Abstract
‘Mini Blues’ highbush blueberry (Vaccinium sp.) was released in 2016 as a high-quality, machine-harvestable alternative to lowbush (V. angustifolium Ait.) or other small-fruited highbush blueberry cultivars for processed markets. A planting was established in Oct. 2015 in western Oregon to evaluate the effects of pruning method on yield, machine-harvest efficiency (MHE), berry weight and total soluble solids (TSS), leaf tissue nutrients, pruning weight, pruning time, and costs. Plants were pruned for shape and to remove flower buds in 2015–16 and 2016–17. Pruning treatments began in 2017–18 and included: 1) conventional highbush pruning (HB); 2) removing one or two of the oldest canes per bush (Speed); 3) leaving plants to grow from 2017 to 2021 (Unpruned) before doing a hard renovation prune in 2021–22 (cutting the plants back to a height of ≈0.3 m and leaving the best 8–10 canes/plant); and 4) hedging after fruit harvest in 2018 (Hedge) and then unpruned afterward until renovation in 2021–22. The pattern of yield progression, observed wood aging, and reduced berry size after 4 years of no pruning indicated renovation was necessary in the unpruned and hedge treatments. Low growth was removed each year in all treatments, and hedging was only done in 2018 because it severely reduced yield the following year and, therefore, was not a viable option. An over-the-row machine harvester was used from 2018 to 2021. Speed-pruned plants, averaged over 4 years, had the greatest potential yield (3.75 kg/plant) compared with the other treatments (averaged 2.99 kg/plant) but had a similar yield as HB because more fruit remained on the bush after harvest with speed pruning. In 2021, speed pruning resulted in the highest yield (4.2 kg/plant), followed by HB (3.8 kg/plant) and the unpruned and hedge methods (averaged 3.1 kg/plant). MHE increased from 43% in 2018 to 74% in 2021, mainly because, as the plants aged, a larger proportion of the canopy was above the catcher plates on the harvester. On average, MHE was highest with HB pruning (70%), intermediate in the unpruned and speed-pruned plants (59%), and lowest in the hedged plants (49%). In 2021, ground drop loss was highest for hedge (18%), lowest for speed (14%), and intermediate for HB and unpruned (averaged 16%) methods. HB-pruned plants had heavier berries (0.64 g) than unpruned and hedge treatments (averaged 0.57 g) and a similar berry weight as the speed-pruned plants (0.61 g). Pruning had no effect on berry TSS. In contrast to leaf K, leaf Mg and Ca concentrations were lowest in HB and higher in all other treatments. In 2020–21, HB pruning required 471 h·ha−1, while speed pruning took 79 h·ha−1; the hedge and unpruned treatments required an average of 60 h·ha−1 to remove low-growing branches that would interfere with machine harvest. In 2021–22, renovation of the unpruned and hedge treatments took 290 h·ha−1. While leaving bushes unpruned during establishment appears to be a promising option for ‘Mini Blues’, further work is needed to evaluate fruit production after renovation and to determine how long the plants could remain unpruned thereafter. Speed pruning is also a good option, reducing pruning costs by 85%.
The North American market for small-fruited (<1 g), processed blueberry is currently mainly fulfilled by lowbush blueberry clones (Vaccinium angustifolium Ait.) and ‘Rubel’, a selection made from a wild northern highbush blueberry (V. corymbosum L.) population in the early 1990s. ‘Rubel’ has an average berry weight of 0.86 g (Ehlenfeldt, 2001) and was commonly planted according to a survey done in 2003 (Finn et al., 2009; Strik and Yarborough, 2005). ‘Mini Blues’ (Vaccinium hybrid: 75% V. corymbosum L. and 25% V. darrowii Camp) was released originally as ‘Baby Blues’ (Finn et al., 2016), but the cultivar name required change during the patenting process (Finn, 2017). ‘Mini Blues’ was released as a viable alternative to ‘Rubel’ and lowbush blueberry for the specialty processed market and was described as a vigorous, high-yielding (similar yield to ‘Aurora’ and ‘Liberty’ blueberry), machine-harvestable cultivar, producing very small fruit (0.9 g compared with 2.7 g for ‘Aurora’, ‘Liberty’, ‘Bluecrop’, and ‘Draper’) with high total soluble solids (TSS) and excellent aromatic flavor and processed quality, exceeding that of ‘Rubel’ (Finn et al., 2016).
