Randle, A. , Extension Agent, University Of Georgia

ABSTRACT

Rejuvenation pruning is a rarely studied technique for restoring productivity by pruning plants near ground level. To determine its effectiveness, we recorded the fruit yield and new cane growth of 30 year-old ‘Tifblue’ rabbiteye blueberries over two years following rejuvenation. Plants were hand-pruned to maintain 6 – 10 canes, 12 – 20 canes, or 24 – 40 canes, and a control treatment in which plants received no additional pruning. Although difference in yield was not significant among the treatments, yields of all rejuvenated plants were nearing average yields of mature ‘Tifblue’ plants (4 years old) after two years. However, new cane growth remained steady in all treatments. These results suggest that rejuvenation pruning is an effective method in promoting production of new canes, but it should not replace an annual pruning regimen. 

Introduction

Pruning is a necessary part of blueberry management to maintain yield and fruit quality over time (Brightwell and Johnston, 1944). This is especially important for rabbiteye blueberries, which have more vigorous growth than highbush or lowbush blueberries (Powell et al., 2002). Pruning controls plant size, increases yield and fruit size, and improves field operations like weed control, mowing and fertilization (Krewer et al., 2004; Williamson et al., 2004). In you-pick or hand-picked operations, removing dead and weak plant material as well as excess canes creates an open framework, and thus an easily picked, aesthetically pleasing plant (Davies, 1983; Williamson et al., 2004). Some hand-pruning is necessary throughout the lifetime of the plant, even in mechanically harvested and pruned plantings (Krewer et al., 2004). Despite the importance of pruning, producers often neglect the task because it is labor-intensive, taking five to fifteen minutes per bush (Davies, 1983). In established rabbiteye plantings, pruning and mowing operations make up the largest portion of variable costs at 21%, about $267 per acre annually (Fonsah et al., 2008).

If pruning is ignored entirely, plants become unproductive and difficult to harvest. When plants reach this state, producers can bring blueberries back into production by a technique called rejuvenation pruning (also called renewal or renovation pruning). Rejuvenation pruning calls for cutting all the plant material back to within a foot or less of the ground (Powell et al., 2002; Williamson et al., 2004). This results in an immediate flush of new cane growth, especially in vigorous cultivars like ‘Tifblue,’ but eliminates yield in the following year (Davies, 1983; Powell et al., 2002). Thinning the resulting cane growth to five to ten canes establishes the new foundation for the rejuvenated plant (Williamson et al., 2004). Although this method causes a dramatic loss in yield, research in other plant species shows that plants return substantial yields after the third year, and overall fruit quality and size increases (Albert et al., 2010; Brightwell and Johnston, 1944; Howell, et al., 1975).

While fruit production literature often mentions rejuvenation pruning, little research exists to show how plants respond to this technique, and no published work discusses rabbiteye blueberries. The goal of this study was to show how yield and plant growth was affected by performing rejuvenation pruning on 30 year-old ‘Tifblue’ blueberries, and determine if this is a viable option for farmers with overgrown blueberry plantings.

 

Materials and Methods

This study was conducted on a you-pick blueberry farm near Auburn, Alabama. In the summer of 2014, a forestry mulcher was used to cut overgrown 30 year-old rabbiteye plants to within one inch of the ground (Figure 1). In the spring of 2015, the plants produced vigorous new growth, providing an opportunity to measure the effectiveness of rejuvenation pruning in mature rabbiteye blueberries.

 

Figure 1. This forestry mulcher cut 30 year old rabbiteye blueberries back to the ground in 2014; by 2016, plants had returned to productivity.

 

To determine if this effort was worthwhile, we selected 24 ‘Tifblue’ plants within the five acre planting. Rows were spaced twelve feet apart and plants six feet apart within the rows. At the beginning of this study in August 2015, plants had grown untouched for one year, and had reached a height of five feet on average. Plants averaged 100 canes each.

The experimental unit consisted of one plant with a guard plant on each side. Four pruning treatments were arranged in a randomized complete block design (RCBD) with six replications. Plants were hand-pruned to maintain a specific number of canes: 6 – 10 canes (approximately a 90% prune), 12 -20 canes (approximately a 75% prune), or 24 – 40 canes (approximately a 50% prune). A fourth treatment served as a control in which rejuvenated plants received no additional pruning.

