Excess nutrient loading to water bodies from urban and agricultural landscapes can accelerate eutrophication, which results in excessive algae growth, death of fish and other aquatic species, and degradation of overall water quality (Howarth, 1988). Burkholder et al. (1992) reported surface-water quality degradation at nitrate (NO3−) levels as low as 0.05 to 0.1 mg·L−1. Excessive or poorly timed fertilization of residential landscapes can result in water quality degradation as nutrients, particularly nitrogen (N) and phosphorus (P), are lost in leachate or runoff. For example, Line et al. (2002) reported that the average total N and P exported from a residential setting was 269% and 302%, respectively, greater than from wooded sites. Although fertilizer is not the sole contributor to N exports from residential landscapes it is the most direct and deliberate addition of these nutrients to the urban ecosystem. As such, improving the fertilizer management practices (e.g., application rates, timing of application, and method of application) of consumers and the commercial green industry is an important step in reducing nonpoint nutrient losses from urban landscapes.
Nationwide, state and local lawmakers have addressed environmental concerns surrounding fertilization of urban landscapes by passing a range of season- and/or formulation- specific fertilization bans (mainly for turfgrass) in areas with impaired water bodies that are linked to watersheds that have elevated nutrient loads (e.g., Chesapeake Bay, Gulf of Mexico delta-Mississippi River basin, Florida Everglades). In 2007, the Florida Legislature appointed the Florida Department of Agriculture and Consumer Services to create the Florida Consumer Fertilizer Task Force, which helped to develop recommendations for statewide policies and programs regarding consumer fertilizer use (Hartman et al., 2008). As Florida’s state agencies worked to implement plans for the statewide protection of surface and groundwater, many local governments began to implement their own preventive measures via county and city-wide fertilizer ordinances. As a result of these local actions, several counties and municipalities in the Tampa Bay region (and other areas of Florida) adopted a summer fertilization blackout period with the goal of decreasing nutrient losses during the rainy season (i.e., 1 June to 30 Sept. in Manatee County, FL; Manatee County Florida, 2012). However, the fertilizer blackout period coincides with the period of active plant growth in Florida. The full impact of these fertilizer blackout periods on plant growth and the environment has yet to be evaluated; scientific studies on this topic are limited in the literature.
Beyond directly limiting fertilization, many industry and government organizations have created standards or best management practices (BMPs) that help guide landscape maintenance efforts. For example, the American National Standard Institute’s (ANSI) A300 Tree Care Standard for Fertilization and the Florida-friendly Landscaping™ Green Industries BMP manual (American National Standards Institute, 2011; Florida Department of Environmental Protection, 2010) provide standard recommended fertilizer application rates and other guidelines for maintaining woody ornamentals. Industry consensus and limited research serve as the basis for these published application rates (Shober et al., 2010). Shober et al. (2013) found that annual N application rates of 98 to 195 kg·ha−1 were sufficient to maintain acceptable plant growth [i.e., volume or size index (SI), chlorophyll content, and dry weight] and visual quality of several woody ornamental landscape plant species grown in Florida. Similarly, Werner and Jull (2009) reported annual N application rates of 49 to 146 kg·ha−1 were able to support the healthy growth of young common hackberry (Celtis occidentalis L.) trees in Wisconsin. Both researchers reported optimal fertilizer application rates within the ANSI’s standard recommendation rate range of 49 to 195 kg·ha−1 annually for woody ornamentals (American National Standards Institute, 2011).
Previous research on fertilization of woody ornamentals in the landscape has focused on root growth (Struve, 2002), shoot growth (Gilman et al., 2000; Struve, 2002), and/or aesthetic plant response (Shober et al., 2013, 2014) to fertilizer applications. Yet, few studies have looked at the environmental impacts when applying fertilizers at rates recommended to optimize shoot growth and aesthetics (Rose, 1999; Shober et al., 2010). Among the studies that evaluate the potential for nutrient losses when fertilizing woody ornamentals, Qin et al. (2013) noted that nutrient losses from urban landscapes (containing turfgrass and ornamentals) were reduced when established woody ornamentals were included in fertilized Florida landscapes. In contrast, Erickson et al. (2008) reported that nutrient losses from mixed ornamental plant beds (once fully established) were similar to those from turfgrass monoculture. Our research expands on previously published work to determine the impacts of application rate, method, and timing on woody ornamental health and N leaching from urban residential planting beds. The overall goal of this study was to refine current fertilization standards and BMPs for woody ornamentals by identifying fertilizer rates, application methods, and timing recommendations that account for both the aesthetics of the ornamentals and the potential for nutrient losses to the surrounding environment.
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