chicagocanine,
Your best bet would be to call Ortho directly to ask, be sure to specify what kinds of weed trees you're talking about. I know from experience that if you are trying to kill buckthorn weed trees, which are found in your area, it is very difficult and I'm not sure if this would do it. There are also other suckering trees that can be a problem.

You also may need to paint it the stumps using one of those cheap disposable painting sponges rather than spraying it on. For me, I've had better coverage/control this way. You also may possibly need to drill some holes in the stumps and just pour a little in to sit there. For stumps a stump killer may work better. These are all suggestions I received from Ortho for various situations. Again, I think it all depends on what weed trees you are trying to kill. I'm in a similar situation, good luck!
G.

Thanks! I have no idea what kind of trees they are :P
They are very small and skinny so it's hard to identify them... I was searching before for a stump killer but could not find any, even after asking the clerks at a couple stores they had no heard of it! All they had was something you spray on to protect a stump, nothing to kill it.

Ask for brush killer not stump killer. Follow directions.