Certified GKH Gardening Expert
Weed trees tend to be an issue because even if you cut them back, they regrow from the roots. The best way to kill the roots after cutting them back is to paint the cuts on the saplings with concentrated RoundUp right after you cut the tops off. The roots will suck the RoundUp in and this will kill the roots.
To keep new saplings from germinating, treat the area with a pre-emergent like Preen or corn meal gluten on a regular basis. This will keep the seeds from growing.