Why Does My Plum Tree Only Crop Heavily Every Other Year?

The classic pattern of biennial bearing — bumper crop one year, little or nothing the next — is a recognised problem with several fruit tree species, and plums are susceptible. A year of very heavy cropping depletes the tree's resources to the point where it cannot form adequate flower buds for the following spring, resulting in an off year. The cycle then repeats. Understanding the mechanism makes it clear why the traditional management fix is so effective.

Why biennial bearing develops

Flower buds for next year's crop form on plum trees from early to midsummer — typically June to July — at the same time as the current year's fruit is swelling and beginning its major growth phase. In a heavy-cropping year, the developing fruits compete intensely with flower bud initiation for the tree's carbohydrate supply. When there is too much fruit, the flower buds for the following year are suppressed or poorly formed. In the on year the tree sets a huge crop; in the off year the buds simply were not formed adequately the previous summer.

How to break the cycle — early thinning

The most reliable method for breaking a biennial bearing cycle is hand-thinning fruitlets aggressively in late May or early June of the heavy-crop year, before the natural June drop. Remove fruitlets to leave one plum every 5–7 cm along each branch. This reduces the competition for carbohydrates at exactly the time flower buds for the following season are forming, allowing the tree to support both a reasonable current crop and adequate flower bud formation.

Timing is critical

Thinning after midsummer has minimal impact on biennial bearing because the critical flower bud formation window has already passed. Thinning must happen in late May to early June to intercept the competition at the right moment. Many growers are reluctant to remove apparently healthy fruitlets, but the long-term gain from a tree in regular annual bearing is considerable.

Other contributing factors

Biennial bearing is more likely to become entrenched after a year when the crop was unusually heavy — perhaps due to ideal pollination conditions — or after a year with no crop due to frost, following which the tree invests all resources in fruit the next season. Maintaining adequate potassium levels supports flower bud formation and provides some buffering.

Recovery takes time

Re-establishing annual bearing from a firmly established biennial pattern may take two to three seasons of consistent early thinning. Persistence pays off — most plum trees respond well over time.