By Raj Vansiya (summer student) and Kathryn Carter, Fruit Specialist, OMAFRA
Spring is upon us, and with it come the colourful flowers of the season. The flowers pollinated now will become tasty fruit in August. Every year growers attempt to maximize the fruit set for their crop. Plums flower in spring with the goal that 50% of the flowers should be pollinated and set fruit. Getting consistent yields in plums can be challenging due to:
• Lower than required level of pollination
• Lack of a compatible pollinizer
• Frost during flowering
• Excessive amounts of nitrogen in the soil
• Biennial bearing
While getting consistent yields in plums can be challenging, there are opportunities to improve fruit set and yields.
Pollination: Plums are one of the earliest blooming tree fruit, and as a result the weather during bloom can be cool and wet, which can have a negative impact on bee activity in the orchard.
• Although there isn’t a lot we can do about the weather, some research suggests that some bee species, such as the blue orchard mason bee, are better suited to pollinating fruit trees. These solitary bees are native to Canada and the US and can help increase pollination in orchards as the new generation of bees emerges in coincidence with apple and peach blooms in early spring.
• Reducing competition for pollination: Research has shown that the flowers of weeds and wild hosts may be more appealing to honey bees than the flowers of tree fruit. Growers should manage other weeds, and flowering plants that are nearby plum orchards to ensure that they do not interfere with the pollination of plums. It is always a good idea to mow the orchard floor prior to bloom to reduce competition with dandelions.
• Protecting pollinators: Most stone fruit growers are well aware of the importance of protecting pollinators from toxic pesticides. Refer to Table 5-7; Activity of Fungicides on Tender Fruit Diseases and Impact on Honeybees and Table 5-8; Activity of Insecticides and Miticides on Tender Fruit Pests and Impact on Honeybees. (OMAFRA, 2019)
• With over 23 native species in Ontario, and commercially available nesting boxes, these bees are an ideal pollinator for early spring crops. Native bees can be encouraged to nest by leaving bare patches of earth, old hollow stems, and old wood as potential habitats (MSU, 2019). Honey bees are also a good early spring pollinator. Commercial growers should have a minimum of 1 hive of honey bees per acre of plum trees.
Pollinizer: Many plum varieties require cross pollination in order to set fruit. A pollinizer variety must be planted in close vicinity of the main plum cultivar, preferably in a 1:8 distribution of pollinizer to main cultivar. There should be at least two pollinizers for self-sterile plum cultivars with one pollinizer beginning to flower two days before and the other two days after the self-sterile cultivar. A list of potential options for pollinizers for plums can be found at http://www.michiganplum.org/pollination/
Frost: Cold temperatures during bloom can damage the blossoms resulting in reduced yields. Wind machines and other frost mitigation strategies may help to reduce the impact of the cold temperatures on yields.
Nitrogen: Research has shown that excess levels of Nitrogen in the soil can have a negative impact on yields. Collect soil samples every 3 years, and leaf samples every other year, to improve fertilizer applications.
Biennial bearing: Plums are biennial bearing and a bumper crop one year, often results in a lack lustre crop the next. There are some options for trying to mitigate these issues. Research has shown that fruit thinning can result in a more consistent crop from year to year. Other research has shown that applying Gibberellic acid and an auxin at 4 and 6 weeks after petal fall has been shown to result in increased fruit set in Victoria plum (Webster and Goldwin, 1981). However, GAA is not registered for this use in Canada.
MSU, 2019. https://pollinators.msu.edu/resources/pollinator-planting/native-bee-habitat/
OMAFRA, 2019. http://www.omafra.gov.on.ca/english/crops/pub360/pub360ch5.pdf
Webster A, Goldwin G.K., Journal of Horticultural Science (1981), The hormonal requirements for improved fruit setting of plum, Prunus domestica L. cv. Victoria. https://www.tandfonline.com/doi/abs/10.1080/00221589.1981.11514962