These have sprung up everywhere along with every other weed (yard back's up the drainage ditch and it is covered with all kinds of weed). I've been fighting their take-over but this year, I'm losing the battle I think they are a type of tree (which DH chopped down). They have a very long tap root and hard to pull up.

Do you have a picture ??

Oops...it would help, wouldn't it?

My first thought was Fatsia japonica but that's just a wild guess.

What about mulberries?

http://ppwsipm.contentsrvr.net...

Looks like Giant Ragweed (Ambrosia trifida) to me: http://en.wikipedia.org/wiki/F...

(unless they really are tree saplings, in which case, I have no idea)

I had the same thought as Ken Giant ragweed
sometimes they grow like trees

Yep! I'm with Kent. It's Ambrosia trifida and is very common around here along with a couple of other ragweeds.