Biennial bearing

If gibberellins produced in seeds are the main cause of flower inhibition and hence of biennial bearing in fruit trees, we might expect to find differences in gibberellin production between strongly biennial and more regular cropping varieties. 

To sum up - it would seem that reduction of the on year crop by blossom or fruit thinning with growth regulators remains the most practical way of controlling biennial bearing in apples but we still need more reliable and consistent fruit thinning agents. 

Yield early cropping, moderately high yield, because of risk of biennial bearing... 

Thinning means the removal of excess blossoms or fruits. The aims of thinning are firstly to prevent biennial bearing and secondly to improve internal and external fruit quality. 

One of the most important aims of thinning is to prevent biennial bearing, i.e. to maintain regular fertility. 

Biennial bearing can also be caused by a late frost, which destroys all the flower buds in spring. Biennial bearing is a common problem in apples, pears and plums, but is virtually unknown in cherries. 

Fig. 4.7. Biennial bearing is one of the biggest problems in organic fruit growing.

Excess buds or blossom clusters are removed before flowering using the thinning machine. In this way the strain on the trees is reduced at an early stage (breaking the biennial bearing cycle). 

Frequently only one fruit is left per blossom cluster. It is better, however, to remove some blossom or fruit clusters completely (because the tree is then likely to produce flower buds at that point) and to leave 2 or 3 fruits on the remaining clusters. In this way the shoots from which all clusters have been removed have the assimilates available for producing flower buds. Nor is there any inhibition of flower bud production by phytohormones (which are synthesized in the seeds of the young fruitlets). Biennial bearing can thus be reduced by hand thinning an at early stage. 

March-April, if few flower buds are produced (biennial bearing), in cultivars with big fruit. 

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