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The objectives of this study were to quantify irrigation volume, runoff volume and nutrient content, and plant growth of container-grown conifers when irrigated based on plant daily water use (DWU) vs. a standard irrigation rate. Four conifer taxa were grown in 10.2-L (no. 3) containers subjected to four irrigation treatments from 23 June to 16 Oct. 2009 and 6 June to 31 Oct. 2010. The taxa were: 1) Chamaecyparis obtusa Sieb. & Zucc. ‘Filicoides’, 2) Chamaecyparis pisifera (Sieb. & Zucc.) Endl. ‘Sungold’, 3) Thuja occidentalis L. ‘Holmstrup’, and 4) Thuja plicata D. Donn ‘Zebrina’. The four irrigation treatments were: 1) control application of 19 mm·d, 2) irrigation applied to replace 100% DWU (100 DWU) per day, 3) applications alternating 100% with 75% DWU in a 2-day cycle (100–75 DWU), and 4) a 3-day application cycle replacing 100% DWU the first day and 75% DWU on the second and third days (100–75–75 DWU). Irrigation treatments did not affect plant growth index {GI= [(H +WNS +WEW)/3]} in 2009. In 2010, GI of C. obtusa ‘Filicoides’ was greater for 100 DWU than the control plants. Seasonal total water applied for 100, 100–75, and 100–75–75 DWU was 22%, 32%, and 56% less, respectively, than the control amount of 117 L per container in 2009 (114 days) and 24%, 18%, and 24% less than the control amount of 165 L per container in 2010 (147 days). Scheduling irrigation based on DWU reduced runoff volumes and (nitratenitrogen) NO3 -N and (phosphate-phosphorous) PO4 -P load compared with the control. Irrigating based on DWU reduced water application and runoff volumes and NO3 -N and PO4 -P load while producing plants of equal or greater size than control plants. Producers must irrigate container-grown nursery plants frequently, often multiple times daily, because of container volume limitations and substrate properties. This increases water extraction demands and places nurseries in competition for water resources with population centers, industry, and other agricultural sectors (Beeson et al., 2004; O’Neill and Dobrowolski, 2011). Since 1992, annual water applications to container nursery stock in Florida (mean annual rainfall = 1100 mm) have been capped at 2290 mm, this amount was further limited to 1830 mm in 2003 for nurseries in direct competition with municipalities for potable water (Beeson, 2004). Nursery water withdrawal policies are also in place in California, Oregon, North Carolina, and Texas (Beeson et al., 2004). One method to aid nurseries in conserving water is to schedule irrigation in response to plant water use. Warsaw et al. (2009a) showed that irrigating based on DWU reduced water applications between 6% and 75%, depending on taxa, compared with a 19 mm·d control without negatively impacting growth of over 20 ornamental shrubs. From a management perspective, scheduling irrigation based on plant water use is more practical when the process is automated. Set point irrigation using automated irrigation systems have been shown to result in plants of similar or greater size when grown at higher substrate volumetric water content (q), typically greater than 0.40 m·m, comparedwith those grown below 0.30m·m (Bayer et al., 2015; Nemali and van Iersel, 2006). Using a similar irrigation system that monitored q using time-domain reflectometry (TDR) sensors, Cornejo et al. (2005) improved irrigation application efficiency 22% compared with a timer-controlled irrigation system. In addition to concerns about the quantity of water used, runoff leaving nurseries may contain NO3 -N and PO4 -P, among other contaminants, at concentrations as high as 8 and 5 mg·L, respectively (Sharma and Bolques, 2007). Although those NO3 -N levels do not exceed the U.S. environmental protection agency (EPA) safe drinking water threshold (10 mg·L), the Michigan Department of Environmental Quality reported that where NO3 -N is already present, just 0.3 mg·L of PO4 -P may promote cyanobacteria blooms that can damage sensitive ecosystems (Anonymous, 2008). In addition to regulating concentration, total maximum daily load (TMDL) standards have been developed on a case-specific basis to set a maximum rate at which nutrients can enter a water body (EPA, 2011). The total average annual 10-year loads of N and P entering the Chesapeake Bay are 1.6 and 1.5 times higher than the 187 and 18 million pound caps for N and P (CBF, 2010). As a result, since 2002, the state of Maryland requires agricultural operations to draft management plans for N and P (Lea-Cox et al., 2001). In Florida, agriculture accounts for 98% of all P imports into the Lake Okeechobee watershed, where a P TMDL of 40 mg·L places agricultural nonpoint sources under intense pressure (FDEP, 2001). Nutrient TMDLs have been implemented in Michigan for P levels in 12 watersheds, although few nurseries are affected in this instance (Anonymous, 2011). Research has shown decreased nutrient loading with DWU-based irrigation management. Warsaw et al. (2009b) found total quantities of NO3 -N in runoff reduced by 38% and 59% in 100% and 75% DWU applications compared with their control. The objectives of this project were to study the effects of four DWU-based irrigation treatments on plant performance and runoff water volume and quality. Plant GI and dry weight, DWU, runoff water volume and NO3 -N and PO4 -P concentrations and loads were determined to compare the irrigation treatments. Materials and Methods Site description. The Michigan State University Horticulture Teaching and Research Center (HTRC), Holt, MI, is located at lat. 42.67 N, long. –84.48 W, and elevation of 264 m. Plants were grown on 3 · 6 m nursery production beds covered with 6-mm impermeable high density polyethylene plastic and overlain by polypropylene woven permeable landscape fabric. The beds were oriented east to west along the long axis, sloped to the center and westward, and channeled runoff water into a collection basin positioned below grade on the west end of each bed. Rectangular basin frames were made of wood Received for publication 9 July 2015. Accepted for publication 10 Aug. 2015. This project was supported by the USDA National Institute of Food and Agriculture, hatch project number MICL02010. We appreciate the financial support provided by MichiganStateUniversityProjectGREEEN,Michigan Department of Agriculture Horticulture Fund, and the in-kind support of plants, substrate and fertilizer from Spring Meadow Nursery, Inc., Renewed Earth LLC, andHarrell’s LLC.We also like to thank Aaron Warsaw, Glenn Jarrell, and Max Braun for their assistance on this project. Corresponding author. E-mail: fernan15@msu.edu. HORTSCIENCE VOL. 50(10) OCTOBER 2015 1553 | SOIL MANAGEMENT, FERTILIZATION, AND IRRIGATION

Irrigating Based on Daily Water Use Reduces Nursery Runoff Volume and Nutrient Load Without Reducing Growth of Four Conifers