I'd be afraid of killing/damaging the tree as well. I would just prune them off. I've heard that they decrease in frequency with regular pruning.

What kind of tree is having the suckers?

It is a crape myrtle that was cut down a year ago to make room for the new deck. I'm trying to kill the suckers that are coming up all over from the roots. The stump is dead and gone. I have plans for a flower bed in this area. The product I read about and cannot find the site again said this would work and also would work on a live tree if desired and would not harm it.

Well, if it is the whole thing you are trying to get rid of, try clipping the suckers in the fall and painting the cut tips with Brush Be Gone (if you don't find the product you are looking for).

Supposedly trees pass nourishment down to the roots in the fall and it is recommended to apply any herbicide to cut sections then.

I have the same problem. The stump appears dead but left over roots have sprouted. If you can find a thicker piece of the root, uncover it. Drill a hole into but not through it and then pour a bit of brush be gone into it. The super strong kind. brush be gone doesn't do much to crepe foliage even after MULTIPLE sprays, lol.

GGG