Most apple trees require a partner to bear fruit, making the distinction between a “pollinator” and a “pollinizer” important. A pollinator is an agent, such as a bee, that physically moves pollen between flowers. The pollinizer is the specific tree variety that provides the compatible pollen needed for fertilization. Since the vast majority of cultivated apple varieties cannot fertilize themselves, a second, compatible tree must be present for a harvest.
Why Apple Trees Need a Partner
The requirement for a partner is due to a natural biological mechanism called gametophytic self-incompatibility (GSI), which prevents the tree from being fertilized by its own pollen. This system is a form of genetic control that ensures cross-pollination, promoting genetic diversity and stronger offspring in the wild. Without pollen from a different, compatible variety, the flower will not develop into a mature apple.
Most apple trees are classified as diploid, meaning they have two sets of chromosomes, and their pollen is viable for other trees. However, a significant group of varieties, known as triploids, possess three sets of chromosomes. Triploid varieties, such as ‘Jonagold’ or ‘Bramley’s Seedling,’ produce pollen that is sterile and cannot serve as a pollinizer for any other apple tree.
A triploid variety still needs pollination from a compatible diploid tree to produce fruit. If planting a triploid, you must ensure two other compatible diploid varieties are nearby. These two trees must cross-pollinate each other and provide the sterile triploid with viable pollen, which is essential for fruit production.
Choosing Compatible Pollinizer Varieties
Selecting a suitable partner for your apple tree moves from biological necessity to horticultural strategy. Two main factors determine compatibility: genetic makeup and the timing of the bloom. The pollinizer variety must not be genetically identical to the main tree, which generally means avoiding planting two trees of the exact same name, like two ‘Honeycrisp’ trees.
Beyond avoiding identical varieties, the S-alleles, which control the self-incompatibility system, must be different enough to allow fertilization. Varieties are categorized into bloom groups, often numbered 1 through 6, based on the timing of their flowering in the spring. A successful pairing requires that the trees’ bloom times overlap significantly to ensure pollen is available when the flowers are receptive.
A tree in one bloom group, for example, a mid-season bloomer, can typically be pollinated by any tree in its own group or the groups directly adjacent to it. Many growers utilize white-flowered crabapple trees as universal pollinizers because they produce abundant, highly viable pollen and their bloom periods often overlap with many standard apple varieties. The pollen from the crabapple will not affect the fruit characteristics of the edible apple variety, only the seeds inside.
How Pollen Transfer Occurs
Once compatible pollinizer trees are established, insects primarily accomplish the physical transfer of pollen. Apple pollen is heavy and sticky, making the trees entomophilous; they rely on biological vectors rather than wind, which is ineffective. Honeybees are the most well-known pollinators, but native species like bumblebees and solitary mason bees are often more efficient.
Insects are attracted to apple blossoms by nectar and inadvertently carry pollen grains on their bodies as they move between trees. For effective cross-pollination, the pollinizer and the main tree should be planted within about 100 feet of each other, which is the typical effective range for bees. Growers encourage pollinator activity by avoiding broad-spectrum pesticides during bloom and planting other bee-friendly flowers nearby.
Hand-pollination remains a viable option in situations where insect activity is low, such as due to poor weather. This involves manually collecting pollen from the pollinizer flower and lightly brushing it onto the stigma of the receiving flower. While time-consuming, this technique ensures successful transfer and is often reserved for small orchards or specific breeding efforts.