Why Is My Plum Tree Bark Splitting?

Bark splitting on a plum tree can look frightening, but the significance varies enormously depending on the location, appearance and timing of the split. Some splits are entirely benign — part of the natural growth cycle of an expanding trunk. Others are early signs of bacterial canker or frost damage that require prompt action to prevent the problem spreading deeper into the tree's vascular system.

Natural growth cracking — clean shallow splits

As plum trunks increase in girth each growing season, the outer bark layer cannot always stretch fast enough to keep pace. This produces shallow longitudinal cracks or plates of lifting bark, usually on the south-facing side of the trunk. The exposed wood underneath looks clean, pale and slightly damp — not sunken or discoloured. This type of splitting needs no intervention. The tree forms a new layer of bark beneath the crack and the split gradually fills in over a couple of seasons.

Frost crack — sudden splits after hard frost

Frost crack occurs when water in the outer bark freezes and expands rapidly, splitting the bark longitudinally. It often happens overnight in hard winters and the crack can extend several centimetres into the sapwood. The edges of the crack are clean and the split often runs straight along the trunk. On mild days the crack may partially close. There is nothing to apply — the tree calluses over naturally if no disease enters. Keep the wound clean and avoid applying wound paint, which research shows can trap moisture and delay healing.

Bacterial canker — sunken, oozing splits

Bacterial canker (Pseudomonas syringae) is the most serious cause of bark splitting in plum trees. The bark develops sunken, wet-looking patches that exude amber or brown gum. The wood underneath is discoloured orange-brown rather than clean white. In autumn and winter the bacteria are most active. Prune out affected branches or pare back cankers on the main trunk to clean wood during dry summer weather when the bacteria are dormant. Apply copper fungicide spray in autumn (August to October) to protect pruning wounds and leaf scars.

Gummosis associated with splitting

Wherever bark splits — whether from frost, growth, or mechanical damage — amber gum may ooze from the exposed site. Gum in itself is not harmful; it is the plum's normal response to wounding. Gum oozing from within sunken, darkened bark is the warning sign of disease, whereas gum at the edge of a clean growth crack is simply a reaction to the wound. Allow natural gum deposits to dry and fall away rather than scraping them off, which reopens the wound.

Mechanical damage at the graft union

Young plum trees grafted onto rootstocks occasionally split at the graft union, particularly if stakes are removed too early and the tree rocks in the wind. Inspect newly planted trees every spring for signs of movement at the union. Maintain a stake and tie for at least the first two to three years until the union has knitted together fully.