This paper investigates a dynamic spectrum access (DSA) scheme for low power wireless sensor networks (WSN) in an open spectrum where multiple systems coexist and interfere with each other. Low transmission power and a simple communication protocol are significant obstacles to the operation of low power WSN in the open spectrum. As a result, available spectrum is more scarce for a low power WSN than for the other coexisting systems. To address this, we are proposing a new DSA scheme with an energy efficient channel switching criterion that can improve the expected throughput by chasing the low interference channel. OPNET-based system level simulation results, designed under the framework of ZigBee and IEEE 802.15.4, demonstrates that the proposed DSA scheme shows improved energy efficiency and throughput compared to conventional ZigBee systems.