A pattern has emerged that I do not understand. On planting a new plum tree, it takes a couple of years to establish and then flowers and bears fruit for several years, whereupon it stops flowering altogether and just puts on growth. The same thing has happened with damsons and greengages, but apples and medlar in the same area are trouble free. My orchard sits on light, sandy, well-drained soil; the ground is manured at planting and a 1m grass-free circle is around each tree for its first five years. Have you any suggestions to get the trees reflowering? NH
There are several possible explanations as to why your plum tree isn’t flowering and fruiting (no pollinated blossom, as I’m presuming you already know, means no fruit). One is that the blossom is being damaged by frost or icy winds before it has a chance to properly open (plum trees flower earlier than other fruit), which can happen in a bad year or if the tree is growing in a very exposed position.
Bear in mind that as a tree matures, it can gradually grow above the protective cover of nearby walls and hedges, with the result that as it gets bigger its blossoming branches can become much more exposed to late spring frosts/cold winds, etc, than when it was a young tree.
Conversely, another possibility – and one that’s increasingly becoming a problem as a result of climate change – is that your plum trees aren’t being sufficiently exposed to cold temperatures during the winter months for them to successfully bloom the following spring.
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Known as chilling hours, a plum tree typically needs approximately between 700-1,000 hours of being exposed to a temperature between 0-8 degrees over the course of winter for its flower buds to properly develop and to open at the right time of year. But, as apple trees also need a similar chilling period and yours are fruiting well, this doesn’t seem to be the cause of your problem.
A more likely possibility is the soil, which you describe as light, sandy and free-draining. This isn’t ideal for plum trees, which like a cool, deep, fertile, moisture-retentive but free-draining, slightly acidic soil. You mention that you added manure to the planting hole, so it may be the case that as your plum trees mature and their roots spread, they are running out of the fertile, humus-rich soil contained in the original planting hole. Light, sandy soils can often also be deficient in key plant nutrients, so I’d suggest spreading a slow-release balanced granular fertiliser around the base of the plant every early spring and then very gently raking it into the soil, along with a light annual mulch of well-rotted home-made garden compost or manure.
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Finally, bear in mind that other factors that can adversely affect a plum tree successfully blossoming and fruiting include bird damage (birds stripping and eating the flower buds), poor pollination due to bad weather or lack of a suitable nearby pollinating partner and incorrect pruning, which, if done at the wrong time of year, will result in removal of the nascent buds. To reduce the risk of silver leaf disease and bacterial canker, plum, damsons and gages should only be lightly pruned in summer, both to encourage an open goblet-type shape and to remove any damaged, diseased or broken branches.
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