Timing of Low Pressure Irrigation Affects Plant Growth and Water Utilization Efficiency

Pine bark based substrates, commonly used in the southeastern United States for container nursery crop protection, have low moisture retention properties; therefore, daily irrigation during the growing season is required to maximize plant growth. Current guidelines state that irrigation should occur during the early morning hours (before 1000 HR). However, limited research indicated that multiple application of water each day resulted in significantly more growth compared to early morning application. The objective of these studies was to evaluate the effects of irrigation timing on plant growth, photosynthesis, water utilization efficiency, and substrate temperature. In experiment 1, the daily total volume of irrigation required to maintain 0.4 leaching fraction (LF) in the early morning application (0300, 0500, and 0700 HR) was divided into three equal parts and applied at the following times: 0300, 0500, and 0700 HR; 1200, 1500, and 1800 HR; 0900, 1200, and 1500 HR; and 0500, 1200, and 1900 HR. In experiment 2, the daily total volume of irrigation to maintain 0.15 LF within each timing was divided into three equal parts and applied at the following times: 0200, 0400, and 0600 HR; 0600, 0900, and 1200 HR; 1200, 1500, and 1800 HR; and 0600, 1200, and 1800 HR. Irrigation applied at 1200, 1500, and 1800 HR produced 57% and 69% greater total plant dry weight in experiments 1 and 2, respectively, compared to plants irrigated during early morning hours. Root: top ratio was unaffected by irrigation timing. In both experiments, irrigation applied at 1200, 1500, and 1800 HR had higher water utilization efficiency compare to irrigation applied at 0300, 0500, and 0700 HR or 0600, 0900, and 1200 HR. In experiment 2, plants irrigated at 1200, 1500, and 1800 HR maintained higher rates of net CO₂ assimilation and stomatal conductance, and lower substrate temperatures from 1800 to 2200 HR compared to plants irrigated at 0300, 0500, and 0700 HR or 0600, 0900, and 1200 HR.