The need for a companion plum tree depends entirely on the specific variety chosen. While some plum trees reliably produce fruit on their own, many others depend on cross-pollination to set a crop. Understanding these pollination needs is the first step in ensuring a plentiful harvest.
Understanding Plum Pollination Requirements
The necessity for a second tree is determined by whether a plum variety is self-pollinating or cross-pollinating. A plum tree is considered self-pollinating, or self-fertile, if it can produce fruit using its own pollen. This means the flowers contain compatible male and female parts, allowing a single tree to set fruit without a partner.
Even self-fertile varieties often benefit from having a different, compatible plum variety planted nearby. Cross-pollination can increase the size of the harvest and improve fruit quality. While one tree is sufficient, planting two may lead to a more abundant yield.
Conversely, a cross-pollinating tree must receive pollen from a different variety to successfully fertilize its flowers and produce fruit. Pollen from the same tree is incompatible and will not lead to fruit development, causing blossoms to drop off. For these varieties, planting a pair of different, compatible trees is mandatory for fruit set. Pollen transfer is mainly accomplished by insects, particularly bees.
Identifying Compatible Plum Types
Plum trees are categorized into two major groups: European and Japanese. This classification indicates their pollination needs. European plums (Prunus domestica) are often self-pollinating, making them popular for gardeners with limited space. Many European varieties, such as ‘Stanley’ and ‘Italian’ prune plums, are self-fertile and produce a crop without a partner tree.
Japanese plums (Prunus salicina) and their hybrids are nearly all cross-pollinating and require a different variety for fruit production. ‘Santa Rosa’ is a common Japanese plum that needs a compatible partner and often serves as a good pollinator for other Japanese plums. European and Japanese plums generally do not cross-pollinate each other because they are different species that bloom at different times.
Compatibility extends beyond simply planting any two trees; the varieties must belong to the same species group, meaning a Japanese plum needs a Japanese pollinator, and a European plum needs a European pollinator. The pollinating partner must also have a synchronized bloom time with the tree it is meant to pollinate, ensuring the pollen is available when the flowers are receptive. If a Japanese plum variety blooms early, its partner must also be an early bloomer.
Optimizing Planting for Successful Fruit Set
For cross-pollinating varieties, the physical distance between the trees is a practical consideration that directly impacts the success of pollen transfer. Pollen is primarily moved by bees, and for the most reliable cross-pollination, the two compatible trees should be planted relatively close together. A distance of less than 100 feet is generally recommended to ensure pollinators can easily travel between the trees during the bloom period.
The synchronization of the flowering period is critical, as pollen must be available at the precise time the receiving tree’s flowers are ready for fertilization. When selecting a pair, checking that their bloom times overlap is more important than simply matching their species. Cool, rainy weather during the spring bloom can suppress insect activity, reducing the chances of successful pollen transfer, so having trees closer together can help maximize the limited window of opportunity.
For gardeners with restricted space who must plant only one tree, there are creative ways to ensure cross-pollination without a separate second tree.
Grafting
One common technique is grafting, where a branch from a compatible pollinator variety is physically attached to the single plum tree. This effectively turns one tree into a self-pollinating unit by providing the necessary foreign pollen source.
Choosing Self-Fertile Varieties
Another solution is to choose a self-fertile variety to begin with, which eliminates the need for a second planting while still offering the possibility of a solid fruit harvest.