Overgrown Briars with never ending roots

Parents getting older and not able to attend the garden like the used too over last few year and have ended up with a mess of weed at end of garden - some of which are briars that had grown wild and were nearly size of a tree

Cut them down last weekend. That was interesting
Digging up the roots is proving to be problem as roots seem really deep and impossible to get them up and seem to be going in all directions not just straight down

Anyone got any ideas how we can kill these briars off wihout digging up the roots. Any products on market that will do that?

Thanks

Rootout or something similar?

Try 'Grazon 90'. Expensive but mighty stuff.

for a cheaper way you need to cut them as close as possible to the ground (which you have already done) and then paint the cut stem with either diesel or old waste oil. Essentially you just need to get the oil or diesel to work its way through the sap down into the roots.

See the pdf on this website - pages 5-5 & 5-6. 5-5 is for a tree but its the same principle.

http://www.northcoastweeds.org.au/site-files/docs/camphor/5-control-methods.pdf