Wild blueberries go through a series of developmental stages before producing a harvestable berry. These stages extend over a two-year period, or production cycle, consisting of a sprout and crop year. During these two years, wild blueberry floral buds undergo three dormancy periods before bud burst and bloom, the flowers then need to be pollinated and germinated, and the seeds need to be fertilized. Read this week’s blog to get an overview of these processes and what is needed to get harvestable wild blueberries.

Wild Blueberries - From Dormant Buds to Ripe Berries

This journey starts with the first dormancy period known as paradormancy. During paradormancy the apical meristem, the growth region at the tip of the stem, has dominance over the lower or lateral meristems, where the floral buds will form [4]. During the fall of the first year, or sprout year, short daylength and cold temperatures drive tip-dieback, or apical abortion. Once apical dominance is broken, then the floral buds can form. The buds then enter a second dormancy period known as endodormancy, which helps the buds survive harsh winter conditions [4]. The buds will acclimate throughout endodormancy as temperatures get colder [1]. This stage can be measured by an accumulation of cold temperatures, known as the chilling requirement. The chilling requirement of wild blueberries is approximately 1000 hours of temperatures less than 0°C [7]. The floral buds will then enter the third dormancy period known as ecodormancy, which prevents the buds from opening too early in the spring [4]. The floral buds deacclimate throughout ecodormancy as the temperatures rise [1]. This stage can be measured by an accumulation of daily mean temperatures, known as growing degree days (GDD). It takes approximately 400 GDDs for the start of bloom, with peak bloom happening around 550 GDDs [6].

Endodormant wild blueberry floral buds (John MacDonald ©, 2024)

Once all three dormancy periods are satisfied, the floral buds will start to burst and these can be measured by the five stages of bud burst, known as T1 to T5 [3].  The T1 stage is known as the bud swell stage when the buds start to expand, and a green tip can be seen. The T2 and T3 stages are known as the early bud burst and bud burst stages, respectfully, when the buds continue to swell and the bud scales are separating. The T4 stage is known as the tight cluster stage when flower pedals become visible and elongated. Finally, the T5 stage is known as early flower which is just before bloom when the flowers are still closed. The next stage is floral bloom when the flowers have fully opened.

The stigma, where the pollen lands on the receiving flower, on individual flowers is only receptive to pollen for up to nine days, where pollination within the first four days will result in the highest fruit set [2]. The bloom period for an entire field is typically three to four weeks [2]. Wild blueberry fields typically contain two species of wild blueberries, Vaccinium angustifolium and Vaccinium myrtilloides, and pollination is incompatible between these species. Wild blueberries are also self-incompatible, so wild blueberry flowers require pollen transfer from a different clone of the same species to get successfully pollinated [2].

Pollination can be one of the most limiting factors in wild blueberry fruit production. Pollination is the transfer from pollen from the anther of one flower to the stigma of another flower on a different clone of the same species. Wild blueberries are entomophilic with respect to pollination, meaning that they need an animal pollinator [2]. The conditions during bloom need to be ideal for the animal pollinators to ensure successful pollination. Wild pollinator populations vary between fields, so managed pollinators are used to get even pollination. Therefore, placement and removal of managed pollinators is crucial in providing efficiencies around pollination.

Bumblebee pollinating wild blueberry flowers (John MacDonald ©, 2024)

Once the pollen is successfully transferred from the anther of one plant to the stigma of another clone of the same species the next stage, known as pollen germination, can start. If conditions are favorable for the flowers, germination typically starts two to three hours after successful pollen transfer [5]. During germination, a pollen tube forms into the ovary at the base of the style, or female flower parts, and extends to an individual ovule. The pollen sperm travels down the pollen tube until they reach the ovule. Pollen germination can take three to four days, if conditions are favorable [5]. The next stage is known as fertilization, when the pollen sperm reaches the eggs and nuclei they are fertilized, and the seeds start forming. Once the seeds are formed, the fruit starts developing around the seed which provides protection and helps with seed dispersal. There are different maturity stages of blueberries from pin head, to green, to red, and then to ripe blue harvestable fruit.

For more information on how to tell if a flower was successfully pollinated, please check back in for the next blog on wild blueberries.

Written by John MacDonald, ATTTA Seasonal Apiculturist

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email attta@perennia.ca

References:

[1] Deslauriers, A., Garcia, L., Charrier, G., Butto, V., Pichette, A., and Pare, M. 2021. Cold acclimation and deacclimation in wild blueberry: Direct and indirect influence of environmental factors and non-structural carbohydrates Agricultural and Forest Meteorology, 301–302: 108349. (ACCLIMATION)

[2] Drummond, F. 2019. Reproductive biology of wild blueberry (Vaccinium angustifolium Ait.). Agriculture, 9(4): 69.

[3] Hildebrand, P.D., and Braun, P.G. 1991. Factors affecting infection of lowbush blueberry by ascospores of Monilinia vaccinii-corymbosi. Canadian Journal of Plant Pathology 13(3): 232–240.

