Pollination is the transfer of pollen from the male part of a flower to the female part, leading to fertilization and fruit development. The vast majority of cultivated apple varieties (Malus domestica) are self-incompatible. This means an apple tree requires pollen from a different, genetically distinct variety—known as cross-pollination—to produce fruit.
The Rule of Self-Incompatibility in Apples
The biological reason most apple varieties cannot pollinate themselves is governed by gametophytic self-incompatibility. This system is controlled by the S-locus, a highly polymorphic region of the genome containing S-alleles. A standard diploid apple variety carries two different S-alleles (e.g., S1 and S2).
For successful fertilization, the S-allele carried by the incoming pollen grain must be different from both S-alleles present in the flower’s female tissue. If the S-alleles match, an incompatibility reaction is triggered. This reaction involves specialized proteins, such as the S-RNase enzyme, which halts the growth of the matching pollen tube as it attempts to grow down the style toward the ovule. This genetic barrier promotes outcrossing and ensures genetic diversity.
The Mechanism of Cross-Pollination
Successful cross-pollination requires genetic compatibility and the physical transfer of pollen between varieties. The process relies heavily on external agents to move compatible pollen from the donor tree (the pollinizer) to the receptive flower. Insects are the dominant and most effective means of transfer, with the domesticated honeybee being the most prevalent pollinator in commercial orchards.
As a bee visits an apple flower, pollen grains adhere to its body and are deposited onto the stigma of the next flower it visits. Wind plays a negligible role in apple pollen dispersal compared to the directed action of insects. Proximity is also necessary, as successful cross-pollination is most likely when compatible trees are planted within 50 to 100 feet of each other.
For the transfer to be effective, the bloom periods of the two different apple varieties must overlap significantly. Varieties are categorized by their bloom time (early, mid, or late season), and a mismatch in flowering prevents the necessary exchange of viable pollen. Once a compatible pollen grain lands on the stigma, it germinates, and the pollen tube grows through the style to the ovule, initiating the fertilization event that stimulates fruit growth.
Practical Steps for Successful Fruit Set
To ensure a reliable harvest, growers must plan for cross-pollination. The fundamental requirement is to plant at least two different, genetically compatible apple varieties that share a similar flowering window. Consulting a pollination chart can confirm the necessary S-allele compatibility and bloom overlap.
An important consideration is the presence of triploid apple varieties, such as ‘Jonagold’ or ‘Mutsu,’ which have three sets of chromosomes instead of the standard two. While triploid trees can receive compatible pollen, their own pollen is sterile and cannot serve as a pollinizer for other trees. If a triploid variety is planted, it requires two separate, non-triploid (diploid) varieties nearby to ensure its own pollination and the pollination of the second diploid tree.
For gardeners with limited space, a crabapple tree serves as an excellent, prolific pollinizer for most standard apple varieties. Other short-term techniques include grafting a small branch from a compatible pollinizer onto the main tree or temporarily hanging buckets of fresh, blooming branches within the canopy. Attracting a healthy population of bees and avoiding the use of pesticides during the bloom period are also steps to maximize fruit set.