Most fruit trees require cross-pollination in order to produce fruit. Cross-pollination is the transfer of pollen from one species or variety to the flower of a different species or variety. "Shiro" plum must be pollinated by a different Japanese plum or a Japanese-American hybrid. The best pollinizers are "Redheart," "Ozark Premier," "Starking Delicious" and "Methley." "Santa Rosa," often considered the standard Japanese plum, will also pollinate a "Shiro" plum. All of these are hardy in USDA zones 5 through 9.
Pollen is carried from on tree to another by bees or other insects. Plum varieties should be planted within 60 to 100 feet of each other for cross-pollination to occur. Dwarf and semi dwarf trees should be planted more closely. The tree can be in a neighboring yard, so if your neighbor has a compatible variety, plant your plum close to their side of the property.
If your property does not have room for two plum trees and there are no other plums in the neighborhood, you can produce plums without planting another tree. The easiest way is to graft a branch of another variety of plum onto a branch of the "Shiro" plum. You can also place cut, flowering branches of another variety in your tree. Replace the branches early each morning during the flowering period.
Poor fruit set can occur even when a compatible pollinator is present if frost occurs during the bloom period. The tree is most susceptible to frost while it is blooming. Even if the temperature does not fall below freezing, bees travel shorter distances when the temperature is below 50 degrees Fahrenheit or when it is wet or rainy. If the pollinizer is close to the maximum distance away, pollination can be poor. Bee diseases or the use of pesticides during the bloom period can also reduce pollination.