Abstract
Hedge pruning has been used in southeastern US pecan (Carya illinoinensis) orchards for ∼10 to 15 years and has become common in the region within the past 5 years. However, questions remain regarding how pecan trees in the southeastern United States will respond to hedge pruning into the hot summer months in a humid environment. Two treatments were evaluated for ‘Creek’ and ‘Caddo’ pecan: dormant hedge-pruned (Jan–Feb 2019 and 2022) and summer hedge-pruned (Jun 2019 and 2020). Summer hedge pruning did not affect pecan yield, nut weight, or percent kernel compared with dormant-season pruning. Length of shoot regrowth was reduced significantly by summer hedge pruning compared with dormant-season hedge pruning. The advantages of hedge pruning ‘Creek’ and ‘Caddo’ from the dormant season through midsummer can help transition southeastern US pecan production to a more profitable and sustainable system.
The southeastern US Coastal Plain accounts for almost 45% of US commercial pecan (Carya illinoinensis) production (Wells 2009). The region is known to have long growing seasons that consist of hot summers with frequent rainfall (Conner 2014). Despite the large volume of pecans produced in Georgia, average nut volume produced per hectare is relatively small, at 600 to 1200 kg⋅ha–1. The increasing cost of production coupled with declining pecan prices and tropical storm/hurricane events have led to significant challenges for pecan producers in the southeastern United States since 2018.
Approximately 65% to 70% of available sunlight is intercepted by mature pecan tree canopies (Wood 1996), with up to 95% light interception in overcrowded, unpruned orchards (Lombardini 2006). This lack of available sunlight leads to increased biennial bearing, reduced pecan quality, and increased disease and insect pressure. It is commonly recommended that at least 50% of the orchard area be covered by tree canopy, and the remainder be open to sunlight for maximum tree performance (Wells 2007). Historically, this issue has been resolved through limb pruning and tree removal (Wells 2007). Mechanical hedge pruning mitigates successfully the effects of orchard shading (Wood and Stahmann 2004) and has become the standard method used for this purpose in many arid production regions (Andales et al. 2006; Herrera and White 2000). Hedge pruning has been used in southeastern US pecan orchards for ∼10 to 15 years, and has become common in the region within the past 5 years (Wells 2015).
The southeastern United States is a relatively low-light environment, with significant cloud cover and atmospheric water vapor common throughout the growing season (Wood 2009). Initial mechanical hedge-pruning studies in such environments did not show significant benefits to pecan production. Worley (1985) determined that annual cuts to one of each of four sides of the canopy and topping at 6 m was not suitable for southeastern US commercial pecan orchards. Lombardini (2006) found that one-time mechanical hedge pruning of nonirrigated pecan trees initially increased light within tree canopies but did not increase orchard productivity, nut yield, or quality. Wood (2009) suggested that moderate-width (2.4 m from the tree axis), short-cycle (annual or biennial pruning) mechanical hedging did not appear suitable for southeastern pecan production in the short term; however, he suggested that, over the long term, the practice would prove superior to unpruned trees as orchards became crowded.
Wells (2018) demonstrated that dormant-season hedge pruning of pecan trees in the southeastern United States can be beneficial to tree water status, nut weight, nut quality, and windstorm resistance. Bock et al. (2017) demonstrated that mechanical hedge pruning can potentially enhance pecan scab disease control. There has been a shift toward the planting of pecan trees at greater density by Georgia pecan producers since 2010 in anticipation of maintaining these densities through hedge pruning (Wells 2014).
Although most hedge pruning is conducted in the dormant season, there is significant interest in extending the hedge-pruning process into the growing season. Wood (2009) found no difference in annual summer vs. annual winter hedge pruning of ‘Desirable’ pecan in terms of in-shell nut yield or percent kernel. However, questions remain regarding how pecan trees in the southeastern US will respond to hedge pruning into the hot, summer months in this humid environment. The objectives of this study were to compare the effects of summer hedge pruning of ‘Creek’ and ‘Caddo’ pecan on yield, nut weight, percent kernel, and length of shoot regrowth after pruning in the southeastern United States compared with the standard practice of dormant-season hedge pruning.
