As I randomly remembered parts of this, it's driving me crazy I don't remember it.
The book is the second in a series I believe.
It follows a boy from a remote town that appears to be looking for a way to save it from a drought or blight. As well as being accompanied be a girl from a nomadic tribe? I am hazy on that detail.
Now here's the most specific detail I remember. He is exploring ruins of an ancient city. It is hard to navigate because of overgrown brambles and trees. And in these ruins he discovers a bag of berries that he brings home and the plants flourish, providing more berries that sustain the town. Inside the ruins he also finds a small ornate statue of an owl.
But the berries turn out to knock out all those that ate them, and the trees come to life and attack the town. Revealing how the ancient city became overgrown.