Cherry trees, like many fruit bearers, must successfully complete a biological process called fruit set to produce a harvest. This process begins with pollination, which is the transfer of pollen from the anther to the stigma of a flower. The necessity of an external pollinator, meaning a second tree or variety, is determined by the specific genetic makeup of the cherry tree planted. Understanding this genetic requirement is the first step in ensuring a successful and reliable crop.
Genetic Requirements: Self-Fruiting vs. Cross-Pollination
Cherry varieties fall into two broad categories defined by their ability to use their own pollen for fertilization. Sour cherries, such as ‘Montmorency’ or ‘North Star,’ are generally self-fruiting, or self-fertile. A single self-fertile tree can produce fruit, though yields often increase with a second compatible variety nearby. Sweet cherries, which include popular varieties like ‘Bing,’ ‘Rainier,’ and ‘Royal Ann,’ are overwhelmingly self-unfruitful and require a partner tree.
The reason for this distinction lies in a mechanism known as gametophytic self-incompatibility (GSI). This system is controlled by the S-locus, which consists of multiple S-alleles within the tree’s genetic code. When a sweet cherry flower receives pollen containing the same S-allele as its own pistil, the flower rejects it, preventing the pollen tube from growing down to fertilize the ovule.
For example, the ‘Bing’ cherry belongs to a specific incompatibility group, meaning it cannot be pollinated by its own pollen or by pollen from another variety within the same group, such as ‘Royal Ann’. The pollen must come from a tree that shares neither of the two S-alleles present in the receiving flower to successfully complete fertilization.
Modern breeding efforts have introduced a solution to this limitation in sweet cherries. Certain new varieties, such as ‘Stella,’ ‘Lapins,’ and ‘Black Gold,’ are considered self-fertile. These varieties contain a special S-allele, sometimes referred to as S4′, which overrides the self-incompatibility system. This genetic modification allows these sweet cherries to set fruit without a separate pollinator, making them desirable for home gardens where space is limited.
The Role of Pollinating Agents and Variety Selection
Even when a cherry tree possesses the genetic capacity for fruit set, the physical movement of pollen must occur. Cherry flowers are structured to rely on insects for pollination, unlike some plants that use wind. The pollen grains are heavy and sticky, making wind dispersal ineffective for ensuring widespread fruit set. Bees are therefore necessary to transfer the pollen from the anthers of one flower to the stigma of another, acting as the primary pollinating agent.
Honeybees are the most common and effective visitors to cherry blossoms in commercial settings, often accounting for over 95% of insect activity. Studies have demonstrated the dramatic difference in yield when bees are present; trees caged to exclude bees had fruit set rates as low as 2%, compared to uncaged trees with rates over 35%. Native bee species, like the blue orchard mason bee, also contribute to this process and are increasingly managed as alternative pollinators.
For self-unfruitful varieties, selecting the correct partner, known as a pollinizer, is a precise requirement. The pollinizer tree must not only be genetically compatible but must also have a bloom period that overlaps with the main variety. A general rule of thumb is that the compatible partner tree should be planted within 100 feet to ensure efficient pollen transfer by foraging insects.
Self-fertile sweet cherry varieties like ‘Stella’ or ‘Lapins’ are often used as universal pollinizers because they can provide compatible pollen to many other sweet cherry types. Growers must consult compatibility charts to identify suitable pairings, as certain combinations, such as ‘Bing’ and ‘Royal Ann,’ are mutually incompatible even though they are different varieties. Successfully pairing an incompatible variety like ‘Rainier’ requires a partner such as ‘Bing’ or a self-fertile type like ‘Sweetheart’ that is blooming at the same time.
Environmental Causes of Poor Fruit Set
Even when genetic needs and pollinating agents are aligned, environmental factors can severely limit fruit production. The most common cause of crop failure is often late spring frost. Cherry blossoms are susceptible to damage when temperatures drop below freezing, with temperatures around -1°C (29°F) or colder capable of killing the delicate pistil inside the flower. This damage may not be visible immediately, as the flower can look normal but the internal reproductive structure will have turned dark brown or black.
Adverse weather during the short blooming window can also interfere with bee activity. Bees are less likely to forage in cold, windy, or rainy conditions, which reduces the rate of pollen transfer. Since the flower’s stigma is only receptive for a few days, a period of poor weather can effectively prevent fertilization across the entire orchard.
Extreme warm temperatures can equally cause problems by accelerating the reproductive cycle. When temperatures rise unexpectedly during bloom, the period of stigma receptivity shortens, and the ovules inside the flower senesce, or lose their viability, more quickly. Beyond weather, cultural practices also play a role, as the application of pesticides during bloom can harm the necessary bee population. Excessive nitrogen fertilizer can promote vegetative growth, favoring leaves and shoots over flower production, ultimately leading to poor fruit set.