Black cottonwood. Towering native deciduous tree liking moist soil conditions. You probably have some older ones with much bigger, broader leaves growing nearby, unless it came in with a newly purchased plant or other material.

Should I let it go as it wants to keep growing. We don't have anything in the neighborhood even remotely like this?? So it may have come from a plant I planted last yr?

Thanks bboy for the id. I just did a google-it may provide shade, & it is kinda pretty in the breeze, but that tree would take up the blacktop we just had laid. Its gotta go. Thanks again

The leaves look too narrow and shiny to be Black Cottonwood, and they look serrated. Black Cottonwood's leaves are more round and heart-shaped, and they are not serrated.Their stems aren't that reddish-brown, either.
I'm not sure what your plant is, but it looks more like willow than cottonwood.
Here's a picture of Black Cottonwood (Populus trichocarpa)leaves: http://oregonstate.edu/dept/ldplants/potri2.htm

Youngsters have comparatively short, slender leaves. That's why I made a point of mentioning older examples nearby would have bigger, broader leaves.

But the twigs are the wrong color. And the leaves are VERY shiny.
If this were true about the young trees, why wouldn't I have learned about this in my Plant ID classes at South Seattle Comm. College? Why doesn't my "Native Plants for Northwest Landscapes" book show or mention this? It does say leaves can be lance-shaped, but it also says "Young trees have smooth, green bark.." It says nothing about reddish-brown.
Why doesn't Oregon state show pictures of younger leaves? You'd think if that were really what young Black cottonwood leaves looked like, the true experts would say so.
I am not convinced.

These come up in nursery pots, edges of paved areas etc. rather often. With all the watering that goes on in nurseries thousands of seedlings may appear if stands of flowering sized specimens are nearby. Go to where you have seen there are new ones coming up or there are likely to be seedlings and have a look.

I was doubting that it was a cottonwood, but can't say for sure. The leaves just don't look right. But the alternating leaf attachment should fit.

If that was a spring photo, I'd be more convinced, because the leaves seem more glossy that time of year.

Doubtless you've both seen them before, but not realised what they were.

Drove by numerous examples today where the small slender leaves could be seen grading up into the big ones right on the same specimen. Go look at a stand of black cottonwoods that is actively reproducing.