I believe they are rich in silicone, which makes them good as pot scourers (that was a bad Victorian idea, eh ! ) They are almost inevitably high in nitrogen as well, like all fast growing evil b******rs.
Don't know about using them as veggie "tea" - as long as they are well dead, I don't see why not, I do it with dockens and so on - but why don't you try using a flame gun on them ? I used a big paraffin one on cloudberry bushes, which have roots that go 15 feet deep and 30 feet sideways (or the other way around, I didn't care as I wasn't going to dig ) and that stopped 'em dead in their tracks. Heat seems to shock the whole plant. I don't know if a twee wee gas gun would work though - maybe a blowlamp ?
Funny to think, when the dinosaurs were around, these things were 90 feet tall and formed huge forests. Then bacteria that could eat lignin evolved, so they died and made fossil fuels. Who is having the last laugh now....?