Blackcurrant Winter Chill through August 2026

Nelson has had a good chill winter!

2025 and 2026 temperature data have been taken from Riwaka, as the Appleby data used previously became unavailable.

Canterbury has done alright for winter chill.

The question might be why 2026 hasn't been the highest chill year Lincoln has had in the recent past, given the amount of frosty nights.

The chill accumulation model doesn't count hours under zero.  This is because when plant tissue is frozen, it doesn't have the metabolic processes ticking along that are required for the plant to "count" cold time.  

Really, we model biology with numbers, but the biology doesn't always behave precisely like the model.  Work done with blackcurrants to work out a more precise chilling model for this species found that each cultivar counted chill slightly differently.  That's way too complicated to be of practical use, so we use the Richardson model which we found to represent the bud break better than a simple count of hours between 0 and 7.  

For those who still like to know the hours 0-7 tally, Riwaka got 1600 and Lincoln got 1680, surprisingly similar.  

In the Richardson model, hours between 2 and 9 have the fastest chill accumulation, hours 0-2 and 9-12 accumulate a bit slower, no hours are added under zero, and we actually lose chill if temperatures go above 16C.

After the 2023 winter, we did some detailed assessment of bud break across Canterbury and Nelson.  We came up with the following approximation for chill requirement by variety for optimal flower bud development:

Of course, the further south you go, the more chill a site tends to get.  Also from that 2023 study:

In 2023, at the end of August when Lincoln had 1633 Richardson Units, Timaru had nearly 2000.  Clearly locations further south had a chill advantage on a mild winter.  

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Interestingly, we tried to get some cuttings to root this winter.  Cuttings taken in June did not root or do anything, they just sat there pouting.  We put the plants in the chiller and tried again a month later, and hey-presto, rooting and bud break happened readily.  I didn't know that ability to root is dependent on winter chill, but apparently it is.  

At any rate, the level of winter chill is not expected to be limiting to blackcurrant bud break or flower development in either Nelson or Canterbury this spring.

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