# A tree dripping sap, sticky drops or white foam: honeydew, slime flux and bleeding wounds

> Why a tree drips or oozes, how to tell insect honeydew from wetwood, rain suds, fermentation or a pruning bleed, what to leave alone, and which signs need help.

Published 2026-10-28. Tags: Decay & damage, How trees work. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/a-tree-dripping-sap-or-foam/

Sticky drops on a car or a walk under a tree are honeydew, the sugary waste of aphids or soft scale insects feeding in the leaves above. A dark, wet streak down the trunk that smells sour is slime flux from wetwood, common on elms, oaks and maples and seldom serious. Suds at the base after a heavy rain are harmless, and so is sap running from a cut made in late winter. White froth that smells of fermentation, pitch with sawdust, or ooze from a sunken patch of bark is the warning of a tree in trouble.

![A dark, wet streak running down a moss-covered trunk from a crack in the bark, in a leafless winter wood](https://bluewhaletrees.com/img/articles/a-tree-dripping-sap-or-foam-1-1000.jpg?v=ba1b68fc "Wetwood on a linden beside the River Ayr in Scotland on March 1, 2016. Slime flux seeps from a crack or wound and stains the bark dark as it runs.")
*Photo: Rosser1954, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0) ([source](https://commons.wikimedia.org/wiki/File%3ABacterial_Wetwood_on_Lime.JPG))*

What drips or oozes, and where, tells which it is:

| What you see | What it is | What it means |
| --- | --- | --- |
| Sticky, shiny drops on leaves, cars and walks below | Honeydew from aphids or soft scales | A nuisance that seldom harms the tree |
| A dark, sour-smelling streak down the trunk | Slime flux from wetwood | Usually harmless, and never drilled or drained |
| Suds piled at the base after heavy rain | Rain frothing as it runs down the bark | Harmless |
| White froth with a beery smell from small bark cracks | Alcoholic flux | A sign of severe stress |
| Clear sap from a cut or a broken twig in late winter or spring | Bleeding | Harmless, and it stops |
| Pitch or sawdust at small holes in the bark | Borers or bark beetles | A closer look |
| Dark ooze from a sunken or cracked patch near the base | A canker, such as bleeding canker | A closer look |

## Sticky drops from above: honeydew

Aphids, soft scales and mealybugs feed through tube-like mouthparts on the sap moving in leaves and thin bark. What they excrete is honeydew, a sugary liquid that falls as clear droplets onto the leaves below and anything parked under them. In a dry spell it builds into a glaze that can pit a car's finish, and it draws ants, bees, wasps and flies. Armored scales make none.

![A beech branch coated in white woolly aphids, with leaves below blackened by a sooty fungus](https://bluewhaletrees.com/img/articles/a-tree-dripping-sap-or-foam-2-1000.jpg?v=7cb3d11b "Beech blight aphids on an American beech branch in Rock Creek Park, Washington, DC, on August 9, 2014. Their honeydew grows the black sooty mold on the leaves.")
*Photo: Katja Schulz, [CC BY 2.0](https://creativecommons.org/licenses/by/2.0) ([source](https://commons.wikimedia.org/wiki/File%3AFagus_grandifolia_-_Flickr_-_treegrow.jpg))*

Left in place, honeydew grows a black sooty mold that blackens leaves, roofs and porches. The mold lives on the honeydew, not the leaf, though a heavy coat blocks light. Oaks, tuliptree and honeylocust drip most dependably, and aphids on maples make sidewalks and cars sticky. [American basswood](https://bluewhaletrees.com/plants/american-basswood/index.html.md) drips aphid honeydew in some summers. Tuliptree scale also feeds on magnolias and lindens, and magnolia scale, which feeds only on magnolias, favors [cucumber magnolia](https://bluewhaletrees.com/plants/cucumber-magnolia/index.html.md) and can kill a heavily infested tree.

Most honeydew is a nuisance, not a threat. Lady beetles and lacewings eat the insects, rain washes the leaves, and a strong jet of water knocks aphids off a small tree. A soil-applied insecticide can poison the pollinators at the flowers and the beneficial insects that drink the honeydew. The surer fix is placement: keep the heavy drippers away from where cars park and people sit.

## A dark, wet streak down the trunk: slime flux

Wetwood is a bacterial infection of the inner wood, and slime flux is the liquid it pushes out. The bacteria live in soil and water and enter through wounds, root injuries among them. As they ferment the sap, gas builds pressure in the trunk and forces the liquid out through cracks, crotches and old pruning cuts. Penn State Extension puts the pressure in an infected trunk at 60 pounds per square inch, against 5 to 10 in a healthy one. It runs clear inside the tree, darkens in the air and smells sour or foul. It can froth white where it leaves the bark, and it dries to a pale gray crust. Grass it reaches at the base dies.