Materials and Methods
The study was conducted in a microsprinkler-irrigated commercial pecan orchard planted in 2006 with a 7.6- × 15.2-m spacing in Berrien County, GA, USA near the town of Ray City (lat. 31°1′27.1″N, long. 83°14′37.1″W) from 2019 to 2022. The orchard consisted of ‘Caddo’ and ‘Creek’ pecan trees arranged in two-row blocks, with each alternating across the width of the orchard. One 56.8 L⋅h–1 microsprinkler was located ∼1.2 m from the tree trunk. Irrigation was scheduled according to the University of Georgia Cooperative Extension irrigation recommendations for pecan production (Wells 2017) (Table 1).
Pecan yield, in-shell nut weight, and percent kernel of dormant and summer hedge-pruned ‘Creek’ and ‘Caddo’ pecan.
Soil type was Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults). The orchard was managed under commercial conditions according to the University of Georgia Cooperative Extension recommendations (Hudson et al. 2017). Foliar Zn was applied as Zn(NO3)2 (17% Zn) at 2-week intervals in four applications per season beginning 2 weeks after budbreak. Soil Zn levels were more than the recommended threshold throughout the study area. A 3.7-m-wide vegetation–free strip was maintained with glyphosate along the tree row in all plots. Row middles consisted of untilled bermudagrass (Cynodon dactylon L.) sod. Trees were fertilized according to the University of Georgia cooperative extension recommendations (Wells 2007).
Treatments were arranged in a randomized complete block design with four blocks. Two treatments were evaluated for each cultivar: dormant hedge-pruned (Jan–Feb 2019 and 2022) and summer hedge-pruned (Jun 2019 and 2020). Each plot consisted of two contiguous tree rows measuring 336 m long. All trees were hedge-pruned twice during the 4-year study. Trees were hedge-pruned on the dates just mentioned with a commercial hedging machine on one side at ∼1.85 m from the trunk and at the tops of the trees. Trees were hedge-pruned on their west side in 2019 and on their east side in 2020 (summer hedge) and 2022 (dormant hedge). Side pruning removed up to 3.15 m of growth from the side of the tree being hedge-pruned in the year of pruning. The tops of hedge-pruned trees were pruned at a 45° angle to a height of 12 m on the same side of the tree on which the hedge pruning was done. Because all treatments were hedged, either in winter or summer, guard rows were not used between plots because row width was wide enough to prevent guard row effects from limb regrowth during the course of the study.
Post-hedging shoot length of current-season growth was measured in the upper canopy from a lift in Dec 2019. This was the only year in which regrowth was measured because it was the only time during the course of the study in which both dormant and summer pruning were conducted in the same year, allowing for direct comparisons. A 50-nut sample was collected from each tree for analysis of individual nut weight and percent kernel. Nuts were shelled and percentage of edible kernel was calculated by dividing the kernel weight for the 50-nut sample by total nut weight.
All plots were mechanically harvested separately. In-shell nuts were dried to 4.5% moisture with a commercial drier and processed in a commercial cleaning plant to remove all sticks, leaves, and debris. All cleaned nuts were then weighed by plot to obtain in-shell nut yield for each plot, which was converted to kilograms per hectare.
Analysis of variance was used to determine differences between cultivars. All statistical analyses were performed using statistical software (SigmaPlot 14; Systat Software Inc, San Jose, CA, USA). All pairwise multiple comparison procedures were performed using Tukey’s least significant difference test (P < 0.05).
Results and Discussion
Previous studies have investigated the effects of hedge pruning pecan trees (Lombardini 2006; Wells 2018; Wood 2009; Worley 1985); however, with the exception of the work of Wood (2009), these studies focused on dormant-season hedge pruning only.
The current study demonstrated no difference in pecan yield, individual nut weight, or percent kernel between summer and dormant hedged trees. Average yield across cultivars and hedging period from 2019 to 2022 was more than 2000 kg⋅ha–1 (Table 1). These results are consistent with those of Wood (2009), who found that summer pruning, relative to dormant-season pruning, did not improve long-term marketable yield of ‘Desirable’ trees.
