We describe the design of a large-scale wireless sensor network for agriculture monitoring. We have deployed a prototype of 64 sensors to monitor a commercial vineyard. The system provides different software modules ranging from filtering raw data to a centralized and a distributed data storage applications. The used protocols ensure reliable and robust communication and load-balancing energy consumption. A backend server provides a user-friendly graphical interface offering two main functionalities: logging communication messages of the employed protocols for diagnostic purposes and providing end-user support for both on-demand and periodic data requests. We highlight the efficiency of such a network in comparison to the traditionally used solution based on individual weather station by deploying both systems in parallel and comparing their reading results. The results clearly demonstrate that wireless sensor networks provide better geographic coverage and an increased spatial resolution compared to traditional solutions. They also enable precise measurements of soil conditions near the plants themselves, increasing the quality of the information available to the farmer.