For effective transfer of viable pollen its growth and the fertilisation of the ovule, it is essential that, where cross-pollination is required, the two varieties flower at approximately the same period.
- At the very least there needs to be an overlap of several days in the flowering periods of the main and pollinating varieties.
- Scion varieties of apple differ in their dormancy characteristics and this can, on occasions, influence the synchrony of flowering between the main variety and its pollinators.
Dormancy and chilling requirements
The timing of bud break and flower opening is influenced by the dormancy characteristics of the scion varieties and the climatic conditions in late autumn, winter and early spring.
- For effective bud break and flowering, flower buds of apples require a period of chilling to break dormancy, followed by a period of higher temperatures (‘forcing’ temperatures) to enable the flowers to complete their development and open.
- In areas where winters are normally too warm, as in the tropics, apples can only be grown at higher altitudes (e.g. above 1,000 metres).
- For most varieties, 1,000 hours of temperatures of 7oC-8oC or less are required to fulfil their chilling requirement (Kronenberg, 1979 see Further reading).
- If this requirement is fully satisfied, the buds will develop and flower normally, if given suitable ‘forcing’ temperatures.
- Where this requirement is only partially met, then flowering may prove erratic and flower quality be lessened.
- The figure of 1,000 hours at temperatures of less than 7oC is somewhat arbitrary and some geographic locations where apples grow quite successfully (Napoli, Roma, Pisa and Barcelona) often fail to record this total of chilling units.
- In most parts of the UK, the chilling requirements should be satisfied usually by mid January in and around the south east of England.
Global warming in the future could influence the date at which the chilling requirement is satisfied for apples grown in the UK. Further studies are warranted on this possibility and its implications to UK apple yields.
Synchrony of flowering necessary for effective fruit set
Apple scion varieties have been classified, rather crudely, on the basis of their average flowering dates recorded over several seasons. Varieties are often grouped into those flowering early, mid-season or late (Dalbro, 1966; Way, 1978).
- One of the problems affecting growers wishing to choose suitable pollinating varieties is that varieties have been grouped according to their average date of flowering.
- Even when the full range of flowering dates are given, as in the information recorded in the fruit trials at Brogdale, Kent, these ranges do not represent the actual length of flowering periods in every year.
- This means that, whilst sufficient overlap of flowering dates of two varieties may occur in many years, it does not necessarily occur in every year, especially when varieties from early and mid-season, or mid-season and late groupings are planted with the aim of mutual pollination.
Research in Denmark (Grauslund, 1996) has shown that the 10 year average length of flowering periods for 11 apple varieties was 16 days.
- Advice in the Netherlands (Kemp and Wertheim, 1992) is that the average flowering ranges of any two varieties should overlap by a minimum of six days.
- However, the Danish research showed that even when the overlap was 11 days it was not always ideal in every year.
- This is because many of the first flowers to open on a variety are those with the best setting and fruit growth potential.
- If, for instance, a late flowering variety has been planted as a pollinator in a block of a mid-season flowering variety, it is unlikely that many of the flowers opening first on the mid-season variety will receive adequate pollination in sufficient time to set fruits.
- If the mid-season variety also produces flowers with short EPPs (like Cox), the problem is exacerbated.
Models for predicting apple flowering dates
Attempts in the Netherlands to develop a model for predicting flowering dates of apple were only partially successful (Kronenberg, 1983; Kronenberg, 1985) and further work is needed under UK conditions.
Research at Long Ashton Research Station also endeavoured to develop a model for apple fruit bud development and hence time of flowering (Landsberg, 1974).