The exploration of bio-physical crop parameters is fundamental for the efficiency of smart agriculture. The leaf area index (LAI) is one of the most important crop parameters and serves as a valuable indicator for yield-limiting processes. It contributes to situational awareness ranging from agricultural optimization to global economy. In this paper, we investigate the potential of Wireless Sensor Networks (WSNs) for the in-field assessment of bio-physical crop parameters. Our experiences using commercial off-the-shelf (COTS) sensor nodes for the indirect and nondestructive LAI estimation are described. Furthermore, we present the design of our measurement architecture and results of various in-field measurements. By directly comparing the results achieved by WSN technology with those of a conventional approach, represented by a widely used standard instrument, we analyze whether bio-physical crop characteristics can be derived from WSN data with a desired accuracy. Moreover, we propose a simple approach to significantly enhance the accuracy of COTS sensor nodes for LAI estimation while, at the same time, reveal open challenges.