Pruning northern highbush blueberry plants annually each winter is important to stimulate vigorous, renewal growth and to maintain a consistent yield, good fruit quality, and a concentrated fruiting season, but pruning method or severity has a large impact on pruning time and costs (Mainland, 1989, 1993; Seifker and Hancock, 1987; Strik et al., 2003). The growth habit of ‘Mini Blues’ is vigorous, with the plants producing many thin fruiting laterals, making it difficult and time consuming to prune by conventional methods used for northern highbush blueberry (Strik, 2020). Unpruned or more lightly pruned ‘Bluecrop’ and ‘Berkeley’ blueberry plants using a speed-pruning method produced smaller fruit than conventional pruning and was considered a disadvantage for hand- or machine-harvest for fresh fruit markets (Strik et al., 2003). However, in a small-fruited cultivar, such as ‘Mini Blues’, there is likely more tolerance for a reasonable reduction in berry size with alternative pruning methods, considering the specialty market is targeting small fruit. Preliminary trials done at Oregon State University’s North Willamette Research and Extension Center (NWREC; Aurora, OR) showed promise for lower input pruning methods without compromising yield or fruit quality. Since pruning is one of the most expensive management components of blueberry production (Sutton and Sterns, 2020), finding ways to reduce this cost without sacrificing yield or fruit quality is critical in this cultivar.
In warmer production regions, such as southeastern United States, blueberry plants are mainly pruned immediately after fruit harvest using mechanical hedging to remove the top and sides of the bush (Mainland, 1993; Phillips and Williamson, 2020). This summer pruning method is possible in these regions as fruit harvest finishes early in the season and there are sufficient heat units after pruning for good shoot growth and flower bud development. Growers in the Pacific Northwest have expressed interest in testing this summer pruning method as it would greatly reduce pruning costs.
Most fruit in North America that are destined for the processed market are harvested by machine (Mainland, 1993; Strik and Yarborough, 2005). Considering the very small fruit size of ‘Mini Blues’, hand harvest would be uneconomical. In machine-harvest simulation trials, ‘Mini Blues’ produced cleaner fruit, with very few pedicels or stems, compared with ‘Draper’ (Finn et al., 2016). In addition, ‘Mini Blues’ fruit came off the bush with less agitation compared with ‘Draper’; higher rates of agitation or vibration of the beater rods on the machine harvester have been correlated with more bush damage and winter cold injury of ‘Jersey’ blueberry in Michigan (Howell et al., 1975a). Machine harvest of northern highbush blueberry typically starts in the fourth to fifth growing season, at which point the plants are considered large enough to withstand the agitation from over-the-row harvesters (Strik and Buller, 2002). For a cultivar that is small-fruited and destined only for the processed market, machine harvest would need to start earlier, such as in the third growing season.
Pruning method may affect compatibility with machine-harvest and impact machine-harvest efficiency (MHE), defined as the proportion of total yield picked using a machine. Losses, which reduce MHE, include fruit that drop to the ground during the harvesting process and fruit that remain on the bush after the last pick of the season is completed. Fruit drop is mainly caused by insufficient closing of the catcher plates around the crown area of the bush, leaving a gap through which shaken fruit can fall to the ground. Fruit can remain on the bush after the last harvest when insufficiently agitated by the vibrating rods on the harvester heads and thus not falling to the catcher plates, when fruit are located below the height of the catcher plates, or when fruit are not ripe enough to release from the bush at the time of harvest. Strik and Buller (2005) reported no effect of planting density and concomitant crown diameter on percent fruit drop, a measure of MHE; however, they found that a simple two-wire trellis system, which kept the blueberry bush canopy in a more upright “V” shape, improved MHE by up to 8% of total yield. Trellises have been quickly adopted by the industry (B. Strik, personal observation) and are now recommended for machine harvest of highbush blueberry (DeVetter et al., 2022).
The objectives of this study were to determine whether lower input pruning methods would be effective at maintaining yield and berry quality during the establishment of ‘Mini Blues’ blueberry. We also assessed the economics of alternative pruning methods relative to conventional northern highbush pruning in the cultivar and examined how each method impacted MHE of fruit intended for a processed market.
Materials and Methods
The study site was within a 0.1-ha block established in Oct. 2015 at NWREC (lat. 45°16′47″N, long. 122°45′23″W). Weather data for this site are available from an AgriMet weather station (U.S. Department of Interior, 2016). The soil is mapped as a Willamette silt loam (a fine-silty, mixed, superactive mesic Pachic Ultic Argixeroll).
Site preparation.