Pecan yield was affected by cultivar in 2019 and 2020 when ‘Creek’ produced significantly (P < 0.05) greater yields than ‘Caddo’ (Table 1). Pecan yield ranged from 1793 to 3365 kg⋅ha–1 for ‘Creek’ and from 856 to 3350 kg⋅ha–1 for ‘Caddo’ over the course of the study. However, when averaged for all years of the study, both cultivars exceeded 2000 kg⋅ha–1 and there was no significant difference in yield. Such yields are a great improvement over previous yields obtained for more traditional cultivars such as Desirable, Schley, and Stuart, which have been commonly grown in the southeastern United States in the past (Sparks 1992). Pecan nut weight was consistently greater for ‘Creek’ and percent kernel was consistently greater for ‘Caddo’ throughout the study (Table 1). There were no significant cultivar × hedging interactions for pecan yield, nut weight, or percent kernel.
Pecan leaf N was reduced significantly by dormant hedging in 2019 compared with summer hedging in both cultivars (Table 2). The 2019 leaf N reduction in dormant-hedged trees may be a result of increased N demand or dilution in longer shoots (Fig. 1) in dormant-hedged trees or the potentially larger canopy volume in dormant-hedged vs. summer-hedged trees in the only year of the study in which both treatments were hedge-pruned in the same year. Similar results were not observed in 2020, a year in which trees in the summer hedging plots were pruned, but those in the dormant-hedged plots were not, nor in 2021, when there was no hedge pruning conducted for either treatment in 2021. However, leaf N for ‘Creek’ was greater (P < 0.05) than that of ‘Caddo’ in 2021.
Leaf N, P, K, and Zn of ‘Creek’ and ‘Caddo’ pecan leaves in summer and dormant-season hedge-pruned pecan trees, 2019 to 2021.
Length of shoot regrowth on summer hedged-pruned and dormant hedge-pruned ‘Creek’ and ‘Caddo’ pecan trees after hedge pruning.
Citation: HortScience 59, 6; 10.21273/HORTSCI17775-24
In 2020, leaf P was significantly (P ≤ 0.05) greater in the summer-hedged than in the dormant-hedged plots. This is likely the result of the cultivar × hedging period interaction, in which summer-hedged ‘Caddo’ had greater (P < 0.05) leaf P than all other treatments. After pruning of both treatments in the same year during 2019, leaf K was reduced (P < 0.05) by summer hedging (Table 2). Leaf Zn was also affected (P < 0.05) by hedging period in 2019 (Table 2). Across both cultivars, leaf Zn was reduced (P < 0.05) by summer hedging. Leaf Zn for summer-hedged trees averaged 77.3 ppm and that of dormant-hedged trees was 83 ppm. Foliar Zn sprays were applied to all trees before June, which may explain some of the reduction in leaf Zn for summer-hedged trees, in which foliage developed after foliar sprays were completed. There was, however, a significant (P < 0.05) cultivar × hedging period interaction in which ‘Creek’ leaf Zn was less in the dormant-hedged plots whereas ‘Caddo’ leaf Zn was reduced in the summer-hedged plots (Table 2).
There was a significant (P < 0.05) reduction in shoot length regrowth following summer pruning compared with dormant-season pruning. Shoot regrowth in summer-hedged trees was 76% shorter than that of dormant-hedged trees for ‘Creek’ and ‘Caddo’ combined. This results from a shorter growing season for those shoots developing after summer hedge pruning. The reduction in shoot length via summer hedge pruning may allow producers to use longer intervals between hedge-pruning events, which could potentially reduce the cost of the hedge-pruning system, but it also presents logistical challenges in the management of debris in the orchard, which may interfere with other orchard activities during the growing season for growers with limited labor and equipment.
However, summer hedge pruning also offers advantages for the producer by extending the hedge-pruning season through June. This is especially advantageous to large producers who cannot prune their entire acreage within the dormant-season window. In addition, summer hedge pruning allows an evaluation of the crop load before making hedge-pruning decisions when used as a tool for management of alternate or biennial bearing.
