My neighbor has a tall very leafy tree. The leaves are kind of heart-shaped, or larger at the top than at the bottom. They are arranged on the stem. I am finding little mini trees growing all over my side yard where his tree grows even though we are separated by a fence. I try to pull them out but they are obviously rooted to his tree. They come up between my stone sidewalks and, I'm worried that they could damage my pool. How can I kill these root suckers growing in my yard?
This sounds like a Quaking Aspen, Populus tremuloides, or other type of suckering tree. You might have to dig a trench between the neighbor's tree and your yard. It should be 18" to 2' deep. Install metal siding or plastic vertically in the trench and backfill. Just pull up the little trees. They won't survive, the roots won't survive without the main tree and any photosynthetic growth to make food. You might have to do this all summer but by next year you shouldn't have a problem.
This is far easier and cheaper than using chemicals. In fact, without the trench, chemicals wouldn't work at all.
Send pictures of the tree, of your yard and the baby trees.