[4] Lang, G.A., Early, J.D., Martin, G.C., and Darnell, R. 1987. Endo-, para-, and ecodormancy: physiological terminology and classification for dormancy research, Horticulture Science, 22(3): 371–377.

[5] Noormets, M., and Olson, A.R. 2005. Observations on the gynoecial pathway for pollen tube growth in sweet lowbush blueberry (Vaccinium angustifolium Ait.), Journal of Applied Botany and Food Quality, 80: 6-13.

[6] White, S.N., Boyd, N.S. and Van Acker, R.C. 2012. Growing degree-day models for predicting lowbush blueberry (Vaccinium angustifolium Ait.) ramet emergence, tip dieback, and flowering in Nova Scotia, Canada, Horticulture Science, 47(8): 1014–1021.

[7] Yarborough, D.E. 2012. Establishment and management of the cultivated lowbush blueberry (Vaccinium angustifolium), International Journal of Fruit Science, 12(1–3): 14–22.

Wild Blueberry Phenology in the Sprout Year

Wild blueberries are managed on a two-year production cycle. The reproductive growth in the second, or crop, year relies on the vegetative growth in the first, or sprout, year to support the development of flowers and fruit. Wild blueberry plants are pruned to promote new vigorous stem growth. The emergence of new stems is driven by warm temperatures in the spring and is controlled by apical dominance, meaning the tallest growing point has dominance over the lower growing points, and will grow upright. The time between emergence of these new stems and when tip-dieback occurs in the fall will determine what the stem can support in the crop year. When the tip dies, the buds are released from dormancy and differentiate to floral buds. With greater knowledge on these stages of the blueberry life cycle, we may be able to predict, in the sprout year, how many flowers the plants will produce and how much fruit these may yield.

Wild Blueberry Phenology in the Sprout Year

Wild blueberry plants are perennial, so they survive for many years. When left alone, the stems will grow taller and will branch more every year. Taller stems with more branches use more resources for vegetative growth.  Therefore, reproductive growth will receive less resources on older stems, resulting in fewer flowers, smaller and less fruit. Wild blueberry producers overcome this through managing the plants on a two-year production by pruning stems after harvesting. By pruning the plant, new growth is stimulated from the underground stems, known as rhizomes. The first year of growth will only be vegetative, but this growth will support the reproductive growth (flowers and fruit) in the second year. 

Pruning wild blueberry plants promotes growth of young vigorous stems, with less branching, and will allow the plant to put more resources into flower and fruit development, resulting in more flowers and greater fruit set. Pruning to a uniform height also allows for more efficient use of farming equipment, such as fruit harvesters and pesticide sprayers. Naturally wild blueberries respond to events such as wildfires through stimulated rhizome growth resulting in increased fruit production. Producers mimic these natural events by burning or mowing crops maintaining a two-year production cycle. Flail mowing is currently the most common practice for pruning wild blueberries.

Flail mower pruning wild blueberry field (credit unknown)

When new stems arise from the rhizome, they have apical dominance. This means that the tallest growing point on the plant has dominance over the lower growing points. Therefore, the buds are in a dormant state, called paradormancy [1]. Abortion of the apical growing point in wild blueberry is a prerequisite to flower bud initiation in the sprout year. Therefore, the time between ramet emergence and tip dieback is crucial in terms of establishment of both crop density and biomass that will support the development of flower buds in the bearing year [2]. Tip-dieback is regulated by short daylength, so during shorter days in late summer, the highest growing point dies, the buds are released from dormancy, and floral buds are formed. The next stages of the wild blueberry plant include leaf cessation, endodormancy, and acclimatizing to winter conditions.

Evaluation of wild blueberry stems during emergence and tip-dieback, during the sprout year, may be the earliest point at which we could determine the number of flowers and fruit wild blueberry stems can support, during the crop year. By learning more about these phenology stages and determining what may be the best duration of time between emergence and tip-dieback for best fruit and flower production, we may be able to make earlier decisions on pollination numbers. If an estimation can be made in the sprout year on how many pollination units may be needed in the crop year, blueberry producers and beekeepers could make earlier decisions and use pollination units more efficiently.

In the coming weeks, the blog will include more information on winter dormancy and wild blueberry phenology stages in the crop year to help predict when to place pollination units. 

Written by John MacDonald, ATTTA Seasonal Apiculturist

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email abyers@perennia.ca

References:

[1] Lang GA, Early JD, Martin GC, Darnell RL. 1987. Endo-, para-, and ecodormancy: Physiological terminology and classification for dormancy research. 22.

[2] White SN, Boyd NS, Van Acker RC (2012). Growing Degree-day Models for Predicting Lowbush Blueberry (Vaccinium angustifolium Ait.) Ramet Emergence, Tip Dieback, and Flowering in Nova Scotia, Canada. HortScience 47(8):1014-1021.