If you scratch your finger in the soil near the base of the stem, you should find the shell if a walnut. That would be one easy way to idenify :-)

Thanks, Mario! Surely enough, I just found a half shell a couple inches below the surface. I'm certain it's California black walnut. Now just the mystery of where the squirrel brought this from, I haven't seen a walnut tree anywhere near us, especially one native to southern California.

We have black walnut in Michigan. At my old house we didn’t have a walnut tree anywhere near us, but I would still get a squirrel transplant in the yard every so often. Now we have some large walnut trees a few doors down from us and I get them all over the place. I don’t mind though. It gives me some plants to play with. I’ve dug them up in the fall/winter to transplant. They seem really vigious, but I haven’t been able to get one to survive yet.

not native to NorCal but certainly grown there.

https://www.calflora.org/cgi-bin/species_query.cgi?where-taxon=Juglans+californica

Your walnut may have been dropped by a crow or planted by a squirrel. The tree maybe from the 1600’s, planted by missionaries. Father Serra may have been responsible for planting walnuts from San Diego to Sonoma, for shade.

Some people put out nuts for squirrels. I saw a squirrel hiding a walnut in a park near me and there are no walnut trees anywhere in the area.

I have two growing in the empty lot next to my home. One has been there over 50 years..the other a seedling from ma walnut. The seedling is crowding the fence line and growing fast on water it steals from my yard. Ma Walnut can't be more than 30 foot tall,maybe less. The seedling has shot up..its going for more on my water.

Interesting is that Crows,big birds this closeup..take the walnuts and drop them in the street in front of my house and let the traffic crack them open. Sometimes,I'll aim for a nut myself to give the