While plum trees are predominantly self-fertile, they do suffer from frost-damaged blossom owing to the fact they flower in March and April.
Fruit trees can be divided into self-fertile and self-sterile varieties. Both require pollen to be transferred from one flower to another in order to produce fruit, but self-sterile varieties require it from another variety. This is usually done by pollinating insects, principally honeybees, but can also be done manually with a paintbrush. Self-fertile varieties will produce fruit without a pollinator, but benefit from a pollination partner for larger fruit size and greater output.
Different varieties open their blossom at different times and are put into groups with group 1 flowering before group 2 and so on. Some sites may provide dates of flowering, but it really depends on your location. Trees will be pollinated by plums in the same or neighbouring flowering groups and as such group 3 will be pollinated by group 2, 3 and 4. Groups beyond this range, either have stopped flowering or haven’t opened their buds, making pollination by insects impossible.
As plums open their leaves early in the year (March-April), they are susceptible to frost-damaged blossom. Frost-damage can prevent a tree from fruiting as female organs in the flower turn into fruit. You can check the health of your tree’s flowers by cutting them vertically. In the center of the flower, you can see an ovary (the bulbous section of the carpel). If it is black or brown, it is probably dead. If it’s green, you’re ok.
During or near the time of the bloom, -2C will result in a 10% loss, while -4C will result in 90% loss. As cold air falls, temperatures are likely to be lowest at the bottom of the tree. To prevent frost-damage, you can throw frost-protection over the top of your tree to retain heat. (Note, accumulated snow can cause trees to branches to break.) Potted trees can moved inside temporarily, but remember they need to be accessible to pollinators at one point. Key for early-flowering trees is planting in full sun and avoiding frost-pockets, low areas where cold air accumulates. Don’t plant fruit trees in those sections of the garden.
It is a little known fact that the first flower colour a bee lands on will be the bee’s preference thereafter. Bees also have an innate preference for blue and violet flowers, which are often high in pollen. They will sometimes ignore the (usually white, sometimes pink) pale blossom of plum trees if more colourful flowers are available. Planting such brightly coloured plants near an orchard may be unwise.
A species is often defined as a population group that is able to breed with one another. Bullaces, damsons, gages, mirabelle and victoria trees are all subspecies of plums and are thus able to pollination each other. Japanese plums (P. salicina and P. mume) are different species altogether, and generally can’t cross-pollinate.
Jorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!
His ideal afternoon would involve a long walk, before settling down for scones.
Jorge is a journeyman gardener with experience in growing crops.