I have one spot on my place that I planted that when I checked it out last summer it was just thick with poison ivy.
The hillside faced north/east and got very good morning sun but then was shaded nicely the rest of the day. There were lots of companion plants growing in there too, ferns, including maiden hair, may apple, bainberry, etc. There is also wild seng growing just down the hollow a bit from that spot.
Last summer I put the round up to it and sprayed it real good and waited 3 weeks and came back and sprayed it agian. I focused on the poision ivy and tried to leave the rest (best I could).
I planted it this past fall/winter and the seedlings are doing great and there are very few weeds in the beds and the poison ivy is gone (for now at least). I hope it don't come back.
I HATE poison Ivy and can't say I would have sprayed that area except for the thick presence of it.
That is one thing that I would definately call a \"weed\" to my seng beds. Anything that really bothers (itches like heck) the grower - most certinaly qualifies as a weed - even if it can be called a companion plant
TNhunter