Plum trees, like most fruit trees, begin their reproductive cycle with a display of blossoms each spring. While the flowers promise a harvest, fruit production is not guaranteed. For the flower’s ovary to develop into a plum, a successful transfer of pollen must occur, a process known as pollination. This biological step determines whether a home gardener with a single tree will actually pick fruit.
Understanding Plum Fertility Self-Pollinating vs. Cross-Pollinating
The requirement for a pollinator in plum trees depends on the specific variety’s fertility status. Plum varieties are broadly categorized as either self-pollinating (self-fertile) or cross-pollinating (self-sterile). Self-pollinating trees possess the capability to set fruit using their own pollen, meaning a single tree can produce a crop without a partner. Even these self-fertile varieties, however, typically yield a much larger and higher-quality harvest when cross-pollinated with a different variety.
The two main groups of cultivated plums, European and Japanese, exhibit different fertility tendencies. European plums (Prunus domestica), including varieties like ‘Stanley’ and ‘Italian Prune,’ are generally self-fertile. Damson plums are also part of this species complex and are reliably self-fertile, making them a popular choice for single-tree planting.
In contrast, most Japanese plums (Prunus salicina), such as ‘Satsuma’ and ‘Shiro,’ are largely self-unfruitful. They require pollen from a different plum variety to produce a crop because pollen from the same variety is genetically incompatible and cannot successfully fertilize the ovule. Therefore, planting a Japanese plum necessitates planting a compatible partner nearby.
Identifying Compatible Pollinators and Varietal Matching
For plum trees requiring a partner, successful cross-pollination relies on two main factors: genetic compatibility and synchronized bloom time. Simply planting any two different plum trees is not sufficient; the chosen varieties must be able to exchange viable pollen and must have their flowers open concurrently. Compatibility charts are necessary resources for determining which varieties can effectively pollinate one another.
In a practical sense, European plums can generally pollinate other European plums, and Japanese plums must be paired with other Japanese plums or suitable hybrids. European and Japanese plums rarely cross-pollinate due to genetic differences and often non-overlapping bloom schedules, as European varieties tend to flower later. A key example of an effective pollinator is the ‘Santa Rosa’ Japanese plum, which is partially self-fertile and also a known universal pollinator for many other Japanese varieties.
The proximity of the trees is also a practical consideration for home growers, as pollen must be physically transferred between the blossoms. For effective pollination, the compatible partner tree should ideally be planted within 50 to 100 feet of the target tree. Gardeners can confirm specific compatibility by checking nursery tags or contacting agricultural extension resources before purchasing, ensuring the pairing will result in fruit set.
The Role of Vectors in Successful Pollination
Once a tree has produced flowers, the transfer of pollen requires a vector. Plum pollen is too heavy to be carried by wind, making insects the primary agents of transfer. Honeybees and various native bee species are the most effective pollinators, attracted by the flower’s nectar and pollen.
As bees forage, pollen grains adhere to the hairs on their bodies and are inadvertently brushed onto the stigmas of subsequent flowers. This bee activity is indispensable; studies show that trees caged from bees have drastically reduced fruit set compared to uncaged trees. The health and presence of local bee populations directly correlate with the quantity and quality of the plum harvest.
Environmental conditions during the bloom period significantly impact the vector’s ability to perform this task. Cold temperatures, heavy rain, or strong winds can prevent bees from flying, effectively halting the pollination process. Since plum trees bloom early in the season, often coinciding with unpredictable weather, a period of poor weather can drastically reduce the number of successful pollen transfers, even if compatible trees are present.
Addressing Pollination Failure Why Your Tree Isn’t Fruiting
When a plum tree fails to set fruit despite having a compatible partner, the cause is often a temporary issue during the bloom period. A late frost can be damaging, as temperatures of just -4°C can destroy the flowers’ female organs necessary for fruit development. This damage is often mistaken for a lack of pollination.
If a lack of pollen transfer is suspected, temporary measures can be employed to encourage fruit set. Hand-pollination involves using a small, soft brush or cotton swab to collect pollen from one tree and gently transfer it to the open flowers of the partner tree. Another technique is to introduce flowering branches from a compatible donor tree, placing them in a bucket of water near the tree that requires pollination, allowing local bees to carry the pollen.
Other factors may result in a poor yield, even after successful pollination. An over-abundance of nitrogen fertilizer can promote lush leaf growth at the expense of fruit production, and a heavy crop one year may lead to the tree resting the following season. Furthermore, young plum trees often do not begin to bear fruit until they are three to six years old, so a lack of fruit may simply be a matter of maturity.