Flower War

Near future. As the earth reaches its end due to environmental pollution, the earth wakes itself up from the ancient sleeping guardian (flower-like biological weapon). They attack mankind, the culprit of environmental pollution, and trying to destroy civilization. Fight against the founding guards of the surviving human-power of the robot defenses. A slum-born new robot pilot, Kay, is assigned to the Moonlight Fox and encounters a different type of guardian during a battle against them.

(Source: MangaDex)