Pruning occurred following fruit harvest: August 7, 2015 and August 12, 2016. Each year, we recorded the number of canes removed from each plant to measure growth of new canes. We measured yield by hand-picking and weighing marketable fruit beginning when 75% of the total fruit on the plants was ripe (June 20, 2016 and June 21, 2017). A second hand-harvest occurred two weeks following (July 4, 2016 and July 6, 2017) to capture an estimated 80% of the total fruit harvest each year. To measure labor needed to prune at each rate, we recorded the time required to prune each plant in minutes.

Throughout the study all plants received standard fertilizer applications and drip irrigation when rainfall was inadequate. Per request of the farmer, we did not use insecticides, but did employ Remedy Ultra (triclopyr) to control woody weeds within rows, including sweetgums, blackberries, red maples, oaks, and green briar. Blueberries were protected by applying herbicide post-harvest as a cut stump application only. Data were subjected to analysis of variance procedures (R Core Team, 2017).

 

Results

As shown in Table 1, yield among pruning treatments was not statistically significant (P =0.091). Yield did increase as pruning severity decreased, as expected (Table 2).

 

Table 1. Analysis of variance table for average total yield and average total canes removed showed no significant difference among treatments.

Source of variation	Df	MS	P-value
Average total yield among  treatments	3	34.54	0.091
Average number of canes removed among treatments	2	362.35	0.432

 

 

Table 2. Effect of pruning severity on yield.

Treatment	Average canes removed		Average pruning time (minutes)		Average total yield (lbs)
	2015	2016	2015	2016	2016	2017
Pruned to 6 – 10 canes	102.2	71.5	21	15	7.8	4.7
Pruned to 12 – 20 canes	74.2	48.7	19	16	8.9	4.0
Pruned to 24 – 40 canes	53.2	41.7	19	15	11.4	5.0
Unpruned	0	0	0	0	8.5	9.3

 

 

 

 

 

 

 

 

 

 

Production of new canes among treatments groups was also not significantly different (P = 0.432). Because production of new canes did not decrease over the two years of the study, pruning labor also did not decrease (Table 2). Previous studies have shown that it takes on average fifteen minutes per plant to hand-prune mature blueberries (Davies, 1983). Even with new cane production from these plants, pruning time remained near this standard. In this study, it took the same amount of time to prune to 6 – 10 canes as the less severe treatments. This occurred because after the initial year most of the new growth canes were small and therefore quickly removed. Also, in bushes with more canes, weed pressure was greater making it harder to identify the appropriate canes to remove.

The cost of rejuvenating an existing planting is illustrated in Table 3. We assume that maintenance costs for a rejuvenated planting would be the same as maintenance costs in a mature planting, with the added cost of mulching plants in Year 1 (Fonsah et al., 2008). In this study, a forestry mulcher was employed for $175/hour and was able to completely mulch an acre of mature blueberries in eight hours. Newly planted plantings incur the cost of land preparation and planting in Year 1, which the rejuvenation technique avoids. In a newly established planting, small harvests will offset costs beginning in Year 2, while a rejuvenated planting will not see harvests offset costs until Year 3. From Year 3 on, yields of rejuvenated plants should compete with mature plantings. 

 

Table 3. Total cost of rejuvenating an existing planting.

	Net rejuvenation cost/acre
Year 1	$3,694.28 (including mulching at $175/hr)
Year 2	$3,166.05 (no harvest to offset costs)
Year 3	$283.65 (full production)
Year 4	$283.65
Net costs	$7,427.63

 

 

Discussion

Although the crop was not a total loss, yield in 2017 was substantially lower than yields in 2016 due to weather conditions. Two consecutive freezes occurred on March 15^th (28° F) and 16^th (26° F) after many plants had begun fruit set. Additionally, a tropical storm hit Alabama within a few days of the designated harvest date. Tropical storm Cindy produced 6.5 inches of rain over four days, causing ripe fruit to split on the plant and knocking unripe fruit off entirely. Unusually high rainfall continued through July, reducing marketable fruit yields. While fruit was still harvested, yields differ dramatically from those of the previous year.

Regardless, all groups were nearing expected production of 3 – 4 year-old mature rabbiteye blueberries. A mature, healthy rabbiteye plant should produce about 15 pounds of fruit annually (Powell et al., 2002). Plants pruned even at the greatest severity, 6 –10 canes, recovered on average half of expected mature production after just two years of regrowth, assuming that harvests captured 80% of total fruit production. Lower pruning severities had higher returns. Only unpruned plants matched expected mature productivity. However, while berry size and quality was not measured in this study, it was noted anecdotally that berry size seemed reduced in unpruned treatments and these plants took longer to harvest and maintain (Figure 2).

