A homogeneous greenhouse climatehas economic advantages due toa more homogeneous crop, lesser diseases and possibilitiesto save energy.The horizontaldistribution oftemperature and humidity,obtainedwitha dense grid oflow-costwireless sensors,opens ways tocontrol homogeneity, either by adapting the greenhouse infrastructure or by selectively operating greenhouse heating and ventilation.Trials were performed with 100 sensors in four commercial greenhouses,to evaluate th practical use,observe climatevariability anddetermineamount of sensorsneeded foran accurate estimate of the spatial and temporalclimatedistribution.We observed long-term averaged spatial differences for temperature and h umidity ofrespectively1.0 �3.4 o C and 10 �40 %, and aspatial differences are larger. The greenhouse s investigated showed that at least 9 sensors per hectare (±33m apart) were necessary to detect long term cold or wet spots