Post-transplant Irrigation Scheduling for Two Native Deciduous Shrub Taxa

in HortScience

The effect of five irrigation scheduling treatments on shoot growth [growth index (GI)] and stem water potential (SWP) of Itea virginica L. ‘Henry's Garnet’ (‘Henry's Garnet’ sweetspire) and Rhododendron austrinum Rehd. (Florida flame azalea) were studied. Plants were transplanted on 13 Mar. 2008 at soil grade level under shade structures in field plots of sandy loam soil on the Auburn University campus in Auburn, AL. Matric potential was continuously measured 7.6 cm from the stem in the root ball and 20.3 cm from the stem in the soil backfill for three plants per treatment per taxa. Irrigation scheduling treatments included (in order of decreasing irrigation frequency): root ball and surrounding soil matric potential maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential dropped to either –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential dropped to either –25 kPa (25S) or –50 kPa (50S). In both taxa, GI increased linearly over time in all five irrigation treatments. For I. virginica ‘Henry's Garnet’, GI increased most in WW and 25S treatments followed by 50S, 50RB, and 75RB. Shoot growth of R. austrinum was similar among treatments. Both I. virginica ‘Henry's Garnet’ and R. austrinum had a larger increase in GI during the first growing season (2008). For I. virginica ‘Henry's Garnet’, SWP was higher in 50S and 75RB treatments than in 50RB, WW, and 25S. For R. austrinum, SWP was not different among treatments. Results indicate that although plant growth might be diminished slightly, irrigation frequency can be reduced without compromising plant visual quality or survival if root ball and soil matric potential is monitored. Additionally, until roots grow into the backfill soil, monitoring both backfill soil and root ball matric potential is important for scheduling and reducing post-transplant irrigation applications.

Abstract

The effect of five irrigation scheduling treatments on shoot growth [growth index (GI)] and stem water potential (SWP) of Itea virginica L. ‘Henry's Garnet’ (‘Henry's Garnet’ sweetspire) and Rhododendron austrinum Rehd. (Florida flame azalea) were studied. Plants were transplanted on 13 Mar. 2008 at soil grade level under shade structures in field plots of sandy loam soil on the Auburn University campus in Auburn, AL. Matric potential was continuously measured 7.6 cm from the stem in the root ball and 20.3 cm from the stem in the soil backfill for three plants per treatment per taxa. Irrigation scheduling treatments included (in order of decreasing irrigation frequency): root ball and surrounding soil matric potential maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential dropped to either –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential dropped to either –25 kPa (25S) or –50 kPa (50S). In both taxa, GI increased linearly over time in all five irrigation treatments. For I. virginica ‘Henry's Garnet’, GI increased most in WW and 25S treatments followed by 50S, 50RB, and 75RB. Shoot growth of R. austrinum was similar among treatments. Both I. virginica ‘Henry's Garnet’ and R. austrinum had a larger increase in GI during the first growing season (2008). For I. virginica ‘Henry's Garnet’, SWP was higher in 50S and 75RB treatments than in 50RB, WW, and 25S. For R. austrinum, SWP was not different among treatments. Results indicate that although plant growth might be diminished slightly, irrigation frequency can be reduced without compromising plant visual quality or survival if root ball and soil matric potential is monitored. Additionally, until roots grow into the backfill soil, monitoring both backfill soil and root ball matric potential is important for scheduling and reducing post-transplant irrigation applications.

Water is considered the most limiting factor in newly transplanted container plants (Costello and Paul, 1975; Scheiber et al., 2007), and the most common cause of death among recently transplanted container-grown plants is water stress (Costello and Paul, 1975; Nelms and Spomer, 1983). This could be attributed to water loss from the original root ball as a result of absorption by roots, evaporation from the soil surface, and movement of water from the root ball into the backfilled soil (Costello and Paul, 1975; Nelms and Spomer, 1983). Moisture in the root ball can be very different from the surrounding soil, and in fact, the root ball may experience drought conditions even when soil is well-watered (Costello and Paul, 1975; Nelms and Spomer, 1983). Water in the surrounding soil does not tend to move into the root ball; thus, until established, water applied to the root ball is critical. In previous research, irrigation outside the original root ball did not aid in rapid establishment of transplanted trees (Gilman et al., 1998). Instead, increasing irrigation frequency was more effective than applying larger volumes of water infrequently to transplanted trees. There appears to be a maximum volume of irrigation water needed for plant functions and, if applied correctly, above which there is no added benefit to the plant (Gilman et al., 1998). This implies that once that volume of water is applied, adding more can be wasteful.

