Plants heal just by getting their foodchunks back up to the maximum level -- kind of like growing their old leaves back. This should only be called by the game engine and so the method is protected by the immutable property.

public override void HealDamage()

Public Overrides Sub HealDamage

public:
virtual void HealDamage() override

abstract HealDamage : unit -> unit 
override HealDamage : unit -> unit