Some species of fruit tree - apples and pears are the prime culprits - can get into the habit of alternating heavy crops one year with carrying little or nothing the year after. next. Some varieties - for example, Blenheim Orange, Bramley and Laxton’s are natural alternate croppers Superb’ but it can happen to just about any other apple or pear as well. In these cases, the usual causes are related to the weather, to watering and/or to the condition fo the soil.
What is Biennial Fruiting?
When a fruit tree produces an excess of flower buds (and a wonderful show of blossom) one year, then the resulting large crop exhausts the tree so it produces no (or few) flower buds the next year and therefore little or no fruit while it rests/recovers.
Reasons for Biennial Fruiting
Apart from varieties that fruit every other year naturally, biennial fruiting is usually provoked when a fruit tree does not get enough water or is undernourished. The other common reason is that a heavy frost in spring can make the blossom unviable. To compensate, the tree flowers and fruits extra heavily the next year and the cycle begins. The "Beast from the East" of 2018" may well cause biennial fruiting in trees that previously cropped every year.