Slime flux is extremely common on mature elms, oaks and mulberries, and it shows less often on maples, paper birch, sycamore and walnut. Nearly all elms and poplars are prone to it, [American elm](https://bluewhaletrees.com/plants/american-elm/index.html.md) among them. Most trees live with it for years, though under severe stress the bacteria can move into the sapwood and bring yellowing, wilting and dieback. There is no cure, and the old remedy of draining the trunk has been dropped:

> "The once common practice of boring a hole into the trunk and inserting a short length of pipe to relieve gas pressure and direct the flow of the fluid is no longer considered to be a useful procedure." (The Morton Arboretum, *Wetwood*)

A drilled hole spreads the bacteria inside the tree and lets decay fungi in. Avoid new wounds, water in dry spells, and keep mowers off the trunk.

## White foam: rain suds or alcoholic flux

Foam piled at the base of a trunk after a heavy rain comes from the bark itself. In dry weather, dust, salts and plant compounds coat the bark. Rain running down the trunk dissolves them into a weak soap and tumbles it over the ridges into suds. It happens on every kind of tree:

> "There is absolutely no need for worry, if foam forms on your trees during or after heavy rains." (Ohio State University Extension, *Why Are My Trees Blowing Bubbles?*)

A white froth that oozes from small cracks in the bark, smells of fermentation and draws flies, yellowjackets and hornets is another matter. Microbes turn the sugary sap into alcohol, and the carbon dioxide they release makes the froth. This alcoholic flux shows most on the trunks of young trees under severe stress, most often from a water problem such as soil kept soaked by irrigation. Bark beetle wounds and wilt diseases can bring it too. Fix the watering at once and have the tree looked at.

## Sap from a cut or a broken twig: bleeding

Maples, walnuts, birches, beech, hornbeam and yellowwood bleed sap from cuts made in late winter and early spring. The flow is unsightly but does the tree no harm. Pruning in the coldest part of winter, or once the leaves are full, avoids it. Among the trees we grow, these bleed from a spring cut:

- [River birch](https://bluewhaletrees.com/plants/river-birch/index.html.md) is pruned in summer or fall. Zones 3 to 9, 40 to 70 feet.
- [Black maple](https://bluewhaletrees.com/plants/black-maple/index.html.md) is best left unpruned in early spring. Zones 3 to 8, 60 to 75 feet.
- [American hornbeam](https://bluewhaletrees.com/plants/american-hornbeam/index.html.md) is pruned in summer, if at all. Zones 3 to 9, 20 to 35 feet.
- [Yellowwood](https://bluewhaletrees.com/plants/yellowwood/index.html.md) is pruned in midsummer. Zones 4 to 8, 30 to 50 feet.
- [Black walnut](https://bluewhaletrees.com/plants/black-walnut/index.html.md) is pruned in late summer. Zones 4 to 9, 70 to 100 feet.
- [Red mulberry](https://bluewhaletrees.com/plants/red-mulberry/index.html.md) is pruned in late fall or midsummer. Zones 4 to 9, 35 to 50 feet.

![Clear drops of sap hanging from the tips of thin, bare birch twigs in early spring](https://bluewhaletrees.com/img/articles/a-tree-dripping-sap-or-foam-3-1000.jpg?v=0c8e5c6f "Sap running down birch twigs where a branch broke off, on March 19, 2022. A birch cut in late winter or spring bleeds sap, which does the tree no harm.")
*Photo: Ben Harris, [CC0](https://creativecommons.org/publicdomain/zero/1.0/deed.en) ([source](https://commons.wikimedia.org/wiki/File%3ASap_dripping_from_a_birch_tree.jpg))*

Sap can also seep from a broken branch or an old wound while the spring flow is strong. Yeasts that settle in it can turn it orange or red and set it bubbling, with a smell like bread. It stops when the flow ends or the wound closes, and it is no sign of trouble inside the tree.

## Pitch, sawdust and ooze from sunken bark

A wounded pine or other conifer bleeds resin rather than sap, and the resin seals the wound. Resin welling from small holes in a pine's trunk marks borers or bark beetles, whose tunnels the tree floods to push them out. On a broadleaf tree, borers start a flow of sap or leave sawdust-like frass in the cracks of the bark. Locust borer larvae, for one, make sap ooze around small holes in [black locust](https://bluewhaletrees.com/plants/black-locust/index.html.md). The signs and the remedies are the subject of [borers in a young or stressed tree](https://bluewhaletrees.com/articles/borers-in-a-young-or-stressed-tree/index.html.md).

Dark brown or black ooze from a patch of dead bark near the base can be bleeding canker, a soil-borne disease of poorly drained ground that kills bark on [beech](https://bluewhaletrees.com/plants/american-beech/index.html.md), maples, magnolias, dogwoods and oaks. A [canker](https://bluewhaletrees.com/articles/glossary/index.html.md#canker) like it sinks or cracks the bark around it. Slime flux and injuries can look the same, and only a lab test of the bark tells them apart. Sap running from neat rows of small holes is the work of a woodpecker, the subject of [sapsucker holes](https://bluewhaletrees.com/articles/sapsucker-holes-and-wasp-damage/index.html.md).

## When to send a photograph

Honeydew, slime flux, rain suds and a bleeding cut can all be left alone. Look closer at white froth with a beery smell, pitch or sawdust at holes in the bark, and ooze from bark that has sunk or cracked. Those signs matter most where leaves are yellowing or limbs are dying back. Blue Whale Trees answers questions about a tree you already have through its [support page](https://bluewhaletrees.com/support/index.html.md). Say what the ooze looks and smells like and when it began, and photos of the trunk and the whole tree can follow by email once we reply.

