I have this weed tree that’s planted itself in my backyard and it grows like crazy. I spent 2 hours cutting it back this weekend and I’m trying to find the best way to kill it. I sprayed the open trunk with weed killer last night but I doubt that will do the trick. 
I noticed the larger stumps left after my cleanup were seeping some liquid after I cut everything back. Any idea what that is? 

I’ve heard pouring gas on the open trunk could kill it but I don’t think dumping gas on my ground is the best option. Do any of you have best practices for killing this thing?