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Almost all apple varieties show a tendency towards biennial bearing. Certain varieties show very strong biennial bearing tendencies. These include Cox's Orange, Braeburn, Pacific Beauty, Pacific Rose, Southern Snap, Red Delicious and Fuji. As the 2000/01 harvest year was an "off" crop year, we will be going into an "on" crop year this spring for many blocks of these biennial bearing-prone varieties.
When you are going into the "on" crop year, the trees are heavily loaded up with very strong fruit bud which, if retained and allowed to set, will result in a massive overcrop of poor quality fruit. This heavy crop will also drain the tree of critical carbohydrate reserves and lead to weakened flowering, or in severe cases of biennial bearing, no flower at all in the following "off" crop year.
Your pruning strategy needs to take cropping potential and flower load into account.
In the winter pruning prior to the "on" crop, the strategy is to use it as an opportunity to clean out the trees of surplus branches and unnecessary structure. For instance, side branches which are blocking ladder or picker access into the tree can be cleaned out to improve work and light access.
This is also the year to do detailed pruning of spurs and weaker fruiting laterals.
As a rule, apples carry their best fruit on well-positioned spurs on two to four or five year old wood, and have particularly poor fruit on the lateral buds of weaker one-year old wood. Stronger one year old wood tends to be more vegetative and less likely to have fruitful lateral buds. Fruitful lateral buds on one-year wood can be differentiated from non-fruiting lateral buds by bud size. Leaf buds which are likely to become flower buds next year are small, more triangular in shape and very flattened against the stem. In contrast, flowering buds tend to be more elongated and not so flattened. By selectively pruning out fruiting one year wood and leaving behind non-fruiting one year wood when pruning prior to the "on" crop year, it is possible to improve your chances of having good flower for the next "off" crop year.
With many biennial blocks, there is a mixture of "off" and "on" trees. In these situations, leaving the detailed pruning until the early stages of bud break will enable the "on" and "off" trees to be readily recognized and pruning adjusted according to flowering density.
In the "off" crop year, your strategy has to be to retain as much flower as possible to try and maintain fruit numbers close to the optimum cropping level. With this objective, you do not want to prune much out at all in the winter prior to the "off" crop.
One strategy worth considering with mature biennial bearing orchards is to only winter prune prior to the "on" crop season. Incidentally, many years ago East Malling Research Station in England completed a study comparing alternate year pruning with pruning each winter. The results they obtained showed little difference in cropping between the two pruning strategies, but a substantial reduction in pruning costs.
In these biennial bearing orchards, the "on" year pruning of the tree structure needs to be aggressive enough to give you the option of carrying out a minimal pruning regime the following year so that you can conserve most of the "off" year flowering potential.
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