## Common questions

### What is the sticky stuff falling from my tree?

Honeydew, the sugary waste of aphids or soft scale insects feeding in the leaves above. Water washes it off, and where it stays a black sooty mold can grow on it.

### What is the white foam on my tree trunk?

Suds at the base after heavy rain are rainwater frothed by what built up on the bark, and they do no harm. A white froth with a beery smell oozing from cracks in the bark is alcoholic flux, a sign of a badly stressed tree.

### Is slime flux killing my tree?

Seldom, since most trees live with wetwood for years. Do not drill or drain it, because a hole spreads the bacteria and lets decay in.

### Should I seal a pruning cut that is bleeding sap?

No: the bleeding does no harm and stops on its own, and pruning sealants are not recommended. The one exception is an oak or an elm cut in the growing season, where a sealant keeps away the beetles that spread disease.

## Sources

- [Slime Flux of Trees](https://purduelandscapereport.org/article/slime-flux-of-trees/): Purdue Landscape Report, Purdue University (Tom Creswell and Gail E. Ruhl), with *Beware the Honeydew List* (Cliff Sadof), *Phytophthora Bleeding Canker* (Tom Creswell) and *Help! My conifer is bleeding!* (John Bonkowski): slime flux as dark, foul-smelling sap forced out by gas from bacteria and yeasts that enter through wounds, its frothy white exit and gray crust, its hosts, no treatment and the harm of drain tubes and boring, wound avoidance, water and mulch; honeydew from aphids, soft scales and mealybugs feeding by tube-like mouthparts, clear droplets that glaze leaves in dry weather, pit car finishes and turn black with sooty mold, natural enemies, systemic insecticides that harm pollinators and beneficial insects, oaks, tuliptree and honeylocust as dependable drippers, and placement away from cars and seats; bleeding canker's dark ooze near the base, its hosts and wet sites, look-alikes and the lab test; and resin from conifer wounds and from borers and bark beetles.
- [Wetwood](https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/wetwood/): The Morton Arboretum, with *Scale insects*, *Aphids*, *How and When to Prune Trees* and *Borers*: wetwood's hosts, its bacteria from the soil and root wounds, liquid that darkens in air, froths and dries to a crust, the quotation on drainage pipes, honeydew that draws ants, bees, wasps and flies, sooty mold blackening roofs and porches, aphids washed off with a jet of water, the bleeder trees and pruning times that avoid bleeding, sealants only for oaks and elms cut in the growing season, and sap and frass at borer holes, among them the locust borer.
- [Bacterial Wetwood or Slime Flux](https://extension.psu.edu/bacterial-wetwood-or-slime-flux): Penn State Extension (Gary W. Moorman), with *Tuliptree Scale* and *Magnolia Scale*: slime flux on elms, oaks, tulip poplar and maples, the pressure of 60 pounds per square inch in an infected trunk against 5 to 10 in a healthy one, little damage in most cases, yellowing, wilting and dieback under severe stress, the liquid killing grass at the base, tuliptree scale on tuliptree, magnolia and linden, and magnolia scale on magnolias only, favoring cucumber magnolia, and able to kill a heavily infested tree.
- [Why Are My Trees Blowing Bubbles?](https://bygl.osu.edu/node/1595): Ohio State University Extension, Buckeye Yard and Garden onLine (Erik Draper, June 2, 2020), with *Leaking Trees* and *Yeasty Beasties, Slim Flux, and Other Natural Wonders* (Joe Boggs, 2019 and 2020): suds at the base after heavy rain from salts, plant compounds and dust on the bark, stemflow mixing, no harm to the tree, the quotation, alcoholic flux as a frothy, fermenting ooze from small bark cracks on stressed young trees, the insects it draws, water stress, bark beetles and wilt diseases behind it, and spring sap flow from wounds, colored by yeasts and stopping when the wound closes.
- [Maple Diseases & Insect Pests](https://hgic.clemson.edu/factsheet/maple-diseases-insect-pests/): Clemson Cooperative Extension, Home & Garden Information Center: aphids on maples whose honeydew makes sidewalks and cars sticky, and soft scales that excrete honeydew while armored scales do not.

## Related

- Next step: [Write to us](https://bluewhaletrees.com/support/index.html.md#article=a-tree-dripping-sap-or-foam)
- [Sapsucker holes and the wasps that follow them](https://bluewhaletrees.com/articles/sapsucker-holes-and-wasp-damage/index.html.md)
- [Fungus on a tree is not a death sentence](https://bluewhaletrees.com/articles/fungus-is-not-a-death-sentence/index.html.md)
- [Drought and an established tree: where to water, how much, leaf scorch, and the year after](https://bluewhaletrees.com/articles/drought-and-an-established-tree/index.html.md)
- [Lichen and moss on bark: harmless, and what they say](https://bluewhaletrees.com/articles/lichen-and-moss-on-bark/index.html.md)