The description of the wither and bloom spell states (Strixhaven: A Curriculum of Chaos, p. 38; emphasis mine):
You invoke both death and life upon a 10-foot-radius sphere centered on a point within range. Each creature of your choice in that area must make a Constitution saving throw, taking 2d6 necrotic damage on a failed save, or half as much damage on a successful one. Nonmagical vegetation in that area withers.
In addition, one creature of your choice in that area can spend and roll one of its unspent Hit Dice and regain a number of hit points equal to the roll plus your spellcasting ability modifier.
There are a few questions on RPG.SE about the wither and bloom spell:
- Does the healing from the Wither and Bloom spell only happen if the target has at least one unspent Hit Die?
- Does your hit die include your Constitution modifier for purpose of Wither and Bloom?
However, neither of these clarify how it is handled if the target of the HP-regain portion of the spell is unconscious or dying.
(There are also questions related to short and long rests, like this one: Can a zero-HP, unconscious and stable character be woken up prematurely?)
Can you use wither and bloom in combat to bring a PC back to > 0 hit points when they are unconscious or dying?