A challenge exists in horticultural greenhouse farming as a result of the current decline in the agriculture labor force in Japan. Although there is a need for cutting-edge technological Information and Communications Technology (ICT) application in agriculture, few small-scale farmers are willing to risk significant capital on sensing technologies. In this study, a simplified smart agriculture system using limited resource expense was developed and applied in a tomato greenhouse. The system’s real-time information capability is composed of commercial inexpensive wireless sensor network (WSN) devices, and a developed web database (web DB) for daily activity data collection was used for crop environment monitoring and management. The crop microclimate environment was monitored on the basis of an agro-climatic index, namely, growing degree day (GDD). Microclimate spatiotemporal distributions using GDD were also determined, which showed well-defined microclimate variations within the greenhouse. Crop calendars were embedded into the developed web DB for tomato farms. We drew conclusions for better crop management and crop yields using our smart agriculture system. The applicability of the system based on initial cost, running costs, and the reliability of WSN data makes it cost-effective. Consequently, it could be used for the accurate crop production planning and decision making of cultivation activities.