Plum trees’ need for a pollination partner is not universal, depending entirely on the specific variety being grown. Successful fruit set requires the transfer of viable pollen, either from the same tree or a compatible partner, followed by fertilization. While some plum varieties can manage this process on their own, a high-quality, abundant harvest is often improved by the presence of a second, compatible tree.
Self-Pollination vs. Cross-Pollination Requirements
Plum trees are broadly categorized into two main groups, which dictates their general pollination needs. The first group is European Plums (Prunus domestica), including varieties like ‘Stanley’ and ‘Green Gage’. Many European plums are “self-fertile,” meaning they can set fruit using their own pollen without a second tree.
The second major group is Japanese Plums (Prunus salicina), such as ‘Santa Rosa’ and ‘Shiro’. Most Japanese plum varieties are “self-sterile,” meaning their pollen is genetically incompatible with their own flowers. They require cross-pollination from a different, compatible variety to produce fruit. Even self-fertile Japanese varieties, like ‘Methley,’ often yield a significantly larger crop when cross-pollinated. Planting a second, distinct variety is a practical requirement for consistent fruiting, especially for Japanese types.
Matching Plum Varieties for Effective Pollination
For cross-pollination to succeed, the two plum varieties must have overlapping bloom times. If bloom times do not align, pollen will not be available when the flowers are receptive. Varieties are often assigned to pollination groups based on their average flowering period, and successful pairing involves varieties within the same or an adjacent group.
Japanese plums must be paired with other Japanese plums, and European plums with other European plums. This is due to genetic differences, such as chromosome count, which prevents successful fertilization between the two major types. The pollinator cannot be a clone of the main tree; two trees of the exact same variety will not cross-pollinate effectively because they lack the necessary genetic distinction.
The Role of Insects and Environmental Factors
Plum trees are insect-pollinated, relying almost entirely on bees, rather than wind, to move pollen between flowers. Honeybees are the primary agents, but native bees and other insects also play a significant role in transferring the sticky, heavy plum pollen. Even self-fertile trees require insects to physically carry the pollen from the anther to the stigma of the flower.
Environmental conditions during the bloom period are important for the success of these insect agents. Bees are less active if temperatures drop below 55 to 60 degrees Fahrenheit, or if there is heavy rain or high winds. Rain is detrimental as it washes viable pollen from the flowers. Cool weather can also inhibit the growth of the pollen tube once the pollen lands on the stigma. Growers often position beehives near orchards to maximize bee activity.
Common Causes of Poor Fruit Set
Even when a compatible pollinator tree is present, a plum tree may still fail to set fruit due to non-pollination factors. One of the most frequent causes is late spring frost, which can kill delicate flower buds and open blossoms, resulting in a complete crop loss for that season. Young plum trees, typically those under three to six years old, often do not produce fruit because they are prioritizing root and canopy establishment over reproductive growth.
Improper nutrient management can also diminish fruit yield. Excessive nitrogen fertilizer, while promoting lush, green leaf growth, can inhibit the formation of flower buds and encourage the tree to put energy into vegetative growth instead of fruit. Furthermore, poor pruning techniques that remove the short, fruit-bearing wood, or insufficient sunlight, can weaken the tree’s ability to sustain a crop. Plums can also exhibit biennial bearing, where a very heavy crop one year is followed by a light crop or no fruit the next, as the tree recovers its energy reserves.