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Issue: February 20, 2010

Now is a good time to prune deciduous trees and pruning sealer will not help and will not stop slime flux.

Q. When is the best time to remove cottonwood tree branches? Is there something to coat the "wound" to stop sap flow? Last spring I cut off a low hanging branch [3 inches in diameter] and it "bled" all summer killing the grass beneath. I have two more branches needing removal; is now a good time before the sap starts to flow? Any "dos" or "don'ts"? Thanks for any advice you can provide.

Paul D.


A. The best time for major pruning of deciduous trees is now, the late dormant season. When necessary for safety or to remove dead branches, any time is the right time. However, when you are cutting large branches, do it during the dormant season. That time will end soon.

When a tree is properly pruned, there is no need for a pruning sealer. The dripping sap is mostly water with some mineral, so the tree just drip irrigates itself. However, your description of the sap "killing the grass beneath"suggests that the tree has a disease called "wet-wood disease" or "slime-flux". This is a bacterial infection of the heart wood that changes the pH of the sap so that it is caustic and damages grass or other things below. The solution is to frequently wash the area to dilute the dripping sap. Periodic washing of the pruning cut with ten percent chlorine bleach solution (1 part bleach to 9 parts water) will help control odors that attract insects. The pruning sealer is not necessary and can actually cause more harm than good. However, if you feel you must put something on the cut, use a cheap (for the sake of your wallet), white, latex (water-based) paint to cover the wound. But, realize that this is for your benefit more than the benefit of the tree.

Proper pruning involves cutting branches in a manner that does not damage the "branch collar". This is to prevent damaging the trees own protective mechanism. NMSU Extension publication H-156: Tree Pruning Techniques (available at http://aces.nmsu.edu/pubs/_h/h-156.pdf) describes proper pruning practices and illustrates the branch collar. This publication gives more of the "dos and don'ts" than I'm able to cover in a single newspaper column.

Now is the time to remove the two remaining branches, but first download the publication described above or get a copy from your local NMSU Cooperative Extension Service office.

For more gardening information, visit the NMSU Extension publications World Wide Web site at http://www.cahe.nmsu.edu/pubs/_h.

Send your gardening questions to:

Yard and Garden, ATTN: Dr. Curtis Smith
NMSU Cooperative Extension Service
9301 Indian School Road, NE, Suite 112
Albuquerque, NM 87112

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.