Flowers on apple trees differ in their potential to set and retain fruits and these differences are usually referred to as differences in flower quality. Flower quality is the ability of a flower to set and carry a fruit. The most common measurement of flower quality is the Effective Pollination Period or EPP.
Types of flower buds
Spur buds: Formed on spurs and short terminal shoots. These begin their initiation shortly after fruit set in the preceding season.
Axillary buds: Formed in the lower (basal) leaf axils on extension shoots later in the season than spur buds, usually as shoot growth slows down in mid to late summer.
The aim must be to encourage the rapid production of high quality flowers on the spurs and short terminal shoots of young trees. Only with a few varieties, such as Gala and its sports, can axillary flowers be relied upon to contribute significantly to cropping on young trees. Even with Gala, axillary flowers are only useful if they are produced on strong wood of approximately 10 mm diameter.
Factors influencing flower quality
Anything that reduces the production of resources by the tree or diverts them away from the developing flower buds will reduce their quality. An imbalance of hormone levels in the tree will also affect flower bud development. The main factors influencing flower quality are:
- Crop load (too many fruits on the tree): Reduce flower bud numbers in the following year especially in varieties with a tendency to biennial cropping. Thin as early as possible to the correct crop load.
- Too much vigour (length of shoots and numbers of shoots) will create an imbalance in the tree and reduce the quality of flower buds: Adopt practices to reduce vigour.
- Lack of light and shading will significantly reduce flower bud quality: Prune to improve light penetration into the canopy.
- Poor leaf quality; poor nutrition and disease (especially mildew) will impact directly on flower bud quality: Mildew should be controlled all the way through the season to prevent infection in developing buds which will carry over to the next season. Nutrient levels should be checked by visual and analytical techniques. It is particularly important to avoid excess nitrogen levels and maintain adequate phosphorus and trace elements to ensure good flower bud development.