Chugger 172,
If you do not want to use wood brush killer the only other option
is to invest in several brush grubbers from Gemplers to rip them
out and renting a tracked backhoe to pull on the heavy chain
and brush grubber and also enable you to dig out the clumps further if needed.
Having more than one brush grubber and several lengths of chain
with hooks and chain loops to pull several of them out at a time
within the extended radius of the tracked backhoes reach.
The brush grubber tugger chain is $72.49. The shrub clump puller is $79.99.