What impacts nectar production?

We know that weather during bloom is one of the biggest factors that determines blackcurrant yield in a given year. A big piece of this is pollination success.

Last May there was an article in Northwest Berry Foundation’s Small Fruit update (31 May 2023) with a lot of detail about nectar flow and weather conditions. The whole article can be accessed here. It’s a good read.

A key paragraph is excerpted here (by authors Monica Borghi and Lisa Wasko Devetter)

“Nectar secretion is impacted by flower age and time of the day. For example,
younger flowers produce more nectar than older flowers and secretion is
greater in the morning and tends to be diminish from early-afternoon and
finally halts at night. If flowers are not pollinated, nectar secretion continues
for one or two days, but it stops as soon as pollination occurs and ovules are
fertilized. Fertilization may take several days after Pollination.”

This was written about blueberries but is equally applicable to Blackcurrants. Geoff Langford has found that individual blackcurrant flowers are receptive to pollen for at most 3 days. Pollen is most effectively moved by a pollinator, typically a species of bee, because the pollen itself is somewhat sticky and not easily blown around in the wind. Once pollen lands on the stigma, it still has a couple days journey (pollen tube growth) down the stigma before it gets to the ovary and fertilizes the ovule.

Blackcurrants are special in that once the pollen gets to the stigma, the pollen tube will continue to grow even if we get cold weather soon after. Let’s count our blessings; plums apparently need warm weather to continue until seed set.

There are some fields of Blackadder and Lewis with a tremendous bloom this spring. The growing conditions last March/April were good for flower bud initiation, with adequate water and mild weather, and healthy blocks have taken advantage of this.

Now that we have blossoms, we want good pollinator activity. Back in 2024 Barry Donovan did some blackcurrant pollinator work, and figured that on a nice warm day without too much wind, if you see 1 honeybee/meter of row, you’ve got enough pollinators working. Bumblebees apparently work 4x harder, so proportionally fewer bumbles are required for the same pollination service.

It’s normal to have only a percentage of the flowers successfully produce a fruit. In fact, if 60% of the flowers turned into a fruit, that would be pretty typical. Thus the parting conclusion from an Estonian study titled “genotype and climate conditions influence the drop off of flowers and premature berries of blackcurrant:”

“Yield depends considerably on the successful initiation of flower bud differentiation, which leads to abundance of flower clusters. In cases of abundant flowering, the drop off of flowers and young berries does not have a considerable influence on the yield.”

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