We always put epsom salts in the stumps to rot them out...kill the roots off. But I am no expert, and that was not on poplar

Use Round-Up on that stump, and wait 2 weeks, even if you plan on removing the stump later. If you don't, you will have the roots sprouting new trees EVERYWHERE! I use Round-Up diluted to HALF-STRENGTH with water. You only need to paint it on the the growth layer (the outer edge of the cut stump, where it is green and alive, and you can use a tiny paintbrush for this, like a child's watercolor brush), and you must do it IMMEDIATELY. Make sure the temperature is warm enough (60 degrees I think) and no rain is predicted for hours. This has worked for me every time. And if you DO get suckers coming up, cut them off and paint Round-Up on the cut surface immediately. I have Aspens and they are aggressive sprouters, so you have to be diligent about cutting and treating them as they come up.