Irrigation Frequency during Container Production Alters Rhododendron Growth, Nutrient Uptake, and Flowering after Transplanting into a Landscape

One deciduous cultivar of Rhododendron L., Gibraltar (AZ), and two evergreen cultivars, P.J.M. Compact (PJM) and English Roseum (ER), were grown in containers for 1 year to determine the effects of irrigation frequency during container production on plant performance the next spring when the plants were transplanted into the landscape. While in the containers, each cultivar was irrigated once or twice daily, using the same amount of water per day, and fertilized with complete nutrient solutions containing 0, 35, 70, or 140 mg·L nitrogen (N). Three months after transplanting into the landscape, nutrient uptake, growth, and flowering were evaluated. In general, the effects of irrigation frequency in containers on performance in the landscape differed between the deciduous cultivar and the evergreen cultivars. In AZ, less frequent irrigation in containers had a pre-conditioning effect that resulted in greater vegetative growth in the landscape but less reproductive growth. In contrast, less frequent irrigation reduced vegetative growth of evergreen cultivars in the landscape and improved flowering. Different growth responses to irrigation frequency between deciduous and evergreen cultivars appeared to be related to differences in timing of nutrient uptake and mobilization. In the deciduous cultivar, less frequent irrigation increased nutrient reserves and improved the ability of the plants to absorb and use nutrients after transplanting, but in the evergreen cultivars, it generally decreased nutrient uptake after transplanting. Less frequent irrigation also altered plant attributes that are important to consumers, including developing a sparser canopy in ER and a fuller canopy in PJM, and producing more but smaller inflorescences in both cultivars. Landscape performance was related to plant nutrition in containers; however, irrigation frequency in containers disrupted relationships between nutrition and performance in all three cultivars. Our results indicate that irrigation frequency during container production of Rhododendron results in a tradeoff between vegetative and reproductive growth the next spring when the plants are in the landscape. Nursery practices for container-grown perennial plants attempt to optimize plant growth and appearance both in the containers and after transplanting in the landscape. Numerous factors influence plant performance after transplanting, including water and nutrient management in the nursery (Arreola et al., 2006; Cabrera and Devereaux, 1999; Cameron et al., 2008; Close et al., 2004; Franco et al., 2006; Sanchez-Blanco et al., 2004). Maintaining available water near 100% container capacity in the growing substrate may optimize plant growth (Beeson, 1992; Fare et al., 1994; Keever and Cobb, 1985; Scagel et al., 2011) but is impracticable in commercial systems (Beeson, 2006) and often leads to inefficient water use and increased nutrient leaching and runoff (Bilderback, 2002). Frequent irrigation can increase nutrient availability and uptake efficiency in containers and reduce the amount of fertilizer required for optimal growth (Raviv et al., 1999; Scagel et al., 2012). Irrigation practices that purposefully result in plant water stress can enhance nursery crop quality by creating plants that are smaller and more able to withstand stressful growing conditions such as those in retail outlets and landscapes, a practice referred to as pre-conditioning (Cameron et al., 2006, 2008). Deficit irrigation regimes where the substrate was allowed to dry to 40% to 75% of container capacity between irrigation events can produce plants with acceptable growth (Beeson, 2006; Koniarski and Matysiak, 2013; Welsh and Zajicek, 1993). However, water limitations during container production may negatively influence nutrient reserves in the plants, which are essential for subsequent plant performance in the landscape (Cabrera and Devereaux, 1999; Scagel et al., 2011, 2012). A combination of reduced irrigation volume and increased irrigation frequency can decrease nutrient and water runoff (Fare et al., 1994). Watering more frequently can also reduce plant water stress when the plants are well fertilized or nutrient deficient (Bi et al., 2007b; Scagel et al., 2007). For example, both high N status and N deficiency increased apparent water stress in containergrown Rhododendron (Scagel et al., 2011) and altering irrigation frequency changed the ability of Rhododendron to absorb nutrients (Scagel et al., 2011, 2012). The effects of irrigation frequency on nutrient uptake in containers therefore have the potential to alter plant performance in the landscape. Knowledge of the potential for irrigation management practices to pre-condition plants in the nursery for future performance is important for managing end-product quality. Nursery plants with a greater ability to function in adverse conditions will have a greater chance of survival and growth after transplanting (Franco et al., 2006; Sanchez-Blanco et al., 2004). Improved irrigation control and monitoring strategies for nursery crops will increase the efficiency of water use in production settings (Lea-Cox et al., 2013). However, more information on how changes in irrigation alter end-product qualities of container-grown nursery plants is needed to implement effective water management. The objective of this study was to determine the effects of irrigation frequency during container production on spring growth, flowering, and nutrient uptake in deciduous and evergreen cultivars of Rhododendron after the plants were transplanted into a landscape. Materials and Methods Two evergreen cultivars of Rhododendron, PJM and ER, and one deciduous cultivar, AZ, were obtained from a commercial nursery as 1-year-old liner stock of clonally propagated plants ( 7 cm · 7-cm cell size). Plants were transplanted on 25 Apr. 2005 into 3.8-L (#1) containers filled with a substrate of bark, sphagnum peatmoss, perlite, vermiculite, dolomitic lime, and gypsum (SB-300; Sun Gro Horticulture, Bellevue, WA) and were grown outdoors in Corvallis, HORTSCIENCE VOL. 49(7) JULY 2014 955 | SOIL MANAGEMENT, FERTILIZATION, AND IRRIGATION