You almost always need two different varieties of apple trees to produce fruit. Most cultivated apples exhibit self-incompatibility, meaning they cannot successfully pollinate themselves or another tree of the exact same genetic makeup. This biological barrier is overcome through cross-pollination, where insects, primarily bees, transfer pollen between two distinct varieties. Even the few types labeled as “self-fertile,” such as ‘Gala’ or ‘Granny Smith,’ produce a significantly better and larger harvest when a compatible partner is nearby.
The Necessity of Cross-Pollination
Two different varieties are required due to the apple tree’s genetic mechanism for preventing self-fertilization. This mechanism, known as gametophytic self-incompatibility, is controlled by specific genes referred to as S-alleles. When pollen lands on a flower, the female part recognizes the pollen’s S-alleles. If the alleles match the flower’s own, the pollen tube growth is rejected and fertilization cannot occur. This system promotes genetic diversity by ensuring fruit is only produced from a cross between two genetically unique individuals.
Fruit formation requires a successful transfer of viable pollen from the anther of one variety to the stigma of another. This transfer is almost entirely dependent on insect activity, with honey bees and orchard mason bees being particularly effective pollinators. Successful fertilization stimulates the ovary to develop into the apple fruit, containing seeds that carry a mix of genes from both parent trees. Without cross-pollination, the unfertilized flowers will simply fall from the tree, resulting in no harvest.
Understanding Variety Compatibility
Successful cross-pollination depends on the two varieties meeting three specific compatibility rules. The first requirement is that the two varieties must have overlapping bloom times. Apple varieties are categorized into flowering groups, and a tree can generally be pollinated by a partner in its own group or one of the adjacent groups. This ensures that viable pollen is available when the flowers on the receiving tree are receptive.
Second, the trees must be genetically distinct enough to bypass the S-allele incompatibility system. Growers must consult compatibility charts to ensure the varieties chosen are not too closely related. Using a chart confirms that the pollen from the donor tree will be accepted by the receiving tree’s flower.
Finally, a specific group of apples known as triploid varieties introduces an additional complication. Triploid apples, which include popular types like ‘Jonagold’ and ‘Gravenstein,’ possess three sets of chromosomes instead of the standard two, making their pollen sterile. Because their pollen is ineffective, triploid varieties cannot pollinate other trees. They also require a compatible pollen donor to set fruit themselves. If a triploid is planted alongside a non-sterile (diploid) partner, the diploid tree will not be pollinated by the triploid. This means two separate, compatible diploid trees are often required to ensure both the triploid and its pollinator produce fruit.
Solutions for Limited Space
For gardeners with limited space, several practical alternatives exist to ensure successful cross-pollination. One effective solution is to introduce a compatible variety via grafting. A small branch, or scion, from a compatible apple type can be grafted onto a limb of the existing tree. This creates a single tree that produces two different varieties and provides the necessary pollen.
Another excellent strategy involves using crabapples, which are highly effective and often universal pollinators for standard apple trees. Many ornamental crabapple varieties, such as ‘Golden Hornet’ or ‘Evereste,’ produce abundant, viable pollen and take up less space than a second full-sized apple tree. Their small size and heavy flowering make them a favored choice for commercial orchards and home gardens.
A short-term solution is the use of a “pollen bouquet.” This involves placing flowering branches from a compatible neighbor’s tree in a bucket of water near the main tree during bloom time. This temporary source of pollen allows bees to transfer the necessary genetic material. If the space constraint is not absolute, a compatible tree planted within a 50 to 100-foot radius is typically sufficient for effective pollination by insects.