Wireless sensor networks are considered an important dimension of the modern information and communication technology-based solutions for greenhouse monitoring. Several sensing and communication technologies are already available and have been used both in pilot and commercial installations. Their attractiveness is based on their deployment flexibility and low cost. In this paper we present the implementation features of a greenhouse monitoring system that is based on a ZigBee wireless sensor network. The system is composed of various sensor nodes that collect environmental conditions' data and transmit them to a remote database. A web application enables the growers to setup electronic cultivation records and monitor the greenhouse conditions as indicated by the collected WSN data. We also present the outcomes of an experimental study in the context of which the implemented system was deployed and operated in a greenhouse complex with hydroponic cultivation of tomato crops. This experimentation intended to explore the level of heterogeneity of the microclimate conditions in the greenhouse environment. The study revealed non negligible differences on the levels of temperature and humidity even across different sectors of compact greenhouse compartments and highlights the importance of dynamic management of the climate conditions on the basis of real time microclimate observations, with an ultimate objective to achieve greenhouse production of high quality and quantity, while using less input resources.