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Now that we are coming to the end of the winter, it is time to evaluate how the winter has gone and consider timing of dormancy breaking sprays such as hydrogen cyanamide (HCN).
To optimize the timing of HCN sprays, it is necessary to understand the dormancy and bud break process.
Dormancy can be considered to have two phases.
The first phase commences at or close to leaf fall, or when the first frost occurs, and requires a period of chilling to break down the growth inhibitor present which prevents bud break. Once sufficient chilling has occurred, the buds begin to come out of dormancy.
At this stage they begin to lose their winter hardiness, and will show a surge in bud size. This stage occurs well before actual bud break. This second stage is heat driven and the rate it proceeds depends on how rapidly temperatures rise in the spring.
It is generally considered that buds need to have moved into this second stage of dormancy to be responsive to HCN sprays.
Working on this theory, we can postulate that buds will become responsive to treatment once they have had a certain period of winter chilling, followed by a period of warming after completion of the initial chilling requirement. Some overseas data suggests that at least 70% of the winter chilling period is necessary before buds become responsive.
Anecdotal evidence here indicates that apple response to HCN shows that there is slight variation between seasons with mild winter years showing better response to later application than earlier application.
As dormancy breaking sprays in pipfruit have their greatest value in compressing the bloom period, the success of a spray can be judged by whether or not bud break of the later flower, such as 1 year wood lateral flower bud, is brought forward to coincide with the spur flower. Applied too early, only the mature, older spur buds are stimulated. If far too early, even these buds fail to respond. With early application, duration of bud break and flowering stages may even be extended.
In recent seasons, we have tried to forecast the application window using winter chill unit calculations and once a set number of chill units accumulated, then using head unit accumulation. Observation of response in previous years suggests that for Hawke's Bay there has been good response over the period 10 to 20 August in most years for Royal Gala in regard to synchronizing flowering of lateral bud one year wood with two year and older spurs. Where early fruit is the primary objective and the fruiting wood on the trees is largely two year and older spurs such as found on mature dwarf rootstocks, there have sometimes been good responses from early August sprays.
Estimating winter chill units in mild winter climates is a tricky business, because these climates often have relatively long periods of time when temperatures are above 70C and daily maximums even above 200C.
This makes the long established hours below 70C method of estimating winter chilling very unreliable and clearly unsuited to lowland North Island locations, because in most years chilling levels measured in this way are well under the published minimum requirement for the fruits we grow, yet these fruits perform well for us.
The Utah or Richardson chill unit model which assumes chilling can occur over a wider temperature range is an improvement on the older hours below 70C model, but has been shown to be somewhat unreliable in mild climates which have long periods of temperatures above 160C because of the negative scoring it allocates to higher temperatures.
Because of the unreliability of these chilling models in locations with mild winter climates, we have been using a model developed in South Africa known as the "daily positive chill" (DPC) model. This is similar to the Richardson model without the negatives.
For Hawke's Bay, the DPC units accumulated over the last four years have been much more uniform than either the hours below 70C model or the Richardson model. The DPC unit variation for chilling up to 25 July showed 12.5% variation between highest and lowest years, whereas the other two methods were both in the order of 40% variation.
On 31 July this year, Hawke's Bay had 985 DPCU. DPCUs for 1998 to 2001 on 31 July were 1000, 938, 1008 and 1010 respectively. On this count we are only just behind the average and have had better chilling than in 1999.
For HCN application, we believe a minimum of 1,000 DPCU are needed, and for uniform compressed bud break probably 1,000 DPCU plus 15-30 Growing Degree Days base 100C should be accumulated.
Weather conditions at the time of HCN application are also thought to affect response. Several days of good, warm weather at the time of application is said to enhance response.
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