Why does my apple tree only fruit every other year?

Fruit buds for next year are formed in early summer while this year's crop is swelling, so the two compete directly. Once a tree carries a huge crop one year, it forms almost no buds and has an empty year, which lets it build reserves and form an enormous number of buds for the following one. Any single event that removes a crop — a frost, a bad pollination season — can flip a tree into the cycle, and some varieties are strongly prone to it.

Step by step

1. Confirm the pattern over three or four years.Biennial bearing is heavy, light, heavy, light. A tree that has simply cropped badly twice for different reasons is a different problem.
2. Identify the variety if you can.Bramley's Seedling, Blenheim Orange, Laxton's Superb, Ellison's Orange and Newton Wonder are all notably biennial. Knowing it is a variety trait tells you to manage it rather than look for a fault.
3. Act in the 'on' year, not the 'off' year.The lever is the size of the heavy crop. In the empty year there is nothing to do beyond normal care.
4. Thin much earlier and much harder than normal.In the heavy year, thin as soon as the fruitlets are large enough to handle rather than waiting for the June drop, and go down to one fruit per two clusters — roughly one fruit every 20cm. This is far more severe than routine thinning.
5. Consider blossom thinning as well.On a strongly biennial tree, remove whole flower clusters at blossom time on part of the tree — commonly about half of them, taking whole clusters rather than individual flowers. It is the most effective intervention there is and the hardest to make yourself do.
6. Feed nitrogen in the heavy year.This is the exception to the low-nitrogen rule. A tree carrying a huge crop needs support, so a balanced feed or sulphate of ammonia in early spring of the 'on' year helps it form buds despite the crop.
7. Water through the summer of the heavy year.Bud formation happens in June and July. Drought at that point is exactly what stops it, so a mulched, watered tree recovers far better.
8. Prune for renewal rather than heavily.Take out some of the oldest, most congested spur systems each winter and leave some young wood unpruned, which spreads flowering across wood of different ages and evens out the cycle.
9. Expect it to take two seasons and to be imperfect.Many trees are moved from heavy-then-nothing to good-then-moderate rather than to a perfectly even crop. That is a success.

Tips

• Trees on dwarfing rootstocks are more prone to it, because they have less capacity to buffer a heavy crop.
• A frost that removes a whole crop is often the trigger event. The following year's enormous crop is what locks it in — so thin that year hard, even though it feels perverse after a year with nothing.
• On a big old tree that cannot be thinned all over, thin one part of it hard and let the rest crop. That branch then flowers in the off year, and the tree ends up cropping something every year.
• Pears and plums do it too, though usually less markedly than apples.

Common mistakes

• Waiting for the off year to intervene — There is nothing to do in a year with no fruit. The whole of the fix happens during the heavy crop.
• Thinning at the normal rate in the 'on' year — Normal thinning is not enough to free up the energy for bud formation. Breaking a biennial cycle needs roughly twice the severity.
• Pruning hard in the off year to stimulate the tree — It responds with vertical growth and no fruit, and the following year's heavy crop happens anyway.

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