Experimental design.

Six native and six non-native species (Table 1) were obtained from a commercial nursery in #3 smooth-sided nursery containers and were planted at three sites in Florida: 1) Plant Science Research and Education Unit located in USDA hardiness zone 8b (Citra, FL; Arredondo sand), 2) Gulf Coast Research and Education Center located in zone 9a (Balm, FL; Zolfo fine sand or Seffner fine sand), and 3) Fort Lauderdale Research and Education Center located in zone 10b (Fort Lauderdale, FL; Margate fine sand). Shrubs appropriate for each site climate were selected from a total of 21 shrub species (10 Florida native and 11 non-native) (Table 1). Six replicates of each shrub species were planted into level plots at each site. Shrubs were installed on the two following planting dates at each site: Dec. 2006 (USDA hardiness zones 8a and 10b) or Dec. 2007 (zone 9a), and June 2007 (zones 8a, 9a, and 10b) as outlined in Shober et al. (2009). Shrubs were grouped in the field by planting date. The average minimum temperature at 2 m above the soil surface collected for each 52-week planting period from the Florida Automated Weather Network stations located within 50 m of the planting sites was as follows: 21.81 and 38.20 °F in zones 8a and 10b, respectively (Dec. 2007); 25.24, 27.35, and 37.21 °F in zones 8a, 9a, and 10b, respectively (June 2007); and 27.35 °F in zone 9a (Dec. 2007).

Table 1.Ten Florida native and 11 non-native species planted in U.S. Department of Agriculture hardiness zones 8b, 9a, and 10b to evaluate a reduced irrigation frequency scheme for establishment of shrubs after transplant into the landscape.

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Irrigation treatments were applied, regardless of rainfall events, to six plant replicates at each location for each planting date with the following frequencies: every 2 d in hardiness zone 10b and every 4 d in zones 8b and 9a. Irrigation treatments were selected based on recommendations for vigor that were developed by evaluating irrigation frequencies for burford holly and pittosporum grown in zones 8b and 9a (Wiese et al., 2009), for wild coffee, copperleaf, and orange jasmine grown in zone 10b (Moore et al., 2009), and for sweet viburnum grown in all three zones (Shober et al., 2009). Each shrub was irrigated with 3 L of water per irrigation event applied to the root ball using three bubbler emitters (Shrubbler® 360°; Antelco®, Longwood, FL). Each emitter was mounted 4 inches above ground level with an emitter located on the east and west side of each plant, 6 inches from the outside of the root ball, and the third emitter was positioned on the root ball. Irrigation frequencies were controlled as separate zones as described by Shober et al. (2009). Irrigation was ended 20 weeks after transplant, after which time supplemental irrigation (3 L per plant by hand) was supplied to all shrubs for a specific planting date over the 2-year post-planting period when signs of severe water stress, such as severe foliage wilting, were observed.

Plant quality and growth index.

Plant quality (plant density and dieback) was visually rated on a scale of 1 (dead plant) to 9 (dense, full canopy with no dieback) at 12, 20, 28, and 52 weeks after transplant (WAT). Growth index (GI) was used as a quantitative indicator of plant growth. GI in cubic meters for each plant was calculated as: GI = H × W1 × W2, where H is the plant height (meters), W1 is the widest width of the plant (meters), and W2 is the width perpendicular to the widest width (meters). Growth index was measured on three plant replicates per treatment at 0 (date of planting), 12, 20, 28, and 52 WAT.

Root-to-canopy spread ratio.

Root spread was measured on three plant replicates per irrigation frequency at 12, 20, 28, and 52 WAT by excavating the longest root (near the soil surface) on the east and west side of the plant and measuring its length from the base of the plant. Root spread radius was calculated as the mean of the east and west root lengths. Root-to-canopy spread ratio was then calculated as: root spread radius/mean canopy radius with mean canopy radius = 1/4 × (W1 + W2).

Data analysis.

Experimental design and data structure required that statistical analysis of growth and quality data be limited to descriptive statistics (mean, median, etc.). Therefore, median values were used to describe the effect of the selected irrigation treatment on plant quality ratings and mean values were used to describe the effects of the selected irrigation treatment on canopy and root growth for shrub species planted in each hardiness zone.