The field was fallow in 2014 and throughout the spring and summer of 2015, and weeds were controlled using two applications of glyphosate before soil preparation. In Aug. 2015, the field was power spaded to a depth of 0.4 m and tilled to a depth of about 0.2 m. A pooled soil sample taken in Sept. 2015 indicated that soil pH (4.9) was within a suitable range (4.5–5.5), while soil B was low (0.38 ppm; the ideal level is 0.5–1.0 ppm B) and other nutrients were at appropriate levels for blueberry (Hart et al., 2006). Before bed-shaping, 2.2 ton·ha−1 dolomite lime was broadcast over the entire site to mitigate declines in soil pH expected from ammonium-N fertilization over the study period, and 0.6 kg·ha−1 of B (Solubor, U.S. Borax, Chicago, IL) was applied to the in-row area. Soil organic matter levels were 2.1% (at least 4% is recommended) and thus an 8- to 10-cm-deep layer (target application rate of 282 m3·ha−1) of Douglas fir sawdust [Pseudotsuga menziesii (Mirb.) Franco] was applied to the in-row area. The nutrients and sawdust were incorporated by tilling them into the soil to a depth of ∼0.2 m. Amending with sawdust before planting is a standard commercial practice in this region (Sutton and Sterns, 2020). A bed shaper was used to create raised beds that were about 0.3-m high and about 0.6- and 1.2-m wide at the top and base, respectively. A composite soil sample was taken from the in-row area in late October to early November each year to monitor soil pH and nutrient levels.
Industry-standard, 18-month-old ‘Mini Blues’ blueberry plants were removed from 2-L pots and transplanted on 16 Oct. 2015. Plant spacing was 0.9 m in the row and 3.0 m between rows (3588 plants/ha), with five plants per plot and 2.7 m between plots to allow for clearing of fruit between treatments when machine harvesting. Treatment plots were arranged in a completely randomized design with five replicates.
Field management.
Plants were irrigated with one line of 1.3-cm-diameter polyethylene drip tubing (Netafim, Fresno, CA) on each side of the plants and four 3.8 L·h−1 emitters were placed ≈10 cm away from the crown (two on each side). Black weed mat (permeation rate of 6.8 L·m−2 per h; weight of 0.11 kg·m−2; TenCate Protective Fabrics, Union City, GA; OBC Northwest Inc., Canby, OR) was then applied to the in-row area as a mulch. A trellis was installed, consisting of in-row, metal T-posts and 0.6-m-long cross arms attached to each post by U-bolt; a post was located near the end of each treatment plot (Strik and Buller, 2002). A 12.5-gauge, high-tensile wire was inserted in slots on the cross arms and tightened with a wire tightener on each side of the row. Cross-arm height was adjusted as plant height increased during establishment.
The planting was irrigated 3–7 d per week, generally from mid- to late May through September each year. Irrigation was adjusted, as needed, based on weather conditions and stage of plant development. Plants were fertilized through the drip irrigation system with urea (46–0–0; 2016, 2017, 2020, and 2021) or ammonium sulfate (21–0–0, 24% S; 2018 and 2019) applied weekly (2016) or twice per week (2017–21) from mid-April through early July. A total of 73, 139, 67, 81, 94, and 94 kg·ha−1 N was applied in 2016 through 2021, respectively. Soil K levels declined from 250 ppm before planting to 89 ppm in Fall 2018; thus, potassium thiosulfate was applied by fertigation in 2019 and 2020 at a total rate of 45 kg·ha−1 K per year. Foliar applications of B (Solubor, U.S. Borax, Chicago, IL) were applied whenever leaf nutrient testing indicated that concentrations were below the sufficiency level for blueberry (30–80 ppm B; Hart et al., 2006; Strik and Davis, 2022b), which included applications just before bloom in Spring 2018, 2019, and 2021, as well as before leaf senescence in Fall 2018 and 2020, at rates of 0.4–1.5 kg·ha−1 B on each occasion.
A permanent rye and fescue grass blend [perennial ryegrass (Lolium perenne L. ‘Shining star’), creeping red fescue (Festuca rubra L. ‘Boreal’), hard fescue (Festuca brevipila R. ‘VNS’)] was grown in the row aisles and mowed during the growing season, as required. Weeds were controlled along the edges of the weed mat using herbicides and were pulled by hand when present around the plant crown, as needed. A bird-scare alarm (Bird Gard LLC, Sisters, OR) was used to reduce fruit depredation from birds. No other aboveground pests were observed or identified in the planting throughout the course of the study. However, a few plants were killed from apparent rodent feeding on roots; data were adjusted for any plant losses per plot.
