Do they flower ever?

Not that I know. They have woody stems

Opposite leaves? I'm thinking Acer negundo.

I agree with lycopus. A suckering Manitoba maple (acer negundo). Is there evidence that one was cut down? I've never heard of one getting black knot, that's a plum tree problem. Can you give another picture from further back?

Here is a link that might be useful: Manitoba maple

I believe you're both right lycopus and OVG, thanks a lot.

This is small lot (8' x 8') and I recall seeing other foliage.
which could be a plum seedling/ sucker, hence the black knot.

However, I focused on the most prolific one, which must be the Acer negundo, as there is one gigantic one growing around 50 yards away.

I don't have other pictures but I'll try to get some tomorrow.

Here is another picture. As you can there are many stuff growing. I just focused on the most prominent leaves.

Poison Ivy?

I think Manitoba maple even more seeing this last picture. Looks like someone has sheared all the plants in that location, and the maple has responded accordingly.

I doubt it's poison ivy, since this far north, it is a low-growing plant.

Thanks OVG, you're most certainly right.

They get sheared by the owner every year :-)

It's indisputably Acer negundo, Manitoba maple.