You can gridle the bark all the way around.

You take a small knife/axe etc and cut into the top layer of wood
throught the bark all the way around each sapling/root.

If you cannot get at it, try Triclopyr. It'll translocate, you can try
glyphosate, the round up.

It's fairly non toxic and degrades rapidly, 1/2 life was 2 days in
aquatic systems.
It binds to soil sediment rapidly and decomposes so it's that good for
aquatic submersed plants etc. Copper may work like describe also,
adding voltage will increase the reduction and this speed the process
also.

Does well on some plants that are somewhat like Alders, like Purple
loosestrife and is much less toxic than many other herbicides.

Give it 2 weeks or so.

The hillbilly approach is rope and a truck/wench.
Yank it out.

Regards,
Tom Barr