Plants were pruned after planting in 2015–16 to shape the bush and to remove the flower buds, preventing any fruit production during the first growing season. While the plants grew vigorously in the first year, flower buds were also removed in Winter 2016–17 to encourage more vegetative growth during the second season (Strik and Buller, 2005). Our goal was to increase the bush size and begin machine harvesting in the third growing season (earlier than is typical for other northern highbush cultivars in this region) without damaging or uprooting the young plants.
Treatments.
Pruning treatments began in Winter 2017–18 (before year 3) and continued annually through 2020–21 (year 6). Treatments included 1) conventional highbush pruning (HB), removing larger canes, and thinning laterals and whips (HB); 2) removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane (Speed; per Strik et al., 2003); 3) leaving plants to grow for several years before doing a hard renovation prune when necessary (Unpruned); and 4) using a mechanical hedger immediately after fruit harvest to remove the top and sides of the bush (Hedge). A gas-powered sickle bar trimmer was used to hedge plants to 0.5–0.6 m tall and to shape the bush into a ‘V’ by removing growth that was more than 8 cm outside each trellis wire. Hedging was performed on 27 July 2018 and was not repeated in subsequent years due to the poor performance of this treatment. In all treatments (including the unpruned), low growth was pruned off in winter to prevent fruit touching the ground and to improve the fit of catcher plates around the crown and thus increasing MHE. Treatments were repeated annually with the exception of hedging. Plants in the unpruned and hedge treatments did not need renovation until the end of the study period (Winter 2021–22). Plants were renovated by cutting all the canes and whips back to a height of ≈0.3 m from the base of the crown or soil level; the remaining wood was then thinned to the best 8–10 canes/plant, depending on plant vigor, with a goal of leaving the remaining canes in a vase shape (Strik, 2020).
Data collection.
Fruit were harvested every 9–14 d, starting in mid–July, using a self-propelled, over-the-row machine harvester (Littau Harvester, Stayton, OR). The harvester was equipped with two, free-wheeling rotary heads (fiberglass rods that vibrate horizontally at a rate controlled by the driver) and standard plastic catcher plates. Ground speed during harvest was maintained at ∼0.8–1.1 km·h−1; rotary heads were typically adjusted to 100–200 rpm for the first harvest and increased to 400–800 rpm for later harvests depending on bush age and picking number per season. The gap between the rotary heads was maintained at the narrowest setting (5–10 cm between rods).
In the first fruiting season (2018), there was only one harvest due to low yield and concentrated fruit ripening in each treatment (Strik et al., 2022). Harvested fruit were weighed from each plot and divided by the number of plants per plot to calculate the yield per plant for each pick. A random subsample of 25 berries was taken from each plot on every harvest date to determine the average berry weight; a weighted seasonal average mass was then calculated. The berries were then homogenized by hand in a zippered plastic bag and measured for TSS (%) using a temperature-compensating digital refractometer (Atago, Bellevue, WA).
After each harvest, fruit that had fallen to the ground were collected from a 1-m-long section of each plot, including the entire raised bed and crown area, to calculate the total mass of dropped fruit per plant each season. Once the harvest was finished for the season, any remaining fruit on one plant per plot (including green and overripe berries) were stripped by hand and weighed; this proportion of total yield was deemed unharvestable by machine because it was either located below the machine’s catcher plates or within the center of the bush and could not be agitated enough by the rods on the harvester. Total potential yield was calculated by adding machine-harvested yield and the weight of dropped and remaining fruit. Percent drop [(drop wt./total potential yield) × 100], percent remaining fruit [(remaining wt./total potential yield) × 100], and MHE (100 – percent drop – percent remaining) were calculated.
Prunings were weighed from each plot and used to calculate the average pruning weight from each plant per year. The time required to prune each plot and the total harvested yield were used to calculate pruning efficiency, as hours per hectare and dollars per kilogram of fruit harvested. Labor was valued at $14.70/h for general pruning labor, which included worker’s compensation, unemployment insurance, and other labor overhead expenses (Sutton and Sterns, 2020). Labor requirements for pruning presented here are based on pruning by one person (B. Strik) and may be faster or more efficient with a commercial pruning crew.
Plant tissue samples (most recent fully expanded leaves) were collected from each plot in late July to early August each year and were sent to a commercial laboratory (Brookside Laboratories, New Bremen, OH) for nutrient analysis. Leaf N was determined using a combustion analyzer with an induction furnace and thermal conductivity detector (Gavlak et al., 1994). Other leaf nutrients, including P, K, Ca, Mg, Al, B, Cu, Mn, Fe, and Zn, were determined using an inductively coupled plasma (ICP) spectrophotometer, after wet ashing the samples in nitric/perchloric acid (Gavlak et al., 1994).
