Tistou of the green thumbs (Tistou les pouces verts), by Maurice Druon, publishing by Hachette Jeunesse, 1968 

Tistou wasn’t born to become an ordinary adult like most, with his head full of ready-made thoughts. His presence on earth is not insignificant.
His life is quiet, in a nice house in the town of Mirepoil, with Mister Father, Madame Mother, Carolus, the man-servant, Madame Amélie, the cook, and his pony Gymnastique.
His father has a cannon factory, and it seems evident to everybody that Tistou will grow up to work in his father’s business.

When Tistou was 8 years old, he went to school for the first time. Before that, he was homeschooled by his mother. After three days at school, he was expelled. «Mister, your child isn’t like everybody else. It’s impossible for us to keep him.» explained the teacher. (p 35) After this experience, his parents decided definitively. «He’ll learn what he must know by seeing it for himself directly.» (p 41) Thus began his different lessons: a lesson on plants with the gardener, Moustache, and lessons about order, misery, geography and manufacturing with Mister Trounadisse, his father’s trusted manager in the factory.

The prison, the little sick girl in the hospital, the town’s squalid housing, and the zoo all profited by Tistou‘s secret talents. His talents were only known by Moustache, the gardener, who discovers that Tistou had extraordinary green thumbs, and by his pony, Gymnastique.
When the Vaazys (Go Ons) and the Vatens (Go Aways) declare war on each other, Mister Father sold cannons to them both. Tistou felt he had something he had to do…
When Moustache disappeared, Tistou wasn’t satisfied with the complicated explanations of the grown ups. His pony told him the truth and went along with Tistou on his last climb up a tree.