Availability of freshwater worldwide will likely continue to decrease as a result of a growing global population, climate change, and increased agricultural and industrial water demand (Jury and Vaux, 2005). Recent drought in the southeastern United States resulted in more than $1 billion in crop losses and strained the water supply (Seager et al., 2009). An increasing number of laws and regulations limiting the amount of water available for use in container nurseries has been implemented in states such as California, Florida, North Carolina, Texas, and Oregon (Beeson et al., 2004). Management of runoff from container nurseries is also facing stricter regulations in areas such as the Chesapeake Bay Watershed and California in efforts to comply with the Clean Water Act (Lea-Cox and Ross, 2001). This has brought attention to the need to understand what impacts water shortages will have on horticultural operations. Adapting to decreasing water supplies will also demand an understanding of the effect of water stress on plant growth and development.
The need to improve irrigation practices and control runoff in container nurseries is well known. Fare et al. (1992) found that irrigation volumes in container nurseries in Alabama varied greatly. Growers surveyed thought they were applying water at a rate of 2.54 cm·h−1, but actual rates ranged from 0.8 to 3.2 cm·h−1. Measured irrigation volumes varied as a result of changing container spacing, canopy interference, overwintering structures, and inefficiencies in irrigation systems. Inefficient and excessive irrigation leads to increased runoff of water and nutrients (Million et al., 2007). Best management practices have been adopted by many nurseries to use water resources more sustainably (Chappell et al., 2012). These methods are beneficial for nursery growers concerned with reducing water use and controlling runoff; however, these irrigation practices are not based on plant water needs. A combination of these methods, along with more sophisticated soil moisture sensor-based irrigation systems that can irrigate based on plant water use, has the potential to reduce the amount of water needed for irrigation and the amount of runoff produced during and immediately after an irrigation event (Wells et al., 2011).
Capacitance soil moisture sensors can accurately measure θ in peat- and bark-based substrates (van Iersel et al., 2009, 2010) indicating potential for use in container nurseries and greenhouses (Majsztrik et al., 2011). The potential of various soil moisture sensors to monitor and/or control substrate/soil water content has been examined in greenhouse and nursery settings with a variety of species, ranging from woody species such as Acer rubrum and Cornus florida (Lea-Cox et al., 2008a), Rhododendron spp. (Lea-Cox et al., 2008b), and Hydrangea (van Iersel et al., 2009) to herbaceous species such as Petunia ×hybrida (van Iersel et al., 2010) and Antirrhinum spp. (Lea-Cox et al., 2009). van Iersel et al. (2009) observed a water savings of 83% using sensor-controlled irrigation compared with the regular nursery irrigation practices in a commercial nursery setting. Through these studies, a better understanding of the potential use of soil moisture sensors as well as a better understanding of plant water requirements is being developed.
Increased monitoring of irrigation applications is necessary to compensate for changing conditions in nurseries and to ensure irrigation is being applied uniformly and efficiently (Fare et al., 1992). Plant size has been found to be a determining factor in plant water use (Knox, 1989) and water use generally increases with plant age (Million et al., 2007). Research has related plant water use to plant growth index and pan evaporation (Knox, 1989) or growth index and leaf area (Nui et al., 2006), which were shown to be good descriptors of water use for multiple species. Maintenance of specific leaching fractions has also been used to adjust irrigation in response to increasing plant size (Owen et al., 2008). However, these methods do not adjust to day-to-day changes in weather. To more effectively reduce water use without reducing plant quality, the relationship between plant growth and substrate θ needs to be quantified. Substrate θ can be monitored by growers, allowing them to adjust irrigation in real time to adapt to current plant water needs.
Our research compared growth and water use of Hibiscus acetosella ‘Panama Red’ maintained at various substrate θ levels using soil moisture sensor-controlled, automated irrigation. The objectives of this project were 1) to quantify the water use of Hibiscus acetosella ‘Panama Red’ in both a controlled greenhouse setting and outdoor nursery settings; 2) to determine which environmental conditions most strongly affect day-to-day changes in water use; and 3) to describe how growth of Hibiscus acetosella ‘Panama Red’ is affected by θ.
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