Statistical analysis.
Data were analyzed using PROC MIXED in SAS software package ver. 9.4 (SAS Institute, Cary, NC), and means were separated at the 5% level using Tukey’s honest significant difference test. PROC UNIVARIATE was used to ensure a normal distribution of data before analysis, and log transformations were applied as necessary. Yield and loss variables, MHE, berry weight and TSS, pruning weight, time and cost per kg fruit harvested, and leaf nutrients were analyzed across years using a split-plot design [year was treated as a main effect in 2018–21 (2017 was not included since the treatments were not implemented yet, and 2021–22 was not included for the pruning weight and time variables) and the pruning treatments were treated as subplots (n = 4)]. Variables were analyzed by year to demonstrate any year × pruning treatment interactions.
Results and Discussion
Yield.
Year, pruning method, and year × pruning had a significant effect on machine-harvested yield (Table 1). Machine-harvested yield increased from 2018 to 2021, except for 2020 when there was heavy bird depredation (A. Davis, personal observation; Fig. 1). When data were analyzed by year, hedged plants had less yield than HB and speed-pruned plants in 2019. In 2021, the highest yield was found in speed-pruned plants, followed by HB, while plants that had not been pruned (except for low growth) since 2017–18 or were summer hedged in 2018 had the lowest yield. Hedging after fruit harvest in 2018 was an unsuccessful pruning method in our study. While hedging plants after fruit harvest is a standard practice for southern highbush blueberries in the southeastern United States (Mainland, 1993; Phillips and Williamson, 2020), in our region, there was insufficient time for shoots that developed after hedging to form flower buds, greatly reducing yield the next year relative to other treatments. Flower bud initiation in northern highbush blueberry requires shoot growth cessation and short daylength conditions (Bañados et al., 2009; Strik, 2010). Northern highbush cultivars also fruit later in the season than southern highbush (in which hedging is commonly done), which delays the time at which hedging can be done. In our study, hedging after fruit harvest thus stimulated growth flushes that continued later in the season compared with unhedged plants, thus limiting yield potential the following year.
Interactive effect of year and pruning method (P = 0.0486) on yield in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
Interactive effect of year and pruning method (P = 0.0486) on yield in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
Interactive effect of year and pruning method (P = 0.0486) on yield in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
Effects of year and pruning method on yield, machine harvest efficiency, and fruit characteristics of ‘Mini Blues’ highbush blueberry from the third (2018) through sixth (2021) growing seasons in Aurora, OR.
The yield increase from 2020 to 2021 was considerably less for the unpruned and hedge treatments compared with HB and speed-pruned plants (Fig. 1). Others have reported a tendency for blueberry bushes to biennially bear low yields when left unpruned (Brightwell and Johnston, 1944; Seifker and Hancock, 1987), but we saw no evidence of such a pattern in our study.
Total potential yield was affected by year (P < 0.0001) and pruning method (P = 0.0019) but was unaffected by an interaction between year and pruning method (P > 0.05). On average, speed-pruned plants had a greater potential yield (3.75 kg/plant) than the other treatments (averaged 2.99 kg/plant), but had a similar machine-harvested yield as HB because there was more fruit remaining on the bush after harvest with speed pruning (Table 1). Our results are different from those reported by Strik et al. (2003), where unpruned plants had a higher yield than those pruned conventionally, and speed-pruned plants had intermediate yield throughout a 4-year study in ‘Bluecrop’. Unlike in this study, the plants were mature when the pruning treatments were initiated, and fruit were hand harvested, leading to fewer losses than with machine harvest.
