Fruit production on a pear tree depends on pollination, the transfer of pollen from male to female flower parts. This reproductive step is necessary for fertilization and subsequent fruit development. For most pear varieties, a second, genetically distinct tree is necessary to ensure consistent and high-quality fruit yields.
The Necessity of Cross-Pollination
Most commercially important European pear varieties, such as ‘Bartlett’ and ‘Bosc,’ are self-incompatible. This means they cannot be fertilized by their own pollen. This biological barrier is controlled by a genetic mechanism that prevents inbreeding by actively rejecting pollen carrying the same genetic markers (S-alleles) as the receiving flower. Pollen must come from a different variety to successfully set fruit.
A self-incompatible tree will bloom heavily, but the flowers will drop unless they receive compatible pollen. Some varieties, like ‘Kieffer’ or ‘Concorde,’ are partially self-fertile and may produce a small crop alone. However, adding a second variety significantly increases the harvest size and quality. Asian pears (Pyrus pyrifolia) are often labeled partially self-fruitful, but they also benefit greatly from cross-pollination. The best yields for all pear types are reliably achieved when two genetically distinct, compatible varieties are planted near each other.
How Pollination Works
Successful fruit set requires the physical transfer of viable, compatible pollen between the flowers of two different pear trees. Pears rely entirely on insects, not wind, for this process. Honeybees, bumblebees, and other native insects are the primary agents responsible for moving pollen from the anthers of one flower to the stigma of another.
Pear tree flowers produce a minimal amount of nectar, meaning a high population of active pollinating insects is necessary during the bloom period to ensure sufficient pollen transfer. For fertilization to occur, the bloom times of the two planted varieties must overlap significantly. If one tree finishes flowering before the other begins, cross-pollination cannot happen, and fruit set will be minimal.
Selecting Compatible Pollinizer Varieties
Selecting a pollinizer tree (the pollen-donor) requires considering three main factors: genetic compatibility, overlapping bloom time, and ploidy level. Genetic compatibility requires the S-alleles to be different between the two trees to avoid the self-incompatibility rejection response. Bloom periods are categorized into early, mid, and late season; compatible partners must be in the same or an adjacent group to ensure simultaneous flowering.
Certain European pear cultivars are categorized as triploids. Triploid varieties, such as some types of ‘Comice’ and ‘Beurre Hardy,’ produce mostly sterile pollen, making them ineffective pollinizers for other trees. If a triploid pear is planted, it needs pollen from another variety, but two additional non-triploid varieties may be required to ensure mutual cross-pollination in the orchard. Common successful pairings include ‘Bartlett’ and ‘Anjou’ or ‘Bosc’ and ‘Comice.’ Varieties like ‘Kieffer’ are often recommended as universal pollinizers for many European pears.
Why Pears Fail to Set Fruit
Even when a compatible pollinizer is present, several external factors can prevent a pear tree from setting fruit. Late spring frost is a frequent cause of crop failure, as cold temperatures can damage the delicate flower parts. The early blooming nature of pear trees makes them particularly susceptible to this hazard.
Poor activity by pollinating insects is another major issue, often caused by unfavorable weather during the bloom period. Bees are less active when temperatures are low, or when it is raining or excessively windy, leading to insufficient pollen transfer. Furthermore, pear trees require a specific number of chilling hours—cold temperatures during winter dormancy—to reliably flower in the spring. If the winter is too mild, the tree may not flower sufficiently. Excessive nitrogen fertilizer application can also promote leaf growth at the expense of flower bud formation, leading to a lack of fruit.