I've had this tree popped up in my yard and I have no idea what it is.

Alder or bitter cherry. Were there any flower blooms this past spring ?

Ash seedling

Ash trees pic below grow in Washington /Oregon . The leaf growth and bark of this young tree looks similar .

Not ash. Ash has opposite leaf arrangement and the shown tree has alternate leaves.

Probably black walnut planted by a neighborhood squirrel.

bumplbea said:Alder or bitter cherry. Were there any flower blooms this past spring ?

Leaves are compound.. one stem several opposite leaflets.
The leaves are alternate on the trunk.

This rules out Alder and bitter cherry which just have simple single leaves.

Maybe a walnut. What do the crushed leaves smell like?

Im also with walnut/hickory.

Thank you for everyone's inputs! The crushed leaves does have a scent but I honestly can't tell what it smells like. I'll leave the tree alone for now and let it grow.

Did the leaf stain your fingers?

I believe you can rule out Black Walnut - Juglans nigra - as the leaflets are not serrate, not even a little.

I think it is much more likely - especially given chiaseede gardens in Seattle - that this could be Juglans regia or some such, given the entire margins on those leaflets. The evident buds seem also to match with Persian/Common/English Walnut.

I do not know that plant well, but I think it is a common plant used in PNW landscapes and for nut production. Take a look around at neighboring trees, from which a squirrel or chipmunk may have foraged and planted it for you.

I would guess it is Tree of Heaven...Ailanthus altisisima a wide spread invasive species from China. Not a bad looking tree but it is a prolific seed producer and really stinks, in my opinion, when it blooms. My neighbor has one.

The "texture" of the leaflets looks too smooth for Tree of Heaven to me.

Paul2032 said:I would guess it is Tree of Heaven...Ailanthus altisisima a wide spread invasive species from China. Not a bad looking tree but it is a prolific seed producer and really stinks, in my opinion, when it blooms. My neighbor has one.

Leaves are serrated and acute on tree of heaven, not round....
also slightly darker...

Definitely not Ailanthus altissima..no sign of any glands on the leaves.

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

Would suggest that you remove this tree asap.
It is going to be enormous.

DaisyI said:Did the leaf stain your fingers?

No, it did not but it could be that I didn't crush it well enough.
I've done a bit of research and based on all of your suggestions, it does look a lot like a walnut tree. I think it could be English walnut based on the size of the leaves. The leaf I had pictured is at least 20 cm long and the black walnut leaves look smaller than that. I'll make sure to take a walk around the neighborhood and keep an eye out for walnut trees. Once I get confirmation, I'll probably chop it down because I've heard they aren't great for the soil. Thank you for everyone's inputs, they are all greatly appreciated!

If you decide to take it out, use your shovel and cut the root a few inches below ground level. Cutting it down will just encourage it to grow.