Scheduling landscape irrigation based primarily on daytime air temperature and the number of days of precipitation is not well correlated with plant water needs (Qualls et al., 2001). Instead, monitoring soil moisture is a technological approach that can more accurately quantify plant water use by measuring the rate of drying of the backfill soil and transplanted root ball. Installing soil moisture sensors in both the soil backfill and the transplanted root ball could be used to efficiently schedule and control irrigation, thereby reducing water use and lowering direct costs associated with irrigation (Dukes et al., 2005; Qualls et al., 2001). The objective of this research was to determine the effect of scheduling irrigation application based on volumetric water content in either the backfill soil or root ball on establishment and growth of two native shrub taxa.

MATERIALS AND METHODS

Plant taxa used in this experiment included Rhododendron austrinum Rehd. (Florida flame azalea) and Itea virginica L. ‘Henry's Garnet’ (‘Henry's Garnet’ sweetspire) in 11.4-L containers. Plants of I. virginica ‘Henry's Garnet’ were obtained from Greene Hill Nursery, Inc. (Lee County, Waverly, AL) in Mar. 2008. These plants were propagated from cuttings from existing nursery stock in 2007 and produced in containers using 9:1 pine bark:sand substrate. Plants of R. austrinum were obtained from Moore & Davis Nursery LLC in (Macon County, Shorter, AL) in Feb. 2008. These plants were propagated during Summer 2006 from cuttings from existing nursery stock and produced in containers in a 9:1 pine bark:sand substrate. All plants were held in an unheated retractable roof greenhouse at Auburn University (Auburn, AL) until installed.

On 13 Mar. 2008, all plants were planted 1.2 m on center in two 6.1 × 7.6-m field plots (one taxa per plot) on the Auburn University campus in Auburn, AL [Maryvn sandy loam (fine-loamy, kaolinitic typic Kanhapludult)]. Each plot was surrounded by a large metal frame structure 30.5 m tall that was covered on the tops and sides with woven shadecloth (Cassco Associates, Montgomery, AL). Plants were planted at soil grade level in holes twice the width of the root ball (root ball diameter was 28 cm; planting hole diameter was 56 cm) with soil used to create a berm around the plant beginning at the outer edge of the root ball. Plants of I. virginica ‘Henry's Garnet’ were planted under 30% shade, and plants of R. austrinum were planted under 47% shade. Initial soil tests (Auburn University Soils Testing Laboratory, Auburn, AL) did not indicate the need for any addition of fertilizer to the soil. A 7.6-cm layer of pine straw (Pinus taeda L., loblolly pine) mulch was applied to the ground between plants and rows. A fresh layer of pine straw 7.6 cm was added again in Sept. 2008 and Apr. 2009. Weed control was by hand-weeding around plants and by herbicide (glyphosate) between rows.

