There's a lot of interest in controlling tree of heaven. Art Gover at Penn State conducted numerous trials on this weed and summarized the information in a great fact sheet.

I can sum up the control recommendations as:

1.  No cut stem treatments -- this stimulates root sprouts.

2.  Foliar or basal bark applications of glyphosate or triclopyr (or a tank mix) will work. Timing is important for glyphosate foliar treatments -- for best results spray in late summer / early fall.

This is a tough perennial weed; one treatment will not finish the job.  It will take several years to control an established infestation of tree of heaven.  For more detail -- see the fact sheet linked above.