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Highbush Blueberry Cultivar Trial in Northwestern Washington

Volume 5 Issue 9

Carter DeGraw (Summer Research Assistant), Sean Watkinson (Technician), and Lisa W. DeVetter (Assistant Professor, Small Fruit Horticulture)
Washington State University (WSU) Northwestern Washington Research and Extension Center


New cultivars of highbush blueberry (Vaccinium corymbosum) are being developed by public and private breeding programs outside of Washington State.  While successful cultivars important to the northwest Washington blueberry industry have originated from breeding programs outside of Washington, cultivars still need to be evaluated for adaptability and suitability within this important production region.  Furthermore, cultivar trials can identify potential management issues for certain cultivars and begin the process of identifying approaches to mitigate such issues.  The Washington State University Small Fruit Horticulture program has begun an initial cultivar trial at the Northwestern Washington Research and Extension Center in Mount Vernon, Washington.  The objective of this trial is to begin the process of evaluating new cultivars developed by public and private breeding programs within the unique climactic conditions of northwest Washington.  A secondary objective is to begin the process of extending this information to the grower community and cooperative breeding programs, so that they may have additional information regarding the performance of new cultivars.  This report presents data from the first harvest of this trial, as well as observations regarding establishment and productivity.


This cultivar trial was established in Spring of 2015 as a randomized complete block design, with three plant plots of each cultivar replicated three times.  Cultivars included in this study are: ‘Top Shelf’, ‘Blue Ribbon’, ‘Calypso’, ‘Clockwork’, ‘Last Call’, ‘Cargo’, and ‘Baby Blues’.  ‘Duke’, ‘Bluecrop’, ‘Liberty’, and ‘Elliott’ were also grown as experimental controls.  In 2016, we planted ‘Baby Blues’ and plan to continue to add more cultivars as they become available from breeding programs.  Blossoms were removed in the first year of growth to allow adequate plant establishment.  Plants were drip irrigated and annually fertilized with 90 lbs N/acre in 2015 and 2016 (30 lbs. of dry N applied in the spring, a few weeks prior to bloom, and 60 lbs N/acre applied throughout the growing season via fertigation).  A woven-polypropylene fabric (“weedmat”) was applied over the drip tape and provided seasonal weed control.  The fabric was removed in the fall and winter months to reduce the incidence of vole (Microtus spp.) damage.  Netting for bird control was not provided in 2016, but bird control will be provided in future years as we had moderate bird depredation.

Plants were hand harvested in 2016, excluding ‘Cargo’ due to poor growth that we will attempt to address through modified pruning.  ‘Baby Blues’ was also not harvested, as blossoms were removed during its first year of growth.  Total yield per plant was determined from fresh berry weights and average berry weight was estimated from a random 30-berry sample per block.  The 30-berry sample of berries were cooled and analyzed for total soluble solids (TSS), pH, and firmness within two weeks of harvest.  TSS (measured as %Brix) and pH were measured from undiluted, extracted juice with a digital refractometer and pH meter (HI96801 Refractometer and HI84532 Mini Titrator and pH meter, respectively; Hanna Instruments, Woonsocket, RI).  Firmness was also measured on a subset of cultivars using a FirmTech II firmness tester (BioWorks, Inc., Wamego, KS).  Due to software issues, not all cultivars were measured and only partial firmness data will be presented in this report.  Data were analyzed using RStudio (RStudio Team, 2015). Tests of significance were done at α ≤ 0.05.

Results and Discussion

‘Top Shelf’ had the greatest yield in 2016, followed by ‘Blue Ribbon’, ‘Calypso’, ‘Duke’, ‘Clockwork’, ‘Last Call’, and ‘Elliott’.  Yields were lowest for ‘Bluecrop’ and ‘Liberty’.  A large amount of variability was observed this harvest year, which we attribute to heavy vole damage in the winter and spring of 2015/2016 (roots that were damaged weakened the plants and subsequent growth) and the drought of 2015.

