Plum tree

Light and exposure

The plum tree needs full sun to flower well and produce sweet fruit. A shaded spot gives a stunted tree, a thin harvest and far more disease, simply because the foliage never gets enough air and light to dry out.

Also pick a spot sheltered from cold winds, and above all not a dip in the ground. Cold air is heavier and pools there on clear spring nights, scorching the blossom that opens at the first mild spell. A gentle slope or slightly raised ground beats a low, enclosed corner of the garden.

Watering

For the first three years after planting, water the young tree regularly to help it root deeply, especially through dry summers. Once established, an adult plum tree largely fends for itself thanks to its deep roots.

The critical moment is while the fruit is swelling, in the weeks that follow fruit set. A dry spell followed by a sudden soaking, whether from watering or heavy rain, splits fruit that is already well formed. Steady, moderate watering beats a last-minute catch-up every time.

Soil and repotting

The plum tree is not fussy and adapts to most soils, though it prefers ground that is deep, moist and well drained, ideally neutral to slightly chalky. Work in some well-rotted compost at planting to give it a good start.

The one real enemy is standing water. Heavy, poorly drained soil suffocates the roots and opens the door to gummosis, the resin ooze typical of a Prunus in distress. On clay soil, plant on a small mound to raise the graft union above the wet.

Temperature and humidity

Cold itself is rarely a problem for the plum tree: it is a hardy tree that shrugs off well below -20 °C once dormant, and down to about -30 °C depending on the variety, that is USDA zones 4 to 9. In exchange it needs a marked winter, 700 to 1000 hours below 7 °C, to break the dormancy of its buds. The real risk lies in spring, not winter.

Flowering comes early, before the apple, at the first mild days after winter, right when late frosts are still common. A single night at -2 or -3 °C is enough to scorch open blossom and wipe out much of that year’s crop, without the tree itself taking any lasting harm.

Pruning and upkeep

One rule is absolute with the plum tree, as with all stone fruit: prune in summer, never in winter. The right time is after harvest, at the end of summer, while sap is still moving and wounds heal quickly. A cut made in winter, deep in dormancy, stays open for months and becomes an easy entry point for brown rot, canker and silver leaf disease.

The plum tree needs little pruning and dislikes heavy cuts. Limit yourself to opening up the centre of the tree, removing dead or diseased wood as soon as you spot it, whatever the season for that particular job, and shortening overly vigorous branches a little. A balanced framework is built over the first few years, not in a single session.

Pests and diseases

Brown rot is the plum tree’s number one problem, encouraged by damp springs and summers. It rots the blossom and then the fruit, which shrivels into mummies that stay hanging on the branches. Pick off and destroy these mummified fruit without fail, they carry the spores that restart the infection the following year.

Plum rust marks the underside of the leaves with orange pustules in late summer, shot hole disease peppers leaves with small holes as if shot with lead pellets, and black aphids curl the young shoots in spring. More seriously, sharka is a regulated viral disease that marks fruit and leaves with ring-shaped blotches: any suspect tree must be reported, there is no cure, and removal is often the only option.

Propagation

The plum tree cannot be reliably propagated from seed if you want to keep the original variety: a planted stone grows into an unpredictable rootstock, close to wild blackthorn, with fruit of little interest. The normal route is grafting onto a suitable rootstock, such as myrobalan plum, done by budding in summer or by cleft grafting at the end of winter.

In the garden, the most useful habit is watching for suckers: the rootstock often throws up shoots at the base of the trunk or further out in the lawn. These are not free baby plum trees but growth from the rootstock, and should be pulled out as soon as they appear so they do not drain the grafted tree.

Month-by-month care calendar

Green Hub tips

• Never prune the plum tree in winter: do it in summer, right after harvest, or the wounds stay open and let in gummosis and canker.
• Check a variety’s self-fertility before buying. Mirabelle de Nancy or Reine-Claude d’Oullins crop on their own, others need a pollinator planted nearby.
• Pick up and destroy fruit mummified by brown rot, never leave it on the tree or in the compost: it restarts the infection the following year.
• Avoid planting in a dip in the ground. Cold air pools there on clear spring nights and scorches the plum tree’s early blossom, ruining the harvest without harming the tree itself.