This paper presents an efficient monitoring solution for greenhouses. The solution implements the Wireless Sensor Network (WSN) monitoring system and then expands the monitoring concept to an Internet of Things (IoT) solution, by employing 3G access, web services, database and a web interface for user interaction. The main advantage of this modular monitoring system relies on the fact that the End-Devices are highly configurable, enabling several monitoring solutions with different types of sensors and including energy harvesting capabilities. This modular development approach allows the system to be highly scalable and reusable. TI's Z-Stack and SDK support all the network nodes' firmware development. The gateway application software was developed in C++ and Sencha Ext JS framework was employed to implement the web services and the web user interface.