Improper fertilization of residential landscapes can result in water quality degradation because of nutrients [mainly nitrogen and phosphorus (P)] in leachate or runoff. For example, Line et al. (2002) reported that average total N and P export from residential land uses was 256% greater than the exports from wooded sites. Although fertilizer is not the sole contributor to N and P exports from residential landscapes, fertilizer management practices (e.g., application rates, timing of application, and method of application) still need to be monitored more closely to ensure that fertilizer application does not exceed the specific N requirements of growing plants (Line et al., 2002). However, there is limited research available regarding the proper fertilization (e.g., rate, timing, and/or application method) of landscape-grown perennials.
Most of the research focused on fertilizer requirements for perennial plant species was focused on achieving maximum growth and was conducted on containerized plants in a nursery or greenhouse setting (Dubois et al., 2000; Hipp et al., 1988; Macz et al., 2001; Nau, 1996). Nau (1996) reported that rates for producing optimal growth of herbaceous perennial plants during production generally fell within the range of 100 to 200 mg·L−1 of N applied at every irrigation event or 100 to 150 mg·L−1 of N applied weekly, depending on the species. Similarly, Hipp et al. (1988) reported maximum growth of container-grown autumn sage (Salvia greggii) was achieved when N was applied weekly at a rate of 200 mg·L−1 and P was applied biweekly at a rate of 50 mg·L−1; plants receiving 150 mg·L−1 N and 30 mg·L−1 P had comparable growth. The authors concluded that tissue nutrient concentrations of autumn sage should be greater than 2.2% N and 0.20% P to avoid limiting growth. Optimal growth of ‘Margarete’ anemone (Anemone ×hybrida) was achieved when a complete fertilizer was applied three times per week at an N rate of 150 mg·L−1 using a fertilizer with a ratio of 1:2 of ammonium to nitrate (Dubois et al., 2000). In addition, Macz et al. (2001) concluded that the minimum acceptable rate for production of container-grown ‘White Diamond’ chrysanthemum (Dendranthema grandiflora) was 100 mg·L−1 N when additional S was supplied at a rate of 10 mg·L−1.
As was reported for containerized perennials, growth of perennial species generally increased with increasing N rate when grown in the landscape (Harbaugh and Overman, 1984; Wright et al., 2009). Wright et al. (2009) reported that the dry weight of ‘Red Hot Sally’ salvia (Salvia splendens) increased as fertilizer rates increased (from 27 to 42 g, 30 to 35 g, and 100 to 108 g dry weight when fertilized at 0, 1, and 2 lb/1000 ft2 N, respectively, depending on year and mulch treatment); however, fertilizer rate did not affect plant aesthetic quality. Similarly, tuber weight and production index of field grown caladium increased when N was applied at 200 or 300 lb/acre compared with an N rate of 100 lb/acre (Harbaugh and Overman, 1984). Cigar plant (Cuphea ignea), ‘New Gold’ lantana, and ‘Goldsturm’ rudbeckia (Rudbeckia fulgida) increased in size when they were supplied with 4 lb/1000 ft2 N compared with 0, 1, or 2 lb/1000 ft2 N at five months after transplant; however, there were no differences in aesthetic quality of species receiving 1, 2, or 4 lb/1000 ft2 N (Chen et al., 2011).
Even without substantial research to support N rate recommendations for landscape-grown perennials, broad recommendations are available. Rose (1999) surveyed existing extension bulletins related to N fertilization for field- and landscape-grown perennials and reported that N rate recommendations generally fell between 48 and 287 kg·ha−1. However, the authors noted that guidelines for selecting a rate within the range for a particular species were lacking (Rose, 1999). Similarly, Park-Brown (2011) recommended fertilizing perennials in the Florida landscape, but noted that optimal rates and timing will likely vary by species and soil type. The current fertilizer N rate recommendations from the state level extension program, University of Florida—Institute of Food and Agricultural Sciences (UF-IFAS) Florida Friendly Landscaping™, are divided into low (0 to 2 lb/1000 ft2), medium (2 to 4 lb/1000 ft2), and high (4 to 6 lb/1000 ft2) maintenance categories with no guidance available for selecting the appropriate category (Florida Department of Environmental Protection, 2010). Moreover, the current Florida Friendly Landscaping™ fertilizer recommendations were based on research evaluating the response of landscape-grown trees and shrubs to supplemental fertilization (Sartain et al., 2009), but the recommendations are routinely applied to all types of landscape-grown ornamentals (e.g., annuals, perennials, shrubs, etc.). Therefore, it is clear that additional research is needed to evaluate current fertilizer rate recommendations for a broader set of ornamental landscape plant types, such as perennials. The objective of this research was to evaluate plant response of selected landscape-grown herbaceous perennial species to N fertilizer applied at various rates. Results of this research will be used to provide more detailed guidance for the N fertilization of perennials in the landscape. The overall goal is to reduce the potential for fertilizers to be applied in excess of plant needs, which will reduce the likelihood that these nutrients impact water quality because of losses in runoff or leachate.
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