We need to determine what can target and/or interact with plants.
Spells and equipment/objects in DnD usually target either creatures or objects.
To what category do plants belong has already been discussed here: Are plants creatures or objects. The answer argues that "ordinary" plants are objects.
There are competing answers that argue otherwise (Can Mending be used on plants?)
Since there is no precise definition of what creature is in the rules (What is the definition of "creature" and is it used consistently?), there is RAW ambiguity about what spells/abilities can interact with "ordinary" nonmagical plants.
Plants as objects
In my opinion, this is a more reasonable option. A common characteristic of creatures is that they have agency and ability to interact with their environment willfully. Plants have minimal ability to do that compared to animals and other things explicitly defined as creatures. Also, there is somewhat of a precedent for this (Gulthias Tree in Tales from the Yawning Portal).
Plants as objects can then be "repaired" by spells that can repair objects. Here mending comes to mind.
They would also follow the rules for interacting with objects, as described in DMG, p. 246.
Plants as creatures
Some magical plants are already creatures (Awakened Tree) and, therefore, can be interacted with as any other creature (this would include healing spells unless the spell specifically states otherwise, or even "feeding" them potions of healing).
Looking at all plants as creatures would work the same way, however, a problem would arise where there are no stat blocks for them and, therefore, no AC/HP values.
Plants as "neither"
Of course, you can also rule that plants are neither objects nor creatures. In that case, the only way to interact with them would be something that specifies plants, like the druidcraft cantrip
You instantly make a flower blossom, a seed pod open, or a leaf bud bloom.
or plant growth. However, I am not aware of any equipment or spells that would specifically repair/heal damage to plants.
Broader lore discussion
The previous explanation talks about what is defined in the mechanics of the game and what a player character (or NPC) can do based on defined spells/abilities/equipment.
When defining a spell PHB, p.201 says
Uncounted thousands of spells have been created over the course of the multiverse's history, and many of them are long forgotten. Some might yet lie recorded in crumbling spellbooks hidden in ancient ruins or trapped in the minds of dead gods. Or they might someday be reinvented by a character who has amassed enough power and wisdom to do so.
The spells defined in PHB are just a fraction of spells available to players or spells that have defined mechanical effect in the game. It is likely that spells that heal plants that druids can use exist, they are just not mechanically defined in the rules of D&D 5e.