The number of floral buds initiated can be significantly influenced by the crop load set on the trees at the time of initiation and in the few weeks following.
- Flower quality, as well as flower abundance, in the subsequent season is strongly influenced by crop loading and the time of thinning.
- Research conducted at Wye College in Kent (Buzzard and Schwabe, 1995) showed that Cox’s Orange Pippin trees, that were cropping very heavily in the subsequent spring, developed smaller flowers which had shorter EPPs and lower fruit set than trees with lower crop loads in the previous season. Anatomical differences in the stigmas of flowers from trees of the two different crop loads were demonstrated.
- Flower bud numbers and their quality can also be influenced by the dates of fruit harvesting in the previous season.
- Research conducted at Long Ashton Research Station in the late 1970s (Williams et al., 1980) showed that delaying harvesting of Bramley’s Seedling fruits reduced flower numbers in the following spring and also the ability of these flowers to set fruits (i.e. their quality).
- Picking in late August gave the best return bloom but reduced total yields significantly; picking in late October reduced flowering number and quality significantly.
- However, in the year of this experiment, the autumn was atypically mild and shoot growth continued very late in the autumn.
- For optimum flower bud quality trees should be thinned early in the previous season to optimum crop loads.
- Fruit harvesting should not be delayed too long as late picking may reduce flower quality on varieties such as Bramley and Braeburn.
Tree pruning/training and flower bud position on the tree
Severe branch pruning in the winter and early spring reduces flower quality as well as floral abundance on apple trees.
- The pruning stimulates vigorous shoot growth which often continues late into the autumn.
- This shoot growth competes strongly with the spurs and other sites of floral bud production for photosynthates and minerals
- The consequence is the production of limited numbers of floral buds, most of which are of relatively poor quality.
- Light pruning generally produces the best flower bud quality on spurs and short terminal shoots.
- Pruning that exposes the sites of floral bud initiation to adequate light is most beneficial.
Bending branches towards the horizontal, or below the horizontal, reduces the rate of extension shoot growth on the branch.
- This in turn results in photosynthates being partitioned more towards the spurs and other sites of flower initiation and away from extension shoot growth.
- These changes have the effect of improving the quality of flowers produced.
Flowers produced on one-year-old extension shoots produced in the previous season (axillary blossoms) tend to have shorter EPPs and hence lower quality than flowers formed on spurs or short terminal shoots.
- These differences are partly attributable to the time of flower initiation in the previous season.
- Flowers on axillary shoots are initiated much later than flowers on spurs or short terminals and develop much less by the onset of dormancy in the late autumn.
- Flowers formed on spurs situated on two- or three-year-old wood often produce better quality flowers than spurs formed on older wood.
- Flower bud quality is usually aided by light, rather than severe winter pruning.
- Summer pruning can aid flower bud quality on spurs by removing shading.
- However, where summer pruning entails heading back extension shoots, this may reduce flower bud quality by stimulating late shoot growth.
- Shoot bending improves flower bud quality, but should not be carried out to excess or the optimal balance of new renewal shoot growth and flower bud production will be lost.
- Pruning or training, which encourages the production of more one-year-old shoots and the associated axillary blossoms, is generally detrimental.
- Only on tip-bearing varieties can such techniques be warranted.
Exposure of the sites of flower initiation to adequate light
Research at East Malling and in several other countries has demonstrated the importance of good light exposure to the production of high quality flower buds.
- This is particularly relevant in climates such as those experienced in the UK.
- Growers should prune and train their trees so as to ensure good light penetration into all parts of the tree canopy.
- A combination of winter and summer pruning is recommended.
Winter chilling and satisfying the dormancy requirements
Apple trees are thought to require an accumulation of an approximate number of hours during the dormant period when temperatures are above freezing but less than 8oC. Figures of 1000 to 1500 hours are often given in the literature.
- Lack of sufficient chilling units is unlikely, on the above evidence, to be a problem in UK apple orchards.
- In most seasons sufficient chilling units have been accumulated by sometime in February.
- Recent studies have been undertaken in Israel, where winters are warm and spring temperatures very hot.
- These have shown that sprays of urea (8%) applied at the stage of green bud swell advanced initial bud break, elevated the total number of flowering buds and significantly increased subsequent yields.
- However, in climatic conditions more suited to apple production, similar benefits of urea sprays have not been recorded.
- Climatic conditions in the UK should provide sufficient chilling units to satisfy the dormancy requirements of traditional apple varieties.
- Problems of insufficient chilling could be experienced if growing within protective structures is contemplated.
Spring temperatures prior to bud burst
Although UK apple trees are thought to receive adequate chilling units to satisfy their dormancy requirements, it has been shown that unfavourable temperatures in the spring, prior to bud break, can have a negative effect on subsequent fruit set and yield.
- These negative effects were not caused by very low temperatures and frost damage to blossoms, as might have been expected, but by atypically warm temperatures in February and March and to a lesser extent April.
- The authors of this work, which was mainly carried out by scientists based at East Malling, constructed a model which predicts yields of Cox based on these early spring temperatures.
