I think any spray that would kill the suckers is also likely to kill the
tree. I would just continue to mow them down or individually pull them
(being careful not to harm the root system). I have crape myrtles that did
the same thing, but they eventually stopped putting out suckers after I cut
them back a few times.
Quaking Aspen send up suckers and there is nothing that will stop them.
They have a shallow root system, and create a whole grove by suckering. I
have suckers come up that do not seem to be anywhere near the Aspens!! If
the parent tree is large and established, hitting the suckers with a spot
spray of Weed-B-Gone will not lead to enough herbicide going into the main
tree to damage it. Do not use Roundup, as it will damage the entire plant.
Otherwise, it is a ongoing job of cutting them off just below the ground. We
have many Aspen in the neighborhood, so the job never really ends.
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