Nursery crop production is an intensively managed form of agriculture, consuming large amounts of water, nutrients, and pesticides (Beeson, 2010; Bethke and Cloyd, 2009). Nursery growers commonly use static, timer-driven irrigation systems that are not responsive to environmental or plant-based demands and this can lead to over irrigation (Fare, 2014). However, potential restrictions on irrigation and regulations on water quality necessitate that the green industry find alternative ways to manage water without negatively impacting production schedules, or crop or environmental quality (Majsztrik et al., 2011). Irrigation scheduling refers to the amount of water to be applied to a plant, as well as the timing and duration of application (Warren and Bilderback, 2005). Irrigation scheduling has a significant impact on water use efficiency (WUE). Scheduling can be relatively static and arbitrary (timer driven), use environmental models such as evapotranspiration (ET), or be designed to estimate periodic water loss using sensors or physical methods (Jones, 2004; van Iersel et al., 2013). Advanced irrigation scheduling methods have to be developed to address the concerns of the green industry.
Environmental models have been used to estimate water use in container-grown nursery crops by using a modification of the Penman–Monteith equation (Bacci et al., 2008; Beeson and Brooks, 2008; Niu et al., 2006). The models are based on meteorological data and plant-related characteristics such as growth phase, plant height, growth index (GI), canopy coverage, plant/container spacing, container surface area (Beeson, 2004, 2012; Grant et al., 2012; Irmak, 2005). Some major limitations of ET-based scheduling models include the need to determine specific crop coefficients for numerous species and cultivars in production (Beeson, 2005) and for each crop at various growth stages (Niu et al., 2006) and time of year (O’Meara et al., 2013). ET estimates also assume that the crop has access to unlimited water resources, which is often not the case in a container-grown crop (Incrocci et al., 2014; Pardossi et al., 2009).
Another method of irrigation scheduling is to apply the volume of water used in ET each day as calculated relative to container capacity, returning the substrate to container capacity (Warsaw et al., 2009). The most direct method for using estimated ET to schedule irrigation is by weighing containers to assess periodic water loss followed by water replacement to bring a container’s substrate water content back to near container capacity (Million et al., 2010). Substrate moisture sensors have also been used for implementing this type of conservative daily water use (DWU)-based irrigation scheduling for production of several evergreen and deciduous shrubs in the Northern United States (Pershey, 2014) and Hydrangea macrophylla ‘Fasan’ and Gardenia jasminoides ‘Radicans’ in Southern United States locations (O’Meara et al., 2013).
User-defined set points for controlling substrate water content and triggering irrigation via automated irrigation systems have also been used for irrigation scheduling (Nemali and van Iersel, 2006). A sensor-driven automated irrigation study in Hibiscus acetosella ‘Panama Red’ tested threshold water contents ranging from 0.10 to 0.45 cm3·cm−3 and found 0.35 cm3·cm−3 was an optimal set point based on the substantial water savings and acceptable plant growth compared with other volumetric water content (VWC) used in the study (Bayer et al., 2013). By maintaining a constant substrate VWC the irrigation system effectively replaces the water that is lost from the substrate by evaporation, transpiration, or leaching, assuring a constant water supply for the plants. However, the issue of selecting an ideal set point based on both plant and environment demands that controls the timing and volume of irrigation still exists.
A plant-demand-based irrigation system evaluates plant response to environmental changes to predict the amount and timing of irrigation. Photosynthesis is closely linked with stomatal conductance (gS) and it can be influenced by root-to-shoot signaling and both are influenced by leaf water potential. Therefore, photosynthesis has been proposed as a sensitive indicator of plant water status (Fulcher, 2010). An irrigation set point was established that reflected the substrate water content at which photosynthesis began to drop (photosynthetic rate was lowered to 90% of maximum), which corresponded to a reduction in gS. By maintaining substrate moisture content just above this set point, a crop could be produced using 27% less water than the control, and without adversely impacting quality or production time (Fulcher et al., 2012). Development of an irrigation system based on photosynthetic rates would require a smaller data collection to establish irrigation set points and could easily be modified for use with other species. The plant on-demand (OD) irrigation scheduling system has since been employed successfully on a number of crops including Hibiscus rosa-sinensis (Fulcher et al., 2012), oakleaf hydrangea (Hydrangea quercifolia) (Hagen et al., 2014), and redbud (Cercis canadensis) (Nambuthiri et al., 2015a) without incurring a growth or quality “penalty.”
In this OD irrigation system, the amount of water delivered at each irrigation is the same (determined by the set point), but irrigation timing and frequency varies based on plant water use and environmental demand. This is in contrast to the DWU method (Pershey, 2014; Warsaw et al., 2009) where the amount of irrigation will vary based on plant water use but the time irrigation occurs is constant (every 24 h). These two soil moisture sensor (SMS)-based methods of nursery irrigation scheduling were compared in a previous study by producing 1 and 3 gallon oakleaf hydrangea (Hydrangea quercifolia) in outdoor and controlled environments. With the OD treatment, there was generally either no or a positive difference in plant growth and water use was considerably lower in treatments using the OD scheduling method (Hagen et al., 2014). However, oakleaf hydrangea is a relatively high-water-use species, so it is important to monitor the impact of these two conservative irrigation scheduling systems in species with relatively moderate and low water use. Therefore, the objective of this study was to evaluate and compare water use and growth metrics using the OD and DWU irrigation scheduling regimes for two container-grown woody plants that differed in their water use demand under different growing environments.
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