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 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
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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.