Figure 2. Two years after mulching, unpruned blueberry plants are forming a hedge and encroaching on surrounding plants; weeds are an issue throughout the orchard.

 

Plants pruned to 6 – 10 canes took the same amount of time to maintain as other treatments pruned at a lower intensity. Because maintaining fewer canes makes berry harvest and weed control easier, and the number of canes left does not significantly impact yield, these results show that pruning to 6 – 10 canes is the most efficient method for maintaining rejuvenated plants.

In existing plantings, the practice of rejuvenation pruning is a viable alternative to establishing new plants. It provides a fast return on investment, with yields approaching mature planting yields beginning in Year 3. Rejuvenation eliminates the cost of establishing a new planting, potentially offsetting the subsequent loss of income.

The problem with unmanaged plantings is that they tend to have many other problems, like substantial weed pressure and unaddressed irrigation issues. In this study, controlling weeds took about as much time as total pruning per plant, and even at the conclusion of this study weed regrowth continued to be an issue. Because of neglect, new irrigation had to be installed. In some situations, producers might avoid the cost of rebuilding irrigation systems.

While rejuvenation is a viable option, success is highly dependent on a few factors. First, the original plants should be in good health with no major disease or pest issues. They should also be an appropriate variety that still competes well with new releases.

Second, the farmer must be willing to sacrifice all yield in the first year following rejuvenation. Depending on the fruit cultivar, the plant may take longer than two years to return to average yields. It is still unknown how long rejuvenated plants will remain productive.

Finally, and most importantly, the farmer must be willing and able to supply skilled labor to maintain a pruning regimen for years. Rejuvenation pruning should not be viewed as a replacement for proper annual pruning, but instead as a one-time recovery option. Subsequent repeated skilled pruning is the cornerstone of this technique.

Rejuvenation pruning provides a valuable tool to reclaim neglected plantings. It may also be a consideration for new and beginning farmers, who could use this method to bring established fruit plantings back into production. Even if a planting has not yet reached the level of deterioration to warrant rejuvenation pruning, farmers must be aware of the importance of annual pruning and make it a priority in their management.

 

References

Albert, T., Karp, K., Starast, M., and Paal, T. (2010). The effect of mulching and pruning on the vegetative growth and yield of the half-high blueberry. Agronomy Research 8(1): 759-769.

Brightwell, W., and Johnston, S. (1944). Pruning the Highbush Blueberry. Michigan State University Agriculture Experiment Station. Retrieved from: https://archive.lib.msu.edu/DMC/extension_publications/TB192/TB192.pdf.

Davies, F. (1983). Pruning, yield, and morphology of 3 rabbiteye blueberry cultivars in Florida. Proceedings of Florida State Horticulture Society 96: 192-195.

Fonsah, E., Krewer, G., Harrison, K., and Stanaland, D. (2008). Economic returns using risk-rated budget analysis for rabbiteye blueberry in Georgia. HortTechnology 18(3): 506-515.

Howell, G., Hansen, C., Bittenbender, H., and Stackhouse, S. (1975). Rejuvenating Highbush Blueberries. Journal of the American Society for Horticultural Science 199: 455-457.

Krewer, G., Stanaland, D., NeSmith, S., and Mullinix, B. (2004). Post-harvest hedging and pruning of three year pruning trial on ‘Climax’ and ‘Tifblue’ rabbiteye blueberry. Proceedings of the Ninth North American Blueberry Research and Extension Workers Conference, pp. 203-212.

Powell, A., Dozier, W., and Himelrick, D. (2002). Commercial blueberry production guide for Alabama. Alabama Cooperative Extension System. Retrieved from: http://www.aces.edu/pubs/docs/A/ANR-0904/index2.tmpl.

R Core Team. (2017). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. Retrieved from: http://www.R-project.org/.

Williamson, J., Davies, F., and Lyrene, P. (2004). Pruning blueberries in Florida. Florida Cooperative Extension Service. Retrieved from: http://ufdcimages.uflib.ufl.edu/IR/00/00/26/82/00001/HS22300.pdf.

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