Dates of the first harvest, average berry weight, TSS, and initial juice pH are presented in Table 1.  Berry size was largest in ‘Top Shelf’ and ‘Calypso’, followed by ‘Blue Ribbon’.  ‘Duke’, ‘Clockwork’, and ‘Elliott’ had the smallest berry size.  Berry size was variable among ‘Last Call’ and ‘Liberty’, but was overall intermediate and tended to be greater than ‘Bluecrop’.  TSS, an indicator of sweetness, was greatest in ‘Clockwork’ at 15.7 %Brix.  Interestingly, an unofficial taste test sponsored by the Small Fruit Horticulture program at the WSU Northwestern Washington Research and Extension Center indicated this cultivar was preferred by 63% of the participants [followed by ‘Top Shelf’ at 17%, ‘Calypso’ at 17%, and ‘Bluecrop’ at 3% (no additional cultivars were evaluated due to lack of fruit at that time); N=30 participants].  Participants noted ‘Clockwork’ was comparably sweeter and had a distinct blueberry flavor.  TSS for ‘Last call’ was similar to ‘Clockwork’ at 15.2 %Brix, while ‘Top Shelf’, ‘Calypso’, ‘Liberty’, and ‘Elliott’ were all slightly lower.  ‘Duke’, ‘Bluecrop’, and ‘Blue Ribbon’ had the lowest TSS, averaging 13.0 to 13.1 %Brix.  Acidity, as measured by initial juice pH, was greatest in ‘Calypso’, ‘Last Call’, and ‘Elliott’.  In contrast, acidity was lowest in ‘Clockwork’, ‘Duke’, and ‘Bluecrop’.  ‘Top Shelf’, ‘Blue Ribbon’, and ‘Liberty’ all had intermediate pH ranging from 3.35 to 3.49.  Firmness data was not collected from all cultivars in this trial due to software issues.  However, it was observed that ‘Top Shelf’, ‘Calypso’, and ‘Last Call’ had comparably firmer fruit than ‘Clockwork’, ‘Liberty’, and ‘Elliott’.  Future trials will compare firmness across all cultivars.

This report presents preliminary information of new cultivars that may be suitable for commercial blueberry production in northwest Washington.  As this trial continues, we will modify our management practices to better suit these new and unique cultivars.  This will allow more refined assessment of these new cultivars and their potential viability within the northwest Washington blueberry industry.  Overall, our general observations were consistent to those reported by public and private breeders.  We did make several observations about the growth of these new cultivars that will influence pruning and other management practices.  For example, ‘Cargo’ produced few canes but many branching laterals.  As such, we will prune to 3 to 4 of the strongest canes for this cultivar.  Additionally, ‘Clockwork’ has a very concentrated harvest and fruit had a pronounced calyx scar.  Growth was upright and we have been advised by Fall Creek to prune this cultivar lightly, with 1 to 2 canes removed from the base of each plant.  ‘Blue Ribbon’ had a sprawling growth habit and pruning will focus on encouraging upright growth.  We have also found trellising to be helpful for ‘Blue Ribbon’.  The fruit size of ‘Top Shelf’ was large, but has the potential to be larger through harder pruning given the high vigor observed in this cultivar.  Growth was also vigorous in ‘Last Call’ and this late season cultivar had more favorable fruit quality attributes when compared to other late season cultivars included in this study.  In conclusion, many of the evaluated cultivars demonstrate promise for the northwest Washington blueberry industry.  Continued research will provide further information regarding the adaptability and role these new cultivars will have in this important industry.


We would like to thank Jim Lovelace and Dick Mombell of Fall Creek Farm & Nursery, Inc., for provision of plants. Additionally, we would like to thank Dr. Chad Finn, United States Department of Agriculture Agricultural Research Service, for provision of plants.  More information about cultivars is available through Fall Creek Nursery.

Graph depicting average yield per plant, 2016

Figure 1. Average yield per plant, 2016.  Bars represent standard error and letters indicate statistical differences, with the same letter indicating yields are not different at P ≤ 0.05.  Yield of ‘Baby Blues’ and ‘Cargo’ is not presented, as these cultivars were not cropped in 2016.

Table including harvest date, average berry weight, soluble solids and ph of blueberries

Table 1. Harvest date, average berry weight, total soluble solids, and pH of blueberries grown at the Washington State University Northwestern Washington Research and Extension Center, 2016.