Japanese knotweed is an invasive herbaceous plant that may grow 8ft tall or more (comparable to the size of saplings) - and has hollow stems.
Alternatively, Elder (Sambucus spp) is a tree in the proper (common) sense, but has stems that may be hollow or filled with soft pith. Actually, there are quite a few plants that might reasonably grow to something called a tree, and might have hollow-ish stems. Forsythia and philadelphus are another couple.
Several of y’all seem to be ignoring the “hollow stem” part of the OP. 
Just how tall were these tree-like weeds? Also, where were they, geographically? Jimson weed has a hollow stem and can get to be pretty dang tall. The aforementioned pokeweed also grows pretty tall. It has a stem with a spongy center.
I couldn’t say how tall they were, looked like 50 feet at least. Geographically, they were growing in a vacant lot in north Philadelphia.
50ft?!
I withdraw everything I suggested.
Yes, that’s pretty sizeable for a hollow stem. However:
Perhaps some sort of temporal rift has opened…
Sure, even extant bamboos can get pretty big, but it just doesn’t seem all that likely someone would describe them as weeds that look like trees.
In common use, “weeds” refer to herbaceous plants, but, as said, any nuisance plant can be considered a weed. The better term for an out of control tree would be “invasive”. Here’s a site from Penn State that details invasive plants, with plenty of links. Maybe you can find the “tree” you’re looking for there. If you can describe the leaves, trunk, and perhaps flowers, I can probably help you narrow down the ID.
If its as big as all that, it’s almost definitely an Ailanthus. A true, and disturbingly hardy, tree. I was looking up ways to kill it and found this link:
http://www.nps.gov/plants/alien/fact/aial1.htm
which among other things, suggests powerful herbicides mixed with fuel oil, gasoline, or diesel. Yikes.
