In the last few years, changing infrastructure and business requirements are forcing enterprises to rethink their networks. Enterprises look for network infrastructures that increase network efficiency, flexibility, and cost reduction. At the same time, the emergence of Cloud and mobile in enterprise networks has introduced tremendous variability in enterprise traffic patterns at the edge. This highly mobile and dynamic traffic presents a need for dynamic capacity management and adaptive traffic steering and appeals for new infrastructures and management solutions. In this context, passive optical networks (PON) have gained attention in the last few years as a promising solution for enterprise networks, as it can offer efficiency, security, and cost reduction. However, network management in PON is not yet automated and needs humain intervention. As such, capabilities for dynamic and adaptive PON are necessary. In this paper, we present a joint solution for PON capacity management both in deployment and in operation, as to maximize peak load tolerance by dynamically allocating capacity to fit varying and migratory traffic loads. To this end, we developed the novel approaches of capacity pool based deployment and dynamic traffic steering in PON. Compared with traditional edge network design, our approach significantly reduces the need for capacity over-provisioning. Compared with generic PON networks, our approach enables dynamic traffic steering through software-defined control. We implemented our design on a production grade PON testbed, and the results demonstrate the feasibility and flexibility of our approach.