I have a large perimeter garden in my back yard which is filled with nice
perennials (various types of hostas and ferns). The problem I am having is
what I call "weed trees" growing in this garden. I try to get to them when
they are small and can simply be pulled out (or dug out) by the root. But I
have let some grow too big, and even digging them out by the root is not a
realistic option as the roots are too big and too deep. It would take too
long, be very hard on my body, and I still wouldn't be able to remove all of
the root. So I've been cutting them down as close to the ground as
possible. The problem is, most of them invariably grow back and in 2 years
I am repeating this very laborious process. I would like to kill these
"weed trees" once and for all.

I know there are root killer and stump killer products that you can put on a
root/stump to kill it and keep it from growing back. Is anyone familiar
with a good product I could use for this problem? Specific product
brand/name and where available would be helpful. Thanks.