Why is sap or liquid oozing from my tree's bark?

Trees leak for several reasons, and the three common ones have very different implications — from a plum tree's routine reaction to a pruning cut, to a symptom that accompanies serious decay in a large tree. The colour, the texture, the smell and the species narrow it quickly.

Safety

Step by step

1. Note the species first.Plum, cherry, peach and almond bleed thick amber gum readily and for many reasons. Horse chestnut, beech, oak, lime and apple bleeding dark liquid down the bark is a different and more serious picture.
2. Look at the material itself.Clear to amber, thick, sticky, hardening into a glassy bead: gum from stone fruit. Dark reddish-brown or black, watery, staining the bark in vertical streaks and often crusting: bleeding canker. Frothy, wet, discoloured and smelling of fermentation or vinegar: slime flux.
3. For gummosis on stone fruit, look for the trigger.Gum is a general response to injury or stress — a pruning cut, frost damage, drought, waterlogging, a canker, insect boring, or simply a heavy crop on a stressed tree. A little gum around a pruning wound is normal and needs nothing.
4. But check gummosis for bacterial canker.Gum oozing from a flattened, sunken patch of bark, with dieback of the shoots above and small shot-holes in the leaves, is bacterial canker rather than a simple stress reaction. That needs affected branches cut out well into clean wood, in dry summer weather.
5. For dark bleeding on a mature tree, mark it and get advice.Bleeding canker on horse chestnut is widespread in the UK. Many affected trees live for years and some recover, so the presence of bleeding is not a death sentence — but it can be accompanied by internal decay, and the question that matters is whether the tree is structurally sound. That is an arboriculturist's assessment, not a gardener's.
6. For slime flux, generally leave it alone.Bacteria fermenting sap inside a wound or a crack produce a foamy, smelly, discoloured ooze that stains the bark and kills the grass beneath. It looks alarming, it usually does no significant harm, and there is no treatment. Do not drill, plug or seal it — old advice to insert drainage tubes is discredited and creates a new wound.
7. Look for insect involvement.Sawdust-like frass mixed with the ooze, or small round exit holes, points at boring insects, which are generally secondary to a tree already in decline. Sticky honeydew dripping from above and blackening leaves is a sap-sucker problem rather than a bark one.
8. Improve the tree's conditions.Almost all of these are worse on stressed trees. Mulch the root zone with composted bark, keeping it clear of the trunk, remove competing grass, water a young or newly planted tree properly through drought, and stop any mechanical damage from mowers and strimmers.
9. Do not seal, paint or bandage it.Wound paints are no longer recommended for any of these. They trap moisture and organisms against the wood and prevent the tree's own compartmentalisation.

Tips

• Prune stone fruit only between late spring and late summer, in dry weather. Winter pruning is the main avoidable cause of bacterial canker and silver leaf.
• A photograph of the trunk with a date, repeated each year, is genuinely useful for judging whether a bleeding lesion is enlarging.
• Ooze at the base of a tree, at ground level, is more concerning than ooze partway up a limb, because that is where root and butt decay show.
• Sap running from a spring pruning cut on birch, maple or walnut is normal and not a disease — those species bleed heavily if cut in late winter, which is why they are pruned in summer.

Common mistakes

• Drilling a hole to drain a slime flux — Discredited advice that creates a fresh wound and a new infection court. Leave it.
• Sealing a bleeding lesion with paint or tar — Traps moisture and pathogens against the wood and stops the tree walling the damage off.
• Assuming amber gum on a plum means the tree is dying — Gum is a general stress response and is extremely common. What matters is whether there is a sunken canker and dieback with it.
• Deciding a large bleeding tree is safe because it is in full leaf — Canopy condition is a poor guide to structural condition. Assessment is about the wood, not the leaves.

Questions people ask