Poplar trees grow tall and straight under most circumstances, and they put out long root systems to help support them. The roots of these trees can surface and cause issues in your yard. One problem with poplar roots is that sucker growth tends to develop. The suckers can grow into new trees if you let them, but they are weaker and need to be eliminated. One way to stop the suckers or to fix a problem with a root is to kill it.
Expose the root if it isn't already above ground. Use a shovel or garden spade to remove the soil.
Chop the root with an axe or the edge of a shovel to cut it. This should kill the root portion that is cut off from the rest of the tree. However, poplars are known to put out sucker growth from the roots.
Remove a section of the cut-off root so that there is a gap between the tree root and the portion you are trying to kill. Use an axe or shovel blade to cut the root.
Spray, pour or brush on a root killer to the end of the root while it is still freshly cut from the tree. The root will absorb the chemical, which will help speed its death.