Diurnal and seasonal water use and the effects of different levels of irrigation on growth and flower production were studied using commercially cultivated Geraldton wax (Chamelaucium uncinatum Schauer cv. Purple Pride) under semi-arid conditions. Water use and growth responses to increased irrigation differed widely with vigour. Vigorous plants had extensive root systems and used more water than less vigorous plants whether irrigated or unirrigated and responded to increased irrigation by producing more shoots, secondary stems and flowers. In vigorous plants, marketable stem length and flower number were increased from 63 to 89 cm and 61 to 226 when irrigation was increased from 25 to 75% of pan replacement respectively. Less vigorous plants had distorted root systems (i.e. curling and circling) and showed no significant growth response to increased irrigation. Increase of stem length to a desirable length of 90 cm in less vigorous plants, which are wide-spread in commercial plantings, was unlikely. The results highlight the significance of good propagation and establishment techniques to aid vigorous growth. The sap flow study shows that Geraldton wax is a high water using species. A single plant can use in excess of 20 L per day under high evaporative demand when root growth is not limiting.