The most common measurement of ‘flower quality’ is the ‘Effective Pollination Period’ or EPP (Williams, 1970). Trials conducted in the 1970s at Long Ashton Research Station (Stott et al.,1973) showed that short EPPs were a common cause of poor fruit set on the early fruiting variety Discovery. Although 9% of flowers set fruits if pollinated on the day of opening (anthesis) only 1% set fruits if pollinated two days after opening.
The EPP is measured by protecting flowers from external uncontrolled pollination by enclosing them in large paper or other bags at the balloon stage of development. After removing the bags (temporarily) some of the flowers are pollinated on the day they open (anthesis). Further flowers are pollinated one, two, three, four or more days later. After each controlled pollination, the bags are replaced until after fruit set.
- Flowers that set fruits only when pollinated on the day of opening, or not at all, have low EPPs and are classed as poor quality.
- In contrast, flowers which can be pollinated 4 or more days after opening and still set and retain fruits have long EPPs and are classed as higher quality.
- In UK conditions, good quality apple flowers should, under orchard conditions, have EPPs of three to five days.
The reasons for these differences in EPPs and flower quality are not fully understood. Most theories relating to apple flower quality focus on differences in the ovules of the flowers.
- Poor quality flowers are thought to have ovules which either have a short life or which develop out of synchrony with the rest of the flower.
- An example of this might be flowers where the ovule becomes receptive to the pollen tube in advance of flower opening but which has begun to degenerate by the time that pollen eventually reaches it.
Flowers that are initiated late in the previous summer, such as those formed as axillaries on one-year-old wood, develop only partially by the autumn and are usually, but not always, of poor quality, with short EPPs in the subsequent spring.
- In this instance incomplete or shortened flower development may explain the quality differences.
- This means that the window of time for effective pollination and fruit set with axillary flowers is much shorter and the risks of poor fruit set much higher than with spur or terminal flowers.
Climatic conditions have a strong influence on apple flower quality. and this is discussed more fully by Williams (1970) and Abbott (1984).
The aim of the apple grower must be to encourage the rapid production of high quality flowers on the spurs and short terminal shoots of young trees.
- With many of the newer varieties, such as Gala, Braeburn, Kanzi, Jazz and Rubens, axillary flowers can be relied upon to contribute significantly to cropping on young trees.
- These axillary flowers are more likely to set and produce good size fruit if they are produced on strong wood of approximately 10 mm diameter.
Growers should strive to produce flowers of high quality with Effective Pollination Periods of three days or more. Flower quality is improved by:
- Minimising the spring and summer use of nitrogenous fertilisers, which encourage strong vigorous shoot growth and extended growth into the late summer and autumn.
- Limiting strong shoot growth by branch bending and brutting techniques especially in Cox and Bramley.
- Improving light penetration into the tree canopy.
- Optimising crop loading by appropriate flower or fruitlet thinning techniques.
- Not delaying fruit harvesting too late in the season.