Plant quality and growth index.

All plants, with the exception of golden dewdrop (Duranta erecta ‘Sapphire’) in zone 8b and gardenia (Gardenia jasminoides ‘Mystery’) in zone 9a, survived and were actively growing 52 WAT when watered with 3 L of water every 4 d (zones 8b and 9a) or every 2 d (zone 10b) for 20 WAT (Tables 2 and 3). In addition, most shrub species maintained high plant quality ratings (i.e., density and dieback) (Table 2). Poor performance of some species was not related to water stress. For example, gardenia planted in zone 9a was plagued by nutrient deficiencies that were related to pockets of alkaline soil (pH near 8.0) in the field (Tables 2 and 3). In addition, some species [firebush (Hamelia patens) in zones 8b and 9a, golden dewdrop in zone 8b] suffered a decline in growth and quality due to freeze damage.

Table 2.Median visual density and dieback (quality) ratings (n = 6) for selected Florida native and non-native shrub species established in the landscape under low-volume [3 L (0.8 gal) per plant] irrigation over two planting dates in U.S. Department of Agriculture hardiness zones 8b, 9a, and 10b.

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Table 3.Mean growth index and root to canopy spread ratio (n = 6) for selected Florida native and non-native shrub species established in the landscape under low-volume irrigation [3 L (0.8 gal) per plant] over two planting dates in U.S. Department of Agriculture hardiness zones 8b, 9a, and 10b.

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Our data also suggest that there were no differences in canopy growth or aesthetic quality of native versus non-native shrubs at any of the planting locations (Tables 2 and 3). Our results are similar to those of Scheiber et al. (2008), who found that neither native species nor non-native species were, as a group, more responsive to irrigation. Scheiber et al. (2008) reported that growth of two of 10 Florida natives and two of 10 non-natives increased when irrigated (daily for 17 d, then every 2 d for 7 weeks, then every 7 d). Enhanced growth of irrigated shrubs seemed to be associated with species habitat preference because all species that increased in growth as a response to irrigation were endemic to wet habitats (e.g., swamps and streams) (Scheiber et al., 2008).

Root spread-to-canopy spread ratio.

Most shrub species evaluated, regardless of geographic origin, approached a root spread-to-canopy spread ratio of 1.0 by 28 WAT and had exceeded 1.0 by 52 WAT. The majority of shrubs planted in zones 8b and 10b had reached a ratio of 1.0 by 20 WAT, which corresponds to the time when irrigation was discontinued. Root spread-to-canopy spread ratio of tree species has been reported to be species dependent (Gilman and Kane, 1991; Kummerow et al., 1977; Rogers, 1934; Rogers and Vyvyan, 1934), but roots generally extended two to three times the distance from trunk to drip line on established nursery-grown trees (Rogers, 1934; Rogers and Vyvyan, 1934) and 1.7 to 3.7 times the drip line for shrubs (Gilman and Kane, 1991). Shrubs may be considered fully established when the root spread-to-canopy spread ratio has stabilized (Gilman and Kane, 1991). Root spread-to-canopy spread ratio of all shrubs evaluated continued to increase throughout the 52-week period of study, which suggests that shrubs, although not fully established, were able to compensate for evapotranspiration losses without additional irrigation under normal or greater rainfall conditions.

Results of our study show that additional shrub species can be established in the predominantly sandy soils of the Florida landscape in USDA hardiness zones 8b, 9a, and 10b when following the irrigation recommendations for plant vigor published by Shober et al. (2009), Moore et al. (2009), and Wiese et al. (2009). A total of 21 commonly planted native and non-native landscape shrubs was established using these procedures. Despite the public perception of an increased drought tolerance of native shrubs, our research suggests that there were no differences in irrigation needs based solely on native range. This suggests that it may be appropriate to adopt the 20-week low-volume irrigation recommendations for the establishment of a wide variety of Florida native and non-native shrubs. The ability to adopt the 20-week low-volume irrigation recommendations for native and non-native landscape shrubs will provide the industry with greater choice of plant material, while continuing to conserve valuable water resources. However, Florida native and non-native shrubs should be monitored for symptoms of drought stress for 2 years after planting.