Pollination is the necessary process for fruit production, involving the transfer of pollen from the male part of a flower to the receptive female part, called the stigma. For a cherry tree to produce fruit, this transfer must be successful, and the pollen must be genetically compatible. Whether cherry trees require a second tree for cross-pollination depends entirely on the specific type of cherry being grown. Knowing the classification of your tree is the most important factor in determining your planting strategy.
The Critical Difference: Sweet Versus Tart Cherries
The requirement for cross-pollination is fundamentally different between the two primary categories of cherries, sweet and tart. Tart cherries, which belong to the species Prunus cerasus, are predominantly self-fertile, meaning they can produce fruit using their own pollen. Varieties such as Montmorency and Morello are considered self-fruiting and do not need a separate pollinator tree to set a crop.
In stark contrast, sweet cherries, Prunus avium, are overwhelmingly self-incompatible, which is a genetic mechanism preventing fertilization by their own pollen. Cultivars like Bing, Rainier, and Lambert possess this self-sterility and require a genetically distinct partner to produce a harvestable yield. This incompatibility is controlled by a set of genes known as S-alleles. If a flower rejects pollen that shares the same two S-alleles, the pollen tube cannot grow down the style to fertilize the ovule, and no fruit will develop.
Selecting Compatible Pollinator Partners
Gardeners who choose to grow self-incompatible sweet cherries must meticulously select a compatible partner, often called a pollinizer. For successful cross-pollination, the pollinizer tree must belong to a different genetic compatibility group than the main variety. For example, popular varieties like Bing, Royal Ann, and Lambert all share the same genetic group, meaning they cannot pollinate each other, despite being different cultivars.
Beyond genetic compatibility, the bloom period of the two trees must significantly overlap, ensuring that the flowers are open and receptive at the same time. Even a genetically compatible pollinizer is ineffective if its flowers have already dropped their petals or have not yet opened. The pollinizer tree must also be planted within a reasonable proximity, ideally within 100 feet of the main tree, to facilitate the transfer of pollen.
Some modern sweet cherry cultivars, such as Stella, Lapins, and Sweetheart, have been bred to be self-fertile, overcoming the need for a separate pollinizer. These self-fertile varieties are often referred to as universal donors because they can provide compatible pollen for many self-incompatible trees, provided the bloom times align.
Agents of Pollen Transfer
Successful cross-pollination relies almost entirely on the movement of pollen by insects, making cherry trees entomophilous. Honeybees (Apis mellifera) are the most commonly managed and effective pollinators in commercial cherry orchards. Native bees, such as mason bees, also play a significant supporting role in transferring pollen between compatible blossoms.
Wind plays a minimal role in cherry pollination because the pollen grains are relatively heavy and sticky, designed for insect transfer rather than airborne travel. The activity of these insect agents is highly dependent on favorable weather conditions during the brief flowering period, which lasts about seven to eight days. Pollinator activity dramatically decreases at cool temperatures, typically below 50 degrees Fahrenheit, and is further inhibited by heavy rain or strong winds. If cold or wet weather persists during the bloom period, a lack of insect activity can lead to poor pollen transfer and a significantly reduced fruit set.