The Law of Ueki(2001)

Ueki no Housoku

In a world of powerful celestial beings, an epic contest is being conducted to select the next king! Each Celestial selects a kid in junior high to be his champion and grants him a special power. The kids battle it out--losers are eliminated, and the winners are granted new talents!

Seemingly ordinary Kosuke Ueki has been chosen to be a contender in the tournament. Granted the power to change trash into trees, Ueki has two disadvantages to overcome--one, he doesn't know he's a participant in the tournament, and two, how the heck can anyone win a battle with the power to turn trash into trees?

(Source: VIZ Media)