Why sweet cherries fail to set fruit

Sweet cherry (Prunus avium) is largely a self-incompatible species, although a number of self-fertile cultivars exist (Lapins, Big Star and others). Orchards that fail to set a satisfactory crop are a common sight. There are many reasons why a cherry orchard may not produce. The main ones are incompatibility between the cultivar and its polleniser, late frost, and too few bees at bloom.

1. Non-self-fertile cherries that have reached bearing age (heavy bloom) need a suitable polleniser present, otherwise fruit set is minimal. The polleniser must carry the right S-alleles — different from those of the main cultivar — and must flower at exactly the same time as it. Where both conditions are met and set is still poor, another cause has to be looked for.

2. Once a suitable polleniser is in place, bloom also needs favourable weather (16 °C–25 °C, no wind) and strong bee activity — at least one hive per 2 strémmata (0.2 ha), that is roughly five hives per hectare, for good pollination. If the bees do not fly enough, there will be no normal crop. If temperatures during bloom exceed 25 °C with wind, the stigmatic fluid dries out and pollination fails.

3. In young plantings the trees may simply fail to come into bearing — that is, they produce no flower buds (spurs). This comes from over-fertilisation and severe pruning. Feeding a young cherry tree is a delicate business that calls for a sound grasp of tree physiology. Over-fertilise a tree and it turns vegetative, throwing nothing but water sprouts, and it may then take 7–10 years to come into production.

4. Cherry is a species that is highly sensitive to frost. Once bud break has begun, cold hardiness falls sharply. At green tip, a frost of about −2 °C lasting only a few hours can destroy a large part of the crop.

5. Finally, planting distances matter. Trees set at the wrong spacing may well yield less after a few good years. Plant a vigorous cultivar on MaxMa 14 or CAB 6P rootstock in a fertile field at less than 3.5 m between trees and 4.8 m between rows, and by the sixth or seventh year the trees will meet, whether trained as a palmette or as an open vase; the pruning that then becomes unavoidable will keep cutting into the productivity of the orchard.