Automated sub-irrigation of potted ornamentals based on physiological data collected in response to precision water content monitoring improves the efficiency with which the greenhouse industry utilizes water resources by minimizing fertilizer inputs and wastewater runoff, white maintaining economic goals of maximizing crop growth and quality. This thesis is an investigation of the effect of substrate moisture content on growth of 'Chrysanthemum morifolium' Ramat., 'Surfine' and 'Gerbera jamesonii' 'Festival Rose' potted in peat-based substrates and allowed to dry-down to pre-determined irrigation thresholds ([theta]t) to trigger sub-irrigation events. 'C. morifolium' irrigated at [theta] t >20% and <40% exhibited an appearance of adequate quality with no differences in growth or daytime leaf gas exchange. 'G. jamesonii ' irrigated at [theta]t = 30 and 40% exhibited superior growth (shoot dry weight, root structure, flower number, scape height, leaf size and number) and compactness. Compared to plants sub-irrigated at a fixed 40% threshold, 'G. jamesonii' grown using a variable threshold (15% to bud stage then 40% to maturity) exhibited equal growth and higher resistance to water stress. 'G. jamesonii' adapted to irrigation thresholds >=22% substrate moisture and wilted when water content fell to <20%.