Five irrigation scheduling treatments were assigned in a randomized complete block design within each of two plots with five blocks per taxa. Plants were irrigated with overhead irrigation (#4 Nozzle mini-Wobbler®; Senninger Irrigation, Inc., Clermont, FL) three times per week with 2.5 cm water until treatments began on 1 Apr. 2008 [19 d after planting (DAP)]. Two rain gauges were installed in each plot (under shade); within a plot, gauges were equidistant from each other and the corners along one diagonal of the square plot. Rain gauge dimensions were 60 cm high × 4.5 cm deep with a 9.5-cm deep opening on the top; gauges were attached to a wooden stake that was in the ground. Dates and amounts of precipitation events throughout the experiment were recorded. No precipitation occurred during Apr. and May 2009. Watermark® soil moisture sensors (Model 900M; Irrometer Company, Inc., Riverside, CA) were installed on 20 Mar. 2008 to measure soil and root ball matric potential. Sensors were installed 7.6 cm from the stem in the root ball (pine bark-based substrate) and 20.3 cm from the stem in the surrounding soil on three plants per treatment within each taxa. Sensors were installed so that the top of the sensor was 5 cm below the root ball substrate or soil surface. Sensors were calibrated before installation for the root ball substrate or the soil (depending on installation location) using the manufacturer's calibration protocol. Matric potential was measured every 4 h using Watermark® Monitor data loggers. Data loggers were checked every morning at 0800 hr. The five irrigation treatments included: 1) root ball and surrounding soil maintained at or above –25 kPa (WW); 2) root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB); 3) root ball and surrounding soil rewatered when root ball matric potential reached –75 kPa (75RB); 4) root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S); and 5) root ball and surrounding soil rewatered when surrounding soil matric potential reached –50 kPa (50S). When rewatered, plants received 7.4 L water per plant, which was uniformly applied (by hand) from the stem outward to a radius of 30.5 cm from the stem (equivalent to 2.5 cm water applied to that area).

Shoot GI [(widest width + width perpendicular + height)/3] was measured for each plant at the beginning of the experiment (initial GI), at the end of the first growing season (7 Oct. 2008; Fall 2008; 208 DAP), at the start of the second growing season (17 Feb. 2009; Spring 2009; 341 DAP), and at experiment termination (final GI; 30 June 2009; Summer 2009; 474 DAP). To track the total change in GI since experiment initiation, cumulative relative growth index (RGI) was calculated at each measurement date: [(GIx – initial GI)/initial GI] where GIx = GI at that particular measurement date. To quantify how GI changed from one measurement date to the next, seasonal RGI was calculated to reflect change in GI since the last measurement: [(GIx – GIx-1)/GIx-1] where GIx = GI at that particular measurement date and GIx-1 = GI at the previous measurement date. Cumulative GI (cumulative GI/DAP) was calculated to reflect cumulative change in GI per day over the course of the experiment: [(GIx – initial GI)/DAPx] where GIx and DAPx are GI and DAP at that particular measurement date. Seasonal GI (seasonal GI/DAP) was calculated to reflect change in shoot GI per day since the last measurement: [(GIx – GIx-1)/DAPx – DAPx-1] where GIx and DAPx are GI and DAP at that particular measurement date and where GIx-1 and DAPx-1 are GI and DAP at the previous measurement date.

On the day plants in a treatment were determined to need rewatering, SWP was measured on that day immediately before rewatering and then again 24 h later. SWP was measured using a pressure chamber (PMS Instruments, Corvallis, OR) during 13 Apr. 2009 to 25 June 2009. Because taking a measurement was based on when it was time to rewater plants in a particular treatment, SWP measurements were not made on the same day for all treatments, but instead were scheduled to coincide with the day a plant was to be rewatered. For comparison, SWP of plants in WW treatments were measured at the same time plants in an irrigation treatment were measured. When measuring stem water potential, one 10-cm terminal stem section was removed from each plant in a treatment (n = 5) at 1000 hr, placed in a plastic bag, put on ice in a cooler, and immediately returned to the laboratory (≈5 min transport). Stem sections were then recut to 7.6 cm, foliage was removed from the basal 2.5 cm of the stem, and SWP was measured using the pressure chamber. After SWP measurement, plants were rewatered at 1200 hr. Data were analyzed using GLM procedures and regression analysis and means separation using least significant difference (P < 0.05) (SAS Institute Inc., Cary, NC).

RESULTS

Itea virginica ‘Henry's Garnet’.

