Native plants for ecosystem restoration are commonly grown in containers in greenhouses. Within greenhouses, overhead irrigation is the most widely used system to irrigate plants (Leskovar, 1998). Overhead irrigation systems are chosen for their simplicity, low cost, and for reducing fertilizer salt buildup, which can be detrimental to plant growth (Argo and Biernbaum, 1995; Biernbaum, 1992; Molitor, 1990).
Unfortunately, overhead irrigation also has potential negative attributes, namely wasted water and the nutrients with it that may harm the environment. Dumroese et al. (1995) found that between 49% and 72% of water applied to a native plant seedling crop using an overhead boom irrigation system was discharged from the nursery. Moreover, such inefficient irrigation systems may pose an unnecessary high cost to growers where quality water is limited. Some states, including California and Arizona, impose restrictions on water use during dry seasons, which can further increase the need for water conservation in nurseries (Oka, 1993).
Waste water discharged from nurseries presents a significant threat to ground and surface water; the primary concern involves the release of nutrients resulting from regular use of water-soluble fertilizers. Because the rate of fertilizer application is higher in greenhouse production than many other forms of agriculture (Molitor, 1990), nitrate and phosphate runoff from greenhouses may contaminate water resources (Biernbaum, 1992; Juntenen et al., 2002). In a leaching study during conifer seedling production, 11% to 19% of applied nitrogen (N) and 16% to 64% of applied phosphorus (P) were recovered in collected leachate (Juntenen et al., 2002). Similarly, 46% to 65% of applied N was recovered in collected leachate (as NO3-N) for overhead irrigation experiments on Ilex crenata Thumb. ‘Compacta’ (Fare et al., 1994). The continuous effect of high nutrient leaching may become a problem over time. Very high N levels may accumulate and persist under commercial greenhouses (McAvoy et al., 1992; Molitor, 1990) threatening groundwater quality. Consequently, discharges may be legally regulated in the future; such restrictions already exist in Oregon (Grey, 1991).
Subirrigation, instead of overhead irrigation, has potential to reduce water use and chemical runoff from nurseries while improving crop uniformity and reducing labor (Uva et al., 1998). This closed system works by permitting water to move from a reservoir tank to an application tray, where water then moves through the growing medium by capillary action (Coggeshall and Van Sambeek, 2001). Once irrigation is complete, any unused water drains into the holding reservoir for later recirculation through the system. Dumroese et al. (2006) demonstrated a 56% water savings over conventional overhead irrigation for Metrosideros polymorpha Gaud., whereas Ahmed et al. (2000) showed a water savings of 86% for food crops.
Subirrigation may improve crop uniformity because plants have access to equal amounts of water thereby reducing or eliminating the edge effect (Neal, 1989). Part of this improvement is because subirrigation avoids problems with canopy interception and redistribution from overhead irrigation systems. In container nurseries, as leaf area and density increase, irrigation application efficiency decreases (Beeson and Knox, 1991). Consequently, container size may also affect irrigation efficiency, e.g., small container sizes at high densities combined with large leaf areas will likely cause a decrease in efficiency. Along the same lines, studies characterizing water use have found improved efficiency in subirrigation versus overhead irrigation systems (Morvant et al., 2001; Santamaria et al., 2003), assuming tanks are not emptied and refilled regularly. Similarly, because no nutrients are lost from the system, nutrient use efficiency has also been shown to be similar or better in subirrigation systems, especially when combined with controlled-release fertilizer (Morvant et al., 2001; Richards and Reed, 2004). Further benefits may include improved growth and flowering, as Yeh et al. (2004) saw with forbs.
Despite the numerous advantages of subirrigation, potential concern exists. One concern is the accumulation of salts within the upper portion of the growing medium profile, especially under increasing fertilizer concentration regimes (Kent and Reed, 1996; Richards and Reed, 2004; Todd and Reed, 1998). Depending on species, these levels may or may not pose problems, including salt burn, reduced growth, and interference with mineral nutrient uptake (Dumroese et al., 2007); Scoggins (2005) summarizes acceptable electrical conductivity (EC) ranges for several herbaceous perennials. In situations where high salinity levels become problematic, leaching through overhead irrigation may help (Todd and Reed, 1998).
Although a number of studies have highlighted the effects of subirrigation and controlled-release fertilizer, few have specifically addressed the potential for native plant production. This study was undertaken to gain a better understanding of subirrigation as a viable and environmentally conscious alternative for native plant propagation. Our study objectives were to quantify the effects of irrigation and container size on plant height, biomass, and survival; nutrient use and efficiency; and fertilizer leaching.
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