Kind of looks like wisteria to me, been finding quite a bit of it all over. I thought at first it was in the bulk mulch I bought this year but this one showed up in one of my pots.

Leaves look wrong for my choices but check out:

Elderberry (there are several types)

Millettia reticulata = Evergreen Wisteria (it is neither evergreen nor is it a wisteria) but your leaf does not look like my plant.

Sumac - leaves don't look quite right for that either, but there are many types of Sumac. (Take precautions as some may give you a rash).

I'm just going to toss out an idea here, because I can't really see the leaf petioles in the image. Did you try Ash, Ronnie? They look very similar, but the leaflets do have short petioles.

Maybe ailanthus altissima...

I think Stone may be right with ailanthus altissima.

Oh gosh, I hope that's not what you have. That plant can send out chemicals that keep many native plants from growing.

http://ontariotrees.com/monday...

Yikes I just googled seedlings and it's a match. I pulled quite a few out...wonder where so many came from...had to be in my mulch

https://www.google.com/search?...

A quick walk around your neighborhood should let you see if the seeds came from a neighboring property.

You could inform the people who supplied the mulch. If they were not aware of it, they should have a chance to correct/change where they obtain their material. If they already know, then they deserve to know that you will 'out' them if they don't make a change.

Here's a good leaf photo from our database for comparison:

Good idea Greene, I will add though a few of the neighbors that would have it wouldn't care whether or not it was a problem
The place I got the mulch wouldn't care either unfortunately.
I know I won't go back next year no matter how much of a bargain it is