Shoot GI increased linearly over time in all five irrigation treatments (Table 1; Fig. 1). At 208 DAP (Fall 2008), GI was highest in WW followed by 50S, 25S, 50RB, and 75RB (P = 0.002) (Fig. 1). At 341 DAP (Spring 2009), GI was highest in WW and 50S treatments followed by 25S, 50RB, and 75RB (P = 0.0001) (Fig. 1). Final GI (474 DAP, experiment termination, Summer 2009) was highest in WW and 25S treatments followed by 50S, 50RB, and 75RB (Table 2; Fig. 1). Cumulative RGI and seasonal RGI were different among treatments (Table 3). Cumulative RGI at all three measurement dates was highest in WW and 25S treatments followed by 50S and 75RB and was lowest in 50RB (Table 3). Seasonal RGI had the highest increase in growth in Fall 2008 (Table 3). Seasonal RGI in Fall 2008 was highest in WW and 25S treatments followed by 50S and 75RB and was lowest in 50RB (Table 3). Spring 2009 seasonal RGI was highest in 50RB, 75RB, and 50S treatments followed by WW and 25S (Table 3). Seasonal RGI shifted again in Summer 2009 with the highest RGI in 50RB, 75RB, and 25S followed by WW and 50S (Table 3). The cumulative change in GI per day (cumulative GI/DAP) and the change in GI per day within each season (seasonal GI/DAP) were different among most treatments (Table 2). Overall, the largest increase in growth per day occurred between planting and Fall 2008 (Table 2). The largest increase in growth per day was in WW, which grew 0.26 cm·d−1 followed by 25S (0.20 cm·d−1) and 50S (0.19 cm·d−1) and was lowest in 50RB (0.13 cm·d−1) and 75RB (0.13 cm·d−1) (Table 2). Between Fall 2008 and Spring 2009, growth in all treatments slowed, and plants grew only 0.03 cm·d−1 (Table 2). Shoot growth per day began to increase again between Spring 2009 and Summer 2009 when the largest increase in GI per day was in WW, 50RB, 75RB, and 25S, which grew 0.10 cm·d−1, whereas 50S grew 0.07 cm·d−1 (Table 2).

Table 1.

Regression equations, R2 values, and significance of regression equations (P value) for the change in growth index [GI (cm) = (widest width + width perpendicular + height)/3] over time as affected by irrigation scheduling (treatment) for Itea virginica ‘Henry's Garnet’ and Rhododendron austrinum grown in field plots.z

Table 1.
Table 2.

Effect of irrigation scheduling (treatment) on growth index [GI (cm) = (widest width + width perpendicular + height)/3] of Itea virginica ‘Henry's Garnet’ and Rhododendron austrinum grown in field plots.z

Table 2.
Table 3.

Effect of irrigation scheduling (treatment) on shoot growth of Itea virginica ‘Henry's Garnet’ and Rhododendron austrinum grown in field plots.z

Table 3.
Fig. 1.
Fig. 1.

Effect of irrigation scheduling (treatment) on growth index [GI (cm) = (widest width + width perpendicular + height)/3] of (A) Itea virginica ‘Henry's Garnet’ and (B) Rhododendron austrinum grown in field plots. GI was measured on 13 Mar. 2008 (initial, at planting), 7 Oct. 2008 [208 d after planting (DAP)], 17 Feb. 2009 (341 DAP), and 30 June 2009 (final, 474 DAP). Treatments include: root ball and surrounding soil maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S) or –50 kPa (50S).

Citation: HortScience horts 45, 11; 10.21273/HORTSCI.45.11.1620

In some irrigation treatments, SWP was different before and after irrigation (Fig. 2). In 75RB, SWP was higher after rewatering than before rewatering. SWP of plants in 50S was higher before rewatering than after; SWP of plants in the 50S treatment before rewatering was similar to that of plants in the WW treatment (Fig. 2).

Fig. 2.
Fig. 2.

Effect of irrigation scheduling (treatment) on stem water potential (SWP) of (A) Itea virginica ‘Henry's Garnet’ and (B) Rhododendron austrinum grown in field plots from 13 Mar. 2008 to 25 June 2009. Treatments included: root ball and surrounding soil maintained at or above –25 kPa (centibar) [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S) or –50 kPa (50S). Stem water potential was measured immediately before irrigation and again 24 h later for all treatments 13 Apr. 2009 to 25 June 2009. Letters represent means separation among treatments within taxa using least significant difference (P < 0.05). If no differences, then letters are omitted.