- Subsequent work in the USA showed that the yields of many of the varieties growing in New York State could be predicted using a similar model.
- Data collected over a number of years indicates that the fruit set of Cox (and its clones) remains high for this variety (30%) in years when the average maximum temperature in February, March and April was no more than 9oC.
- Average temperatures of 10oC or 11oC were associated with much reduced levels of set (<10%).
- The reasons for the negative effect of these early spring temperatures on flower quality are not yet fully understood.
- Subsequent experiments conducted using semi-controlled environments showed that trees experiencing spring temperatures, such that they received 106 day degrees above ambient, set fruits much more poorly than trees receiving 40 day degrees below the ambient (Miller et al., 1986).
- Fruit setting percentages were lower in the ‘warmed’ trees irrespective of how soon after opening the flowers were pollinated.
- The EPPs were also very much poorer in the flowers of these trees.
Growers should take note of the average maximum temperatures in February, March and April in their Cox orchards.
- Where temperatures are higher than desired average maxima (i.e. 10oC or higher), they are advised to intensify their efforts to secure good pollination and flower fertilisation.
- This can be achieved by supplementing bee populations, providing increased shelter and boosting supplies of compatible pollinating varieties.
- Where temperatures are of the desired average maximum, or lower, growers will need to consider implementing appropriate thinning strategies.
Overcoming biennial bearing
It is fortunate that most of the commercial dessert and culinary varieties of apple now grown in the UK have only weak biennial bearing tendencies.
- This is in contrast to several of the popular varieties of cider apples (Tremlett’s Bitter, Vilberie, Reine de Hâtives and Néhou) and to some of the older dessert varieties, such as Blenheim Orange, Miller’s Seedling and Laxton’s Superb, all of which suffered from this problem.
- Amongst popular dessert varieties that are currently grown, only Elstar shows pronounced biennial bearing tendencies.
- Biennial bearing varieties characteristically produce abundant flowers in what is called the ‘on’ season and set heavy crop loads.
- In the subsequent season, the ‘off’ season, they produce few flowers and fruits.
As the initiation of what are likely to be the best quality flowers begins only 3 to 6 weeks after petal fall, the importance of early blossom (flower) thinning is obvious. Whilst the more popular thinning of fruitlets at the 12 mm fruitlet diameter stage will give good benefits in improving flowering in the subsequent season, even better flower quality should be achieved by earlier blossom thinning.
It is interesting to note that the variety Cox and its clones is described as having slight tendencies to biennial bearing in several countries (e.g. New Zealand).
- If the UK climate continues to get warmer, resulting in heavier crops that are not thinned correctly, biennial bearing may become more of a problem.
The usual methods employed to overcome biennial bearing are:
- Pruning of flower spurs in the winter prior to the ‘on’ year.
- Thinning of flower using ATS.
- Thinning of fruitlets early in the season.
Other strategies that have been tested in research trials involve:
- Treatments with plant growth regulators.
Pruning of flower spurs in the winter prior to the ‘on’ year
The strategy employed here is to remove surplus flower clusters, usually spurs, so that excessive numbers of fruits cannot set in the subsequent spring.
- Knowledge of the variety is essential and the correct degree of spur pruning is only achieved with experience.
- It is also important not to prune the trees so severely so as to induce excessive regrowth in the following year.
- The aim is to achieve the optimum balance of fruit set and growth.
- Prune away excessive spurs in the winter prior to an ‘on’ year.
Treatments with plant growth-regulating chemicals
Most of the growth regulating chemicals that have been tested for their efficacy in reducing the problem of biennial bearing have brought about their effect by flower or fruitlet thinning. However, there are a few isolated cases of other chemicals inducing favourable responses.
- Trials conducted at Long Ashton Research Station in 1977 showed that sprays of the chemical bromouracil (50-75 ppm) to biennial-bearing cider apple varieties in the ‘off’ year reduced biennial cropping.
Sprinkling apple trees in the spring with water
The evaporative cooling effect of water can be used to delay the flowering of apple trees in the spring. Research conducted at East Malling in the 1970s showed that, by using overhead sprinkler systems, blossoming of Cox’s Orange Pippin could be delayed by up to 14 days (Hamer and Boustred). In further research, the authors attempted to construct a computer model to evaluate the effects of evaporative cooling on apple bud dormancy (see Hamer and Boustred, 1980).
- In these trials, Cox trees were misted with water via overhead sprinklers when ambient air temperatures exceeded 4.5oC.
- The misting commenced in mid February and continued until mid April.
- The buds were cooled by as much as 5oC and this induced a delay in flowering of eight days in comparison with non-misted controls.
- Fruit setting potential (as determined by hand pollination) was significantly improved on the misted trees and these trees produced approximately 30% more yield.
- The misting was shown to have beneficial effects on ovule maturity in the flowers, by reducing the proportion of flowers with over-mature ovules.
- Care is needed when using this strategy.
- However, an application of excessive misting, whilst maximising bloom delays, increases the water content of the buds, such that they are more, rather than less, sensitive to frost damage (Hamer and Newman, 1981).
Further research may be warranted to examine misting techniques to delay flowering times of apple varieties.