Container-grown woody ornamental plants may not develop sufficient root systems to compensate for evapotranspiration losses if not properly irrigated during establishment in the landscape (Barnett, 1986; Gilman et al., 1996; Montegue et al., 2000). Some species of containerized shrubs can dry out quickly after planting because soil water potentials can reach levels that inhibit root growth within 2 d after planting (Costello and Paul, 1975). Therefore, the imposition of mandatory water restrictions may result in the failure of newly transplanted shrubs and trees to establish adequate root systems, leading to plant death and significant economic losses to nurseries and the landscaping industry.
Factors such as widespread installation of in-ground irrigation systems, rapid population growth, and frequent drought conditions have led to the adoption of mandatory outdoor water restrictions. In Florida, most water use restrictions allow daily irrigation of newly installed landscape plants for 30 to 60 d regardless of plant type (e.g., turfgrass, shrubs, trees, and bedding plants) (South Florida Water Management District, 2008; Southwest Florida Water Management District, 2008; St. Johns River Water Management District, 2008). This 30- to 60-d allowance is shorter than the period required for the successful establishment of trees and shrubs in the landscape. For example, the University of Florida–Institute of Food and Agricultural Sciences suggests that woody plants installed from 11.4-L (# 3) containers should receive supplemental irrigation for 6 to 12 months after planting (Trenholm et al., 2002). A recent study conducted in Apopka, FL (USDA hardiness Zone 9b) indicated that Viburnum odoratissimum Ker-Gawl., Ilex cornuta Lindl. & Paxt. ‘Burfordii Nana’, and Pittosporum tobira [Dryand] ‘Variegata’ grown in a rainout shelter required irrigation for a period of 16 to 20 weeks after planting (WAP) to ensure successful establishment in the landscape (Scheiber et al., 2007).
Several studies have shown that increasing the total volume of irrigation may not increase woody plant growth or quality. For example, Gilman et al. (1998) reported no difference in growth or stem water potential of transplanted Quercus virginiana Mill. when irrigated with 11, 22, or 33 L of water. Similarly, Welsh et al. (1991) reported no growth response of Photinia ×fraseri in response to increased irrigation volume (50%, 75%, or 100% replacement of actual water use). In contrast, increasing irrigation frequency, with a concurrent increase in total irrigation volume, has been shown to affect growth of woody shrubs and trees (Gilman et al., 1996, 2009). Studies examining the effects of irrigation frequency have consistently shown an increase in woody plant growth in response to increased irrigation frequency and volume (Barnett, 1986; Gilman et al., 1996; Marshall and Gilman, 1998; Stabler and Martin, 2000). For example, canopy growth of Ligustrum vulgare L. increased from 0.03 to 0.14 m3 (1.0 to 4.9 ft3) when plants were irrigated every 5 d instead of every 10 d after an initial 28-d period of daily irrigation (Barnett, 1986). Stabler and Martin (2000) reported a similar increase in growth of Caesalpinia pulcherrima L. and Cercidium floridum (Benth. ex A. Gray) S. Watson subsp. florida as irrigation frequency increased. Marshall and Gilman (1998) found an increase in both trunk diameter and height growth and root mass of Acer rubrum L. as irrigation frequency increased. Increased shoot growth as a response to increased irrigation frequency was also reported for Ilex cornuta ‘Burfordii Nana’, but root growth from the bottom of the root ball decreased (Gilman et al., 1996).
Although many studies have shown an increase in plant growth in response to increased irrigation frequency, most of the research in the southeastern United States has focused on trees, whereas shrub research has mainly been conducted in the western United States. There is a need to determine the impact of irrigation frequency on plant survival and aesthetic quality. The specific objective of this study was to determine the effect of irrigation frequency during establishment of V. odoratissimum on canopy growth, root growth, and aesthetic quality in the landscape in three USDA hardiness zones in Florida.
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