Citation: HortScience horts 45, 11; 10.21273/HORTSCI.45.11.1620

Rhododendron austrinum.

Shoot GI increased linearly over time in all five irrigation treatments (Table 1; Fig. 1) and was not different among treatments at any measurement date (Fig. 1). Cumulative and seasonal RGI were affected by treatments (Table 3). Cumulative and seasonal RGI in Fall 2008 and cumulative RGI in Spring 2009 were higher in 75RB than in 25S or 50S treatments; values for WW and 50RB were similar to each other and those in other treatments (Table 3). Cumulative RGI in Summer 2009 was higher in 50RB than in 50S; values for WW, 75RB, and 25S were similar to each other and those in other treatments (Table 3). Seasonal RGI in Spring 2009 was highest in WW, 50RB, 25S, and 50S treatments and lowest in 75RB (Table 3). Seasonal RGI in Summer 2009 was not different among treatments (Table 3). Both the cumulative change in GI per day (cumulative GI/DAP) and the seasonal change in GI per day (seasonal GI/DAP) were affected by treatments (Table 2). Overall, the largest increase in growth occurred between planting and Fall 2008 (Table 2). The largest increase in GI was in 75RB (0.15 cm·d−1) and WW and 50RB (both 0.13 cm·d−1) followed by 50S and 25S (both 0.11 cm·d−1) (Table 2). Between Fall 2008 and Spring 2009, growth in all treatments slowed, and plants grew 0.03 cm·d−1 (Table 2). Shoot growth had a large increase between Spring 2009 and Summer 2009 when plants in WW, 50RB, 75RB, and 25S treatments grew 0.10 cm·d−1, whereas those in 50S grew 0.10 cm·d−1 (Table 2). Within plants in the 75RB treatments, SWP was higher after rewatering than before (Fig. 2).

DISCUSSION

Both I. virginica ‘Henry's Garnet’ and R. austrinum had the largest increase in GI during the first growing season (with the exception of R. austrinum in 50RB and 25S treatments in which plants had similar growth rates during the second growing season; Tables 2 and 3). During the first growing season, with the exception of plants in WW treatments, RGI was approximately two times higher in I. virginica ‘Henry's Garnet’ than R. austrinum in irrigation treatments based on soil matric potential (not root ball matric potential); in WW treatments, RGI was approximately three times higher in I. virginica ‘Henry's Garnet’ than R. austrinum (Table 3). Cumulative RGI continued to be two times higher in I. virginica ‘Henry's Garnet’ than in R. austrinum in irrigation treatments based on soil matric potential (not root ball matric potential) at the following two measurement dates (Table 3). Seasonal RGI of I. virginica ‘Henry's Garnet’ was two times higher than R. austrinum in the first growing season in irrigation treatments based on soil matric potential (not root ball matric potential); however, at the following two measurement dates, seasonal RGI was similar between taxa (Table 3). After Fall 2008, treatment differences became smaller, which could be attributed to increased precipitation (Fig. 3), which decreased soil drying and resulted in fewer scheduled irrigation applications (Figs. 4 and 5).

Fig. 3.
Fig. 3.

Precipitation amounts from June 2008 to June 2009 averaged from four rain gauges in field plots.

Citation: HortScience horts 45, 11; 10.21273/HORTSCI.45.11.1620

Fig. 4.
Fig. 4.

Effect of irrigation scheduling (treatment) soil matric potential of Itea virginica ‘Henry's Garnet’ grown in field plots from 13 Mar. 2008 to 25 June 2009. Treatments include: root ball and surrounding soil maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S) or –50 kPa (50S). Data for 50RB from Sept. 2008 to Nov. 2008 were lost as a result of data logger malfunction.

Citation: HortScience horts 45, 11; 10.21273/HORTSCI.45.11.1620

Fig. 5.
Fig. 5.

