst others. It is the divine creative power through nature.
illustration from Scivias
The following is from Scivias. Beautiful.
Understanding corresponds to the greeness [viriditas] of the branches and leaves, will corresponds to the flowers and blossoms, feeling is like the first fruit sprouting forth, and intellect like the fully ripened fruit. Sense perception is like the height and breadth of the tree. Thus, the soul provides the inner solidity and strength of the body.
For Hildegard, there was a harmony of correspondence between the macrocosm and microcosm.