The number of plum trees necessary for fruit production depends entirely on the genetics of the specific variety being grown. Successful fruiting requires understanding the tree’s particular pollination requirement, which dictates whether a single tree is sufficient or if a partner tree must be planted nearby. This difference in reproductive strategy is the most significant factor in a plum tree’s ability to set a crop.
Defining Self-Fertility and Cross-Pollination
Plum trees fall into two fundamental categories that determine their need for a planting partner. The first is self-fertile, meaning the tree can produce a crop using its own pollen. In these varieties, pollen from one flower successfully fertilizes the ovule of another flower on the same tree, leading to fruit development.
The second category is self-unfruitful, requiring cross-pollination. These trees possess a genetic barrier that prevents their own pollen from successfully fertilizing their flowers. For a self-unfruitful plum to produce fruit, it must receive pollen from a different, genetically distinct variety to complete fertilization. This scenario strictly requires planting a second plum tree.
Even varieties classified as self-fertile frequently benefit from a compatible partner nearby. While a single self-fertile tree produces fruit, the presence of a different pollinator typically increases the quantity and size of the harvest. This improvement in yield is due to the greater genetic diversity provided by cross-pollination.
Pollination Needs of Major Plum Types
The two primary types of plums grown in North America, European and Japanese, have distinct pollination needs. European plums (Prunus domestica) are often self-fertile. Many varieties, such as ‘Stanley,’ ‘Damson,’ and ‘Italian Prune,’ can reliably set fruit with just a single tree. Most European plums bloom around the same time, simplifying the selection of a compatible partner within the species.
In contrast, Japanese plums (Prunus salicina) and their hybrids are almost universally self-unfruitful and require a second tree for successful fruit set. They must be planted near another Japanese plum that flowers simultaneously. The European and Japanese plum species generally do not cross-pollinate each other due to genetic incompatibility and different bloom times.
Many modern hybrid plums, such as Pluots, are crosses between Japanese plums and other stone fruits, and they also require cross-pollination. These hybrids often have complex compatibility requirements. The need for a partner is the rule for most widely cultivated Japanese and hybrid plums.
Practical Steps for Successful Fruit Production
When a second tree is necessary, the most important consideration is ensuring the two varieties have an overlap in their blooming periods. The pollinator variety must be actively flowering when the main tree’s receptive flowers are open. Nurseries often categorize plums into flowering groups to simplify this pairing, indicating which varieties bloom together. This synchronization is more critical than the exact distance between the trees.
The physical transfer of pollen is almost exclusively carried out by insects, primarily honeybees, bumblebees, and orchard bees. For successful transfer, the pollinator tree should be planted within about 100 feet of the main tree to encourage foraging insects. Closer proximity, ideally 30 to 50 feet, maximizes the chance of effective pollen exchange.
For gardeners with limited space, a second full-sized tree is not the only solution. Grafting a small branch from a compatible pollinator variety onto the main plum tree creates a multi-grafted tree. This technique allows a single tree to produce two or more varieties, solving the pollination problem compactly. Another temporary solution is placing a bouquet of cut flowering branches from a compatible variety in a water-filled container near the main tree during the bloom period.