Effect of irrigation scheduling (treatment) on soil matric potential of R. austrinum grown in field plots from 13 Mar. 2008 to 25 June 2009. Treatments include: root ball and surrounding soil maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S) or –50 kPa (50S). Data from Oct. 2008 to Apr. 2009 for WW and from Dec. 2008 to Apr. 2009 for 75RB and 50S Nov. 2008 were lost as a result of data logger malfunction.

Citation: HortScience horts 45, 11; 10.21273/HORTSCI.45.11.1620

Shoot GI increased in all treatments and both taxa. Overall, irrigation scheduling affected relative shoot growth more in I. virginica ‘Henry's Garnet’ than R. austrinum (Table 3; Fig. 1). Initial GI of I. virginica ‘Henry's Garnet’ and R. austrinum were 45 cm (18 in.) and 58 cm (23 in.), respectively. Initially, I. virginica ‘Henry's Garnet’ was smaller than R. austrinum, which may explain why I. virginica ‘Henry's Garnet’ RGI was much higher than R. austrinum (Table 3). Furthermore, similar centimeters per day growth rates indicated that I. virginica ‘Henry's Garnet’ grew more relative to its initial size than R. austrinum (Table 2). The apparent difference in shoot growth may be attributed to the fast growth rate of I. virginica ‘Henry's Garnet’. Because R. austrinum has a slower growth rate, shoot growth was likely not as affected by fewer irrigation events.

Within a treatment, there were more differences in SWP before and after irrigation and between WW and other treatments in I. virginica ‘Henry's Garnet’ than in R. austrinum (Fig. 2). Fluctuations in I. virginica ‘Henry's Garnet’ may be a result of the succulent new shoot growth, which was used in sampling (terminal stem sections), whereas R. austrinum terminal stem samples were more woody. Itea virginica ‘Henry's Garnet’ plants in treatments that were irrigated more frequently had similar SWP, whereas those receiving the least amount of irrigation (75RB and 50S) had more fluctuations in SWP (Fig. 2). In 50S, the driest treatment, it is not clear why after irrigation, plants had a lower SWP than before irrigation (Fig. 2). It is possible that fissures or channels existed in the dry soil that prevented uniform penetration of irrigation water within the rhizosphere. In 25S, the treatment most frequently irrigated after WW, SWP was not different between before and after irrigation or from WW (Fig. 2).

With the exception of WW, irrigation frequency increased over time in all other treatments (Figs. 4 and 5). For example, days between irrigation decreased in 25S from once every 8 d to once every 4 d (Fig. 4) for I. virginica ‘Henry's Garnet’ and decreased from once every 9 d to once every 5 d in I. virginica and R. austrinum (Fig. 5). For I. virginica ‘Henry's Garnet’ in 50RB, days between irrigation decreased from once every 6 d to once every 4 d (Fig. 4). In general, plants in both taxa that were WW received water once every 4 to 5 d throughout each growing season (Figs. 4 and 5). During winter months, neither taxa required additional irrigation, and root ball and soil matric potential remained high (Figs. 4 and 5).

Itea virginica ‘Henry's Garnet’ plants in the WW treatment and in the treatments based on soil matric potential had the most growth and received the most irrigation because roots were growing into the surrounding soil, likely at a faster rate than roots of R. austrinum. In greenhouse studies, I. virginica ‘Henry's Garnet’ had faster root growth rates than R. austrinum (unpublished data). At planting, I. virginica ‘Henry's Garnet’ had fewer roots in the original root ball than R. austrinum in which the root ball was completely filled with roots. This may explain why I. virginica ‘Henry's Garnet’ in treatments based on root ball matric potential were irrigated less frequently than R. austrinum in treatments based on root ball matric potential. Rate of root growth after transplanting is a critical factor for transplant establishment and survival (Watson and Himelick, 1997). The slower rates of root growth for R. austrinum were typical of plants with fibrous, hair-like root systems (Price et al., 2009; Wright and Wright, 2004; Wright et al., 2007).