Year, pruning method, and year × pruning had a significant effect on the weights of dropped and remaining fruit, as well as the percentage of total potential yield remaining on the bush after the last harvest (Table 1). Fruit drop was greater with speed pruning (1.27 kg/plant) than with other treatments (averaged 0.80 kg/plant) in 2019 (P = 0.0026) but was similar among treatments in other years. Percent fruit drop was also affected by an interaction between year and pruning treatment. There was no apparent pattern in percent fruit drop as bushes aged (Table 1). Pruning only affected the percent drop in 2021 (P = 0.0466), with the highest level for plants hedged in 2018 and then left unpruned thereafter (18%), the lowest for speed-pruned plants (14%), and intermediate levels for HB and unpruned (averaged 16%). There was no apparent relationship between drop weight and the percentage of total yield that dropped on the ground during the harvest season. Crown diameter at catcher plate height is related to MHE, as narrower crowns and the removal of interfering low growth both minimize gaps that fruit can drop through (DeVetter et al., 2022; Howell et al., 1975b; Rohrbach and Mainland, 1989). It is possible that there was little treatment difference in crown diameter at the height of the machine harvester’s catcher plates during this study period. However, in a study of planting density in ‘Bluecrop’, where the crown diameter was much smaller for closely spaced plants, there was no effect of plant spacing on percent fruit drop, which averaged 10% to 15% when a trellis was used (Strik and Buller, 2002). These values may have been lower than in our study because ‘Mini Blues’ has smaller fruit that fall through the catcher plates more easily. Ground losses when using over-the-row machine harvesters have varied considerably in other studies, averaging 12% to 16% for commercial blueberries in Michigan (Cargill and Ledebuhr, 1983), 18% for ‘Bluecrop’ in British Columbia, Canada (van Dalfsen and Gaye, 1999), and 26% in North Carolina (Mainland et al., 1975). In our study, pruning method had little effect on percent fruit drop, and therefore, yield losses on the ground are not a major consideration in choosing a pruning technique for ‘Mini Blues’. However, HB-pruned plants had the least amount of fruit remaining after the last harvest each year (0.17–0.44 kg/plant), and the range was much higher in speed-pruned plants (0.37–1.05 kg/plant) than in any other treatment (data not shown).
The percent of total yield remaining on the bush after harvest dropped significantly after the first harvest year (Table 1) and was lowest in all years for HB (Fig. 2A). Speed pruning also typically resulted in a lower percentage of fruit remaining on the bush compared with unpruned and hedge treatments, particularly in later years of the study (Fig. 2A). The detailed pruning done in the HB treatment resulted in earlier and more concentrated ripening in 2019 and 2020 (Strik et al., 2022), with a similar pattern in 2021 (data not shown), leading to more efficient machine harvest. Strik et al. (2003) also reported a more concentrated fruiting season in ‘Bluecrop’ with HB compared with unpruned bushes and reported a 51% reduction in handpicking efficiency from the reduced berry weight and extended fruiting season for unpruned bushes.
Interactive effects of year and pruning method on (A) the percent of fruit remaining on the plant after harvest and (B) machine harvest efficiency (the percent of total fruit production that was harvested) in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
Interactive effects of year and pruning method on (A) the percent of fruit remaining on the plant after harvest and (B) machine harvest efficiency (the percent of total fruit production that was harvested) in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
Interactive effects of year and pruning method on (A) the percent of fruit remaining on the plant after harvest and (B) machine harvest efficiency (the percent of total fruit production that was harvested) in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
MHE significantly increased from 43% in 2018 to 74% in 2021 (P < 0.0001), mainly because a larger proportion of the canopy was above the height of the machine’s catcher plates as the plants matured. A significant interaction between year and pruning (P = 0.0015) indicated that MHE was higher in pruned plants (HB and speed pruned) than in unpruned plants, including the hedge treatment (Fig. 2B). However, in 2018, unpruned and HB plants had a similar MHE, while speed- and hedge-pruning had lower MHE, even though unpruned and hedged plants were pruned the same before the 2018 fruiting season. These differences in 2018 were likely a result of being the first harvest year after pruning treatments were initiated (relatively small differences in plant canopy structure among treatments). Strik et al. (2003) noted that dead or nonfruitful wood present in unpruned, mature blueberry bushes after 4 years would likely lead to breakage of the canes and excess debris during machine harvest; however, we did not observe this in our study.
Fruit characteristics.
Berry weight was considerably less in 2021 than in the other years of the study (Table 1). This was likely due to an extreme heat event that occurred on 26–28 June 2021, where maximum and minimum daily temperatures ranged from 41 to 45 °C and 19 to 29 °C, respectively, and relative humidity in the late afternoon was unusually low, ranging from 13% to 16%. These unusual conditions occurred during fruit ripening, reducing berry weight and probably potential yield as well. The heat event also reduced berry weight and yield in commercial fields of highbush blueberry in Oregon (B. Strik, personal observation).
Berry weight was also affected by the pruning treatments (Table 1). On average, HB-pruned plants had heavier berries than unpruned and hedge treatments, while speed-pruned plants had a similar berry weight to HB. In mature ‘Bluecrop’, both unpruned and speed pruning methods reduced berry weight compared with conventional pruning in 3 of the 4 years (Strik et al., 2003).
