To automate greenhouse irrigation, microclimate variations of the greenhouse environment and plant water requirements must be understood. Variation in greenhouse microclimate was monitored and quantified using pan evaporation (Bosman, 1990). The growing substrate volumetric water content (VWC) and growth performance of two crops, basil (Ocimum basilicum 'Genovese Gigante') and bellflower (Campanula portenschlagiana ‘Get Mee’), were monitored. The variation was not significantly related to growth performance, indicating plants can tolerate a range of growing substrate VWC for acceptable growth performance. Thresholds of 0.25 m3·m-3 for basil and 0.15 m3·m-3 for bellflower were determined to be the optimal growing substrate VWC at which irrigation should occur. A decision support system (DSS) was designed to control irrigation of a crop of bellflower at a commercial greenhouse, alongside the grower’s crop. At the end of the production cycle, the plants managed by the DSS the same quality as those managed by the commercial grower.