We have about a dozen pine trees that need to be cleaned out around the trunk. We have whacked down the tall weeds, which are now growing back. We plan on killing them off with glyphosate in the next week or so. I'll apply it with aa paint roller so there is no drift to hurt the trees. Then we would like to regularly put down the pre-emergent granules (Treflan) we use in the ornamental beds. We will also mulch, but weeds always find a way to come through mulch!
I seem to remember awhile back that someone mentioned pine trees could be harmed or killed with this type of chemical treatment. Can you confirm or dispute this? Thanks.
Mulching under pine trees
As long as you are using the Roundup type products I would not worry about any effect on the trees. Some broadleaf herbicides can effect the roots of trees.
As for the Preen products (Treflan) there are several formulations. Most are safe around trees.
In most cases if you keep a 3 to 4 inch layer of mulch around the tree you will have no weeds. This would be my option instead of using a herbicide. The mulch will provide many other benefits.
Weeds normally are not the big of problem in the shade.
Dennis - Johnson County Extension