Interest. Power idea being plant absorption. Upon touching a plant, his body can become like it. Plotting on it being an "always on" thing, his body retaining the abilities and appearance of the last thing he touched. Depending on desired power-level, may be instant, or may take time/plant volume to be able to switch.

Possible secondary ability of plant healing, literally absorbing plants to heal, plant size being related to how much he could heal from it. Blade of grass? Might heal a paper cut. California Giant Redwood? ....Wolverine-like regeneration from a single cell (laughably unlikely/impractical, etc). Generic urban tree? Heal a broken bone, or something.