It takes several months for the herbicide to work. The best time to use it is right before the weather starts to cool down, since the tree will be in a dormant period anyway.
[header] How to kill aspen shoots [title] Treat the shoots in late summer or early fall, if you can. [step] Do this before the leaves change color. Herbicides are the best way to eliminate the shoots, though they will also kill the original tree.