Consistently, the biggest difference between root ball and soil matric potential occurred when irrigation scheduling was based on backfill soil matric potential rather than root ball matric potential. This illustrates the rate at which the root ball can dry, which is much faster than the soil (Costello and Paul, 1975; Nelms and Spomer, 1983). Regardless of the presence of plant roots in the soil, the root ball still dried at a faster rate than the soil; however, as roots grew into the soil, differences between root ball and soil matric potential decreased. When scheduling irrigation based on soil matric potential, the volume of water applied may need to be increased, because root ball matric potential tends to be lower than soil (Figs. 4 and 5).

Results suggest that monitoring root ball matric potential is more effective for irrigation scheduling than monitoring the surrounding soil matric potential with respect to increased shoot growth and initial establishment of a plant. However, all plants in this study in all treatments survived, grew, appeared to have good visual quality, and would be considered successfully established by the end of the second growing season. Precipitation rates were unusually high during the second year of establishment; it is likely that irrigation would normally have been needed during the second year of establishment to ensure survival. At the very least, root ball and soil matric potential should continue to be monitored. Results indicate that irrigation frequency can be reduced if soil or root ball matric potential is monitored to ensure adequate moisture. This is in agreement with other research, which found that irrigation outside the original root ball did not aid in quick establishment of transplanted trees (Gilman et al., 1998), and after transplanting, root ball moisture can be significantly lower than backfill soil (Costello and Paul, 1975; Nelms and Spomer, 1983). In all cases, it appears that until roots grow into the backfill soil, monitoring both backfill soil and root ball matric potential is important for scheduling post-transplant irrigation.

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Contributor Notes

Part of a colloquium (The Efficient Use of Alternative Water and Traditional Irrigation Sources in Horticulture) presented 25 July 2009 at ASHS-2009, St. Louis, MO; sponsored by the Water Utilization and Management (WUM) Working Group.

To whom reprint requests should be addressed; e-mail awright@auburn.edu.

Article Sections

Article Figures

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    Effect of irrigation scheduling (treatment) on growth index [GI (cm) = (widest width + width perpendicular + height)/3] of (A) Itea virginica ‘Henry's Garnet’ and (B) Rhododendron austrinum grown in field plots. GI was measured on 13 Mar. 2008 (initial, at planting), 7 Oct. 2008 [208 d after planting (DAP)], 17 Feb. 2009 (341 DAP), and 30 June 2009 (final, 474 DAP). Treatments include: root ball and surrounding soil maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S) or –50 kPa (50S).

  • View in gallery

    Effect of irrigation scheduling (treatment) on stem water potential (SWP) of (A) Itea virginica ‘Henry's Garnet’ and (B) Rhododendron austrinum grown in field plots from 13 Mar. 2008 to 25 June 2009. Treatments included: root ball and surrounding soil maintained at or above –25 kPa (centibar) [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S) or –50 kPa (50S). Stem water potential was measured immediately before irrigation and again 24 h later for all treatments 13 Apr. 2009 to 25 June 2009. Letters represent means separation among treatments within taxa using least significant difference (P < 0.05). If no differences, then letters are omitted.

  • View in gallery

    Precipitation amounts from June 2008 to June 2009 averaged from four rain gauges in field plots.

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    Effect of irrigation scheduling (treatment) soil matric potential of Itea virginica ‘Henry's Garnet’ grown in field plots from 13 Mar. 2008 to 25 June 2009. Treatments include: root ball and surrounding soil maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S) or –50 kPa (50S). Data for 50RB from Sept. 2008 to Nov. 2008 were lost as a result of data logger malfunction.

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    Effect of irrigation scheduling (treatment) on soil matric potential of R. austrinum grown in field plots from 13 Mar. 2008 to 25 June 2009. Treatments include: root ball and surrounding soil maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential reached –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential reached –25 kPa (25S) or –50 kPa (50S). Data from Oct. 2008 to Apr. 2009 for WW and from Dec. 2008 to Apr. 2009 for 75RB and 50S Nov. 2008 were lost as a result of data logger malfunction.

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