Impact of Cyclic Irrigation Timing on Plant Growth and Daily Water Use in Eastern Redbud Grown in a Pot–in–Pot Container Nursery

Current best management practices recommend single irrigation to occur during early morning hours to reduce drift and evaporative loss of water for container grown nursery plants. A pot-in-pot (PIP) study was conducted at the University of Kentucky Horticulture Research Farm in Lexington, KY to evaluate optimal timing of daily cyclic irrigation in eastern redbud (Cercis canadensis ’Forest Pansy‘) growth and daily water use. Liners were grown in either 7-gallon or 15 gallon containers filled with 85% pine bark: 15% peat (vol/vol) in PIP systems in a completely randomized experiment design. Substrate moisture content was continuously monitored using EC5 (Decagon, IL) moisture sensors inserted into three representative containers per irrigation treatment. Irrigation was scheduled to replace 100% daily water use applied in three equal amounts and applied at the following times: cyclic irrigation starting at (i) 7, 8, and 9 am; (ii) 12, 1, and 2 pm; or (iii) at 5, 6,and 7 pm.  Water use was approximately double in plants grown in 15-gal containers compared to 7-gal containers. The timing of cyclic irrigation impacted total and daily use in 7-gal, but not 15-gal containers. In the 7-gal containers, the least amount of water was used in the 7AM cyclic irrigation schedule. Containers required greater irrigation volumes when irrigation was scheduled at noon (19%) and at 4PM (5%) compared to the 7AM irrigation.

Plant physiological measurements as well as plant water status were collected just before the start of cyclic irrigation event and it varied on an average from about 9 μmol CO₂ m^‐2·s^‐1 in the morning to 11 μmol CO₂ m^‐2·s^‐1 in the noon and to 13 μmol CO₂ m^‐2·s^‐1  in the afternoon irrespective of irrigation timing. Sap flow varied from about 23 cm hr^-1 in the morning and to 54 cm hr^-1 in the afternoon for plants grown under various irrigation treatments. Leaf water potential became more negative as day progresses irrespective of cyclic irrigation timing as observed right before the morning (-7 kPa), noon   (-16 kPa) and afternoon irrigation (-22 kPa) events. The study highlights the water savings under sensor based cyclic irrigation and that when water is not limiting, environmental variables such as air temperature, relative humidity and solar radiation are more closely coupled to changes in plant physiological characteristics and water status.