WILD ARMS Hananusubito

Flower Thieves takes place thousands of years after a war between humans and demons destroyed much of the life on the planet, turning the world into a scorched wasteland. Set in a dystopian future, the manga features a large group of humans on their last leg, living in the overcrowded city of Upper Hose where flowers and other flora are rare and valuable. When a plant-eating monster known as a "Flower-Thief" attacks a mysterious girl named Jechika, a young boy, Maxi, must use a forbidden ARM weapon to save her, and is subsequently expelled from the city for using illegal technology. Traveling into the wilderness with Jechika and a florist named Gi, Maxi sets off on a quest to restore the balance of nature throughout the world and make the earth habitable again.

(Source: Wild Arms Wiki)