My mulberry tree was leaking sap earlier and it left ugly black stains down the trunk. The tree has deeply furrowed bark. Can I remove this?


3 Answers 3


That leaking sap & stained bark appears to be caused by bacterial wetwood, also known as slime flux. This is a common bacterial infection that creates slightly caustic sap & raises pressure within the wood. The pressure forces the sap out at weak points, staining the bark. The sap smells sort of fermented, right? That's the result of the bacteria having chowed down on the sugars in the sap. You may also notice bugs showing up for a party.

You shouldn't try to remove the bark. You can remove extra sap by infrequent gentle washing with plain water (rain is free, don't buy a hose for this.) Removing the discolored bark or trying to plug/patch will just cause additional spots where sap will leak instead, and may interrupt any compartmenting the tree's done on its own. The best management of the situation is to keep the tree free from additional stress, and living with what it looks (and smells) like.

The good news about slime flux is it can actually prevent other sorts of rot! It also generally does not kill trees. (I've had one infected in this way for 10 years, and it's healthier & larger than ever aside from the flux.)

  • 2
    Yup! You're the first to answer this right! (And welcome!)
    – J. Musser
    Commented Sep 26, 2016 at 1:01
  • FWIW my reference tree suffering bacterial wetwood is still growing strong (15 years now), the original leakage area completely healed, with no bark or sap removal, no treatment, and only minimal bark staining remaining there. The tree continues to show symptom (excessive sap leaking, fermentation odor) whenever pruned, however as the tree grows prune points are inevitably higher up, so odor and staining are not as obvious as when the tree was younger.
    – waded
    Commented Aug 26, 2022 at 16:52

First, I can't stress this enough, before considering to remove the sap from the trunk, it would be best to take the time to identify the reason for the leaking sap ie Your tree may very well have bigger problems than just sap stains on the trunk...

Second, I would be very wary of removing sap from a tree trunk, doing so can lead to inflicting even more damage on the tree.

Third, keep in mind it will be almost impossible to remove all the sap from the trunk .

To remove the majority of the sap, use a (very) sharp, large blade (2inch/50mm) woodworking chisel that has been cleaned with something like rubbing alcohol.

Keeping the chisel as near parallel to the trunk as you can, very! carefully slice, pry off the sap. Whatever you do, do not let the chisel cut into the bark, or allow the sap to pull away the bark from the trunk. If the sap is really hard, use a hammer to very! carefully insert the chisel into the sap.

Once you've "very! carefully" finished removing as much of the sap as you can from the trunk with the chisel, take a clean (with rubbing alcohol or similar) woodworkers card scraper or decorators/painters hand scraper and very! carefully (this time keeping the tool perpendicular to the trunk) scrape off as much of the remaining sap as you can without damaging the bark.

I recommend stopping there, don't try to remove anymore sap.

If you felt compelled to try and remove even more sap, the following may work (disclaimer, I've never tried it).

Mix up some eco-friendly "degreasing" cleaning solution (read the label to ensure it is safe to use around plants, etc), then apply directly to the sap via a (new) stiff bristle cleaning brush. Allow the solution to "work into" the sap as per the instructions on the label. Then thoroughly rinse off with cold water.

  • Yeah the bigger problem would be bacterial wetwood.
    – J. Musser
    Commented Oct 18, 2014 at 1:01

Prune the tree at the correct time of the year for that type of tree to avoid the stain from a leaking wound.

Over several years the stain does go away as the wound heals closing the leak. It's not a sick tree, it's just healing the wound!

  • 1
    What is the correct time of year to prune this type of tree?
    – Niall C.
    Commented Sep 21, 2015 at 21:18
  • Hint: It is a sick tree (badly infected with slime flux). I posted this as a filler question, still waiting on someone to post the right answer ;)
    – J. Musser
    Commented Sep 22, 2015 at 3:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.