Conclusion
The timing of hedge pruning did not affect pecan yield or quality significantly. Summer pruning reduced shoot regrowth in both cultivars, which may potentially extend the period between hedge prunings, but also creates logistical issues related to cleanup of debris during the growing season when other orchard activities are necessary. The reduced tree size of hedge-pruned trees offers many advantages, including improved spray coverage for better disease and insect control, reduced water stress, heavier individual nut weight, greater percent kernel, more effective management of crop load, and reduced susceptibility to wind injury (Wells 2018). These advantages coupled with cultivars such as Creek and Caddo, which have moderate to high levels of pecan scab resistance and greater yield potential (in the range of 2200 kg⋅ha–1 or more) can help transition southeastern US pecan production to a more profitable and sustainable system.
References Cited
Andales A, Wang J, Sammis TW, Mexal JG, Simmons LJ, Miller DR, Gutschick VP. 2006. A model of pecan tree growth for the management of pruning and irrigation. Agric Water Manag. 84:77–88. https://doi.org/10.1016/j.agwat.2006.02.012.
Bock CH, Hotchkiss MW, Brenneman TB, Stevenson KL, Goff WD, Smith MW, Wells L, Wood BW. 2017. Severity of scab and its effects on fruit weight in mechanically hedge-pruned and topped pecan trees. Plant Dis. 101:785–793. https://doi.org/10.1094/PDIS-10-16-1473-RE.
Conner PJ. 2014. Performance of nine pecan cultivars in southern Georgia. J Am Pomol Soc. 68:118–124.
Herrera E, White JM. 2000. Pruning mature trees. New Mexico State University guide H-629. New Mexico State University, Las Cruces, NM, USA.
Hudson W, Brock J, Culpepper S, Wells L. 2017. Georgia pecan pest management guide. Univ. Georgia Coop. Ext. Bull. 841, Athens, GA, USA.
Lombardini L. 2006. One-time pruning of pecan trees induced limited and short term benefits in canopy light penetration, yield, and nut quality. HortScience. 41:1469–1473. https://doi.org/10.21273/HORTSCI.41.6.1469.
Sparks D. 1992 Pecan cultivars: The orchard’s foundation. Pecan Production Innovations, Watkinsville, GA, USA.
Wells ML (ed). 2007. Southeastern pecan growers handbook. Pub. 1327. Univ. Georgia Coop. Ext. Ser., Athens, GA, USA.
Wells ML. 2009. Pecan nutrient element status and orchard soil fertility in the southeastern coastal plain of the United States. HortTechnology. 19(2):432–438. https://doi.org/10.21273/HORTTECH.19.2.432.
Wells ML. 2014. Pecan planting trends in Georgia. HortTechnology. 24:475–479. https://doi.org/10.21273/HORTTECH.24.4.475.
Wells ML. 2015. Irrigation water management for pecans in humid climates. HortScience. 50:1070–1074. https://doi.org/10.21273/HORTSCI.50.7.1070.
Wells L. 2017. Pecan water requirements and irrigation scheduling. Circ. 1106, Univ. Georgia Coop. Ext. Ser., Athens, GA, USA.
Wells L. 2018. Mechanical hedge pruning affects nut size, nut quality, wind damage, and stem water potential of pecan in humid conditions. HortScience. 53:1203–1207. https://doi.org/10.21273/HORTSCI13217-18.
Wood BW. 1996. Establishing pecan transplants. HortTechnology. 6:276–279.
Wood BW. 2009. Mechanical hedge pruning of pecan in a relatively low-light environment. HortScience. 44:68–72. https://doi.org/10.21273/HORTSCI.44.1.68.
Wood BW, Stahmann D. 2004. Hedge pruning pecan. HortTechnology. 14:63–72. https://doi.org/10.21273/HORTTECH.14.1.0063.
Worley RE. 1985. Effects of hedging and selective limb pruning of Elliott, Desirable, and Farley pecan trees under three irrigation regimes. J Am Soc Hortic Sci. 110:12–16. https://doi.org/10.21273/JASHS.110.1.12.