Plum Orchard Management
Pruning: Plums grown for fresh market require annual pruning to renew the fruit wood, remove “water sprouts” (vigorous shoots) and maintain good light interception in the lower part of the canopy. They are most commonly trained to an open center, or “vase” shaped system.
Irrigation: In some stone fruit species, including peach and nectarine, water stress in early summer often leads to fruit developmental disorders including doubling and the formation of deep sutures. However, plum is less susceptible to fruit disorders when exposed to water stress (Johnson et al 1994). Plums are typically drip or micro sprinkler irrigated with some flood irrigation in Central California. (M. Norton and S. Johnson, pers comm).
For more detailed information and research reports focused on orchard management please see the UC Fruit Report website: http://ucanr.edu/sites/fruitreport/
Cross pollination requirements: Unlike peach and nectarine, most Japanese plum cultivars are “self incompatible” and require cross pollination from another compatible cultivar to set fruit. Some cultivars can set up to 30 – 40% of flowers without the presence of a pollinizer cultivar, however most are limited to 1-3% fruit set which is inadequate for commercial production (LaRue and Norton 1989). Additionally, fruit set in the absence of pollinizers is known to vary among geographic regions (for example Northern vs. Central California).
Self-incompatibility in Japanese plum has important implications for orchard design, establishment, management, and harvest.
When selecting cultivars to plant in an orchard, growers must balance many factors including local climate, soils, pests, fruit characteristics, harvest timing and pollination compatibility. Many commercial nurseries in California provide suggestions for compatible cultivars. Pollinizer cultivars are typically intermixed with the primary cultivar in an orchard. The ratio of pollinizer to primary trees varies depending on the fruit quality of the pollinizer tree and the extent to which the primary cultivar requires pollen to set fruit. For example, if the both the pollinizer and primary cultivars produce desirable fruit they can be planted in alternating four row blocks to maximize harvest and irrigation efficiency throughout the year. Alternatively, if the pollinizer cultivar produces unmarketable fruit and is not required in high abundance individual pollinizer trees may be planted at regular intervals throughout a block of the primary cultivar or limbs of a pollinizer cultivar can be grafted onto the primary cultivar in the first 1 – 2 years.