Asked April 05, 2015, 5:18 PM EDT
Ramsey County Minnesota
It can be a long process to eliminate poplars from the landscape. These new trees popping up are all clones of the original - large trees. Most all of them originate from the root systems of the older trees, (as opposed to seedlings). If you have chosen to retain any of the large trees, they will continue to show up over the years.
I had a similar situation in the yard of our recreational property and was able to suppress most the the new growth by simply continuing to cut out, or mow over the newly growing trees. This may not be a desirable fix for you if these trees are showing up in the lawn, because the stubs of the destroyed trees are woody and can create an uneven, uncomfortable walking surface, but it's the easiest, and least toxic method. This process will have to continue as long as you have healthy older poplar trees in the area.
Otherwise, if you want to eliminate All of the poplars you will have to cut down all of the older trees and immediately treat all of the exposed freshly cut surfaces with a strong brush killer. This is best done during the fall, as that is when the trees are drawing down the fluids and nutrients to make it through the winter. The brush killer will migrate throughout the root system and discourage any new growth. You may still see some new growth for a year or two if all of the roots have not been killed. Simply cut off the new growth and apply the brush killer to the fresh cuts.
If you simply treat the cut surfaces of the newly sprouted trees, the chemical will migrate back to the parent plant and weaken, but not kill it. They will become more prone to insects and disease and eventually will turn into "hazard trees" that will have to be removed before they fall, unpredictably.
Ask at the garden center for the most appropriate brush killer for these trees, and follow the directions carefully. The chemical will kill any, and all other plants that it touches. There may be some small danger to the roots of other, desirable trees in the area if the roots of the trees have become intertwined and grafted onto each other.
I hope this is helpful. Please contact AaE again if you have further questions.