Berry TSS was highest in 2018, when the plants were young (Table 1), which is common for establishing blueberry (Strik et al., 2017b; Strik and Davis, 2022a). Berry TSS was also higher in 2021 than in 2019 or 2020, likely due to the concentration of sugars in the smaller berries. Berry TSS in this study was higher in 2 of the 4 years than the mean reported for this cultivar by Finn et al. (2016; 16.4%) and was generally higher than typical for northern highbush blueberry cultivars in the region (Cai et al., 2021; Strik and Davis, 2021, 2022a; Strik et al., 2017a, 2017b). Pruning method had no effect on berry TSS.
Pruning weight.
Year (2017–18 through 2020–21), pruning method, and year × pruning had a significant effect on pruning weight and time (all P < 0.0001; data not shown). Pruning weight increased from the first (0.1 kg/plant) through the fourth (0.4 kg/plant) dormant season. On average, there was no difference in time to prune between 2019–20 and 2020–21 nor between pruning weight or time to prune the hedge and unpruned treatments (data not shown). However, in the HB treatment, both pruning weight and pruning time increased from 2017–18 to 2020–21, whereas with speed pruning, pruning weight, and pruning time were similar in the latter 2 years (data not shown).
When analyzed by year, there was a similar weight of wood removed when dormant pruning the HB and speed treatments in all years except for 2020–21, when more wood was removed from HB-pruned plants (Table 2). This may be a result of plant canopies increasing in size during establishment, so the proportion of wood removed using the speed technique was lower. Pruning the hedging treatment after fruit harvest in 2018 removed an average of 0.4 kg/plant of fresh shoots and leaves (Strik et al., 2022). The weight of low growth removed in winter for hedged and unpruned plants ranged from 0.04 to 0.13 kg/plant and is estimated to have been a similar value for HB- and speed-pruned plants, although low growth was included in the total pruning weights reported (Table 2).
Effect of pruning method on dormant pruning weight and time required to prune ‘Mini Blues’ highbush blueberry from the beginning of the third (2017–18) through seventh (2021–22) growing seasons in Aurora, OR.
The pattern of yield progression (Fig. 1), wood aging in the bush (B. Strik, personal observation), reduced berry size (Table 1), and extended ripening season (Strik et al., 2022) all indicated it was time to renovate the unpruned and hedge treatment plants in Winter 2021–22. Renovation has been documented to rejuvenate blueberry plants, where cutting them back to a height of 0.3–0.6 m (Stafne and Smith, 2021; Strik, 2020) can return the plants back into full production sooner than cutting them to the ground (Howell et al., 1975b; Stafne and Smith, 2021). Renovation pruning greatly increased the total weight of wood removed in the unpruned and hedge treatments relative to HB- and speed-pruned plants (Table 2).
Leaf nutrient concentrations.
All leaf nutrient concentrations were affected by year, and leaf Mg, K, Ca, B, Mn, and Cu were affected by pruning treatment (Table 3). Leaf nutrient concentrations were within recently published sufficiency levels for northern highbush blueberry (Strik and Davis, 2022b), except for leaf P (<0.08%), Mn (>300 ppm despite soil pH being above the recommended range), Cu (<3 ppm), and Zn (<8 ppm) in several years. Leaf P tended to decline as plants matured, similar to what was reported for ‘Duke’ and ‘Liberty’ blueberry grown organically (Strik et al., 2019) and for ‘Legacy’ blueberry grown conventionally (Strik and Davis, 2022a). Several leaf nutrients (N, P, K, S, and Cu) were lower in 2021 than in other years, which could have been a result of the heat event that occurred in late June that year. Nutrient uptake and movement to leaves decreases during periods of low transpiration (Ragel et al., 2019), such as during a heat event that causes stomates to close. However, unlike findings in ‘Legacy’ that year (Strik and Davis, 2022a), Ca was higher in ‘Mini Blues’ in 2021 compared with prior years in this study. There was a year × treatment interaction for leaf K concentration since all treatments had similar concentrations in 2018 and 2021, while in 2019 and 2020, HB-pruned plants had higher leaf K than the other treatments (data not shown), similar to results reported by Strik et al. (2003) for conventional highbush pruned vs. unpruned ‘Bluecrop’. In contrast to leaf K, leaf Mg and Ca were lower in HB than the other treatments, which could be due to competition between the cations as shown previously (Strik et al., 2019). Leaf B remained low in each treatment throughout the study, despite applying foliar B, as found in other studies in Oregon (e.g., Strik et al., 2019).
Effects of year and pruning method on leaf nutrient concentrations of ‘Mini Blues’ highbush blueberry in Aurora, OR.
Costs of pruning.
The HB method required the most time to prune each year and continued to increase as plants matured, whereas speed pruning was more consistent over time, starting in 2019–20 (Table 2). By the end of this study, ‘Mini Blues’ took twice as long to prune as the industry standard, ‘Duke’ (247 h·ha−1; Sutton and Sterns, 2020). In all years except 2017–18 (year 1 of pruning treatments), time to prune using the speed method was not statistically different from unpruned until the renovation was required in 2021–22. Removing one or two canes in addition to low growth when speed pruning took little extra time compared with the large time differential required for HB pruning. Once the plants were mature, the time required to speed prune was 85% less than HB, which is a similar time saving as reported for speed pruning other cultivars of highbush blueberry (Strik et al., 2003).
In 2021–22, renovation of unpruned and the hedge treatments required about three times as much time as speed pruning, leading to ≈20% higher cumulative pruning time in 2017–18 through 2021–22 (totaling 436 h·ha−1 compared with 360 h·ha−1 for speed pruning). In contrast, HB pruning was more than five times as time consuming as speed pruning over the 5-year study, requiring a total of 1971 h·ha−1.
When calculating the cost of pruning per kg of fruit harvested, HB pruning was considerably higher ($0.70/kg) than any of the reduced input pruning techniques (averaged $0.13/kg). Conventional HB pruning of ‘Mini Blues’ costs considerably more than for ‘Duke’ ($0.30/kg of harvested fruit; Sutton and Sterns, 2020). There was a year × treatment interaction for pruning costs per kg of fruit resulting from plants maturing and losses to bird depredation in 2020 (Fig. 3). In the first harvest season (2018), yields were lower (Table 1, Fig. 1), which increased the cost of pruning per kg of fruit. However, there was no difference in cost per kg for the hedge, unpruned, or speed methods in any year. Speed pruning required more time than the unpruned or hedge treatments, but it also resulted in higher yields in 2 out of 4 years. Strik et al. (2022) reported annual and cumulative costs to prune per hectare for 2018–20, and similar costs were required in 2021, thus resulting in total pruning costs (2018–21) of $19,581, $2095, and $3901 per ha for the HB, unpruned (not including the renovation during 2021–22), and speed methods, respectively. Harvest costs were estimated to be $146/ha at each harvest (Strik et al., 2022), with two to three harvests required per year, depending on the treatment. There were fewer harvests in HB-pruned plants, since the fruiting season was more concentrated than for the speed-pruned or unpruned plants.
Interactive effects of year and pruning method on the cost of pruning per kg of fruit harvested in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
Interactive effects of year and pruning method on the cost of pruning per kg of fruit harvested in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
Interactive effects of year and pruning method on the cost of pruning per kg of fruit harvested in the third (2018) through sixth (2021) growing seasons of ‘Mini Blues’ highbush blueberry in Aurora, OR. Pruning treatments were HB: conventional highbush pruning with removal of larger canes along with lateral and whip thinning; Hedge: using a mechanical hedger immediately after fruit harvest in 2018 to remove the top and sides of the bush and left unpruned until renovation in 2021–22; Unpruned: leaving plants to grow through 2021 after which they were renovated; and Speed: removing one or two of the oldest canes from the base of the plant or pruning back to a more vigorous section of the cane. Low growth was removed from all treatments to increase machine harvest efficiency. Error bars represent ±1 se.
Citation: HortScience 57, 10; 10.21273/HORTSCI16703-22
Conclusion
Hedge pruning after fruit harvest was not a viable method for ‘Mini Blues’ in our region because there was insufficient time for the shoot and flower buds to develop afterward. Speed pruning, on the other hand, was a successful method, reducing pruning costs by 85% compared with the conventional method of pruning northern highbush blueberry. Leaving bushes unpruned for 4 years during establishment was also very promising for this small-fruited cultivar; however, a decline in yield from 2020 to 2021 (the fifth to sixth growing season) relative to that with HB and speed pruning, coupled with observed wood aging, reduced berry size, and an extended ripening season, indicated that the plants in this treatment needed to be renovated after six growing seasons. Long-term viability of the method will depend on how long it takes the plants to go from renovation to full production, as well as how long the plants can go without pruning afterward. In all pruning methods, it is important to prune off flower buds at planting and the following winter to delay harvest until year 3 and to achieve enough plant vigor to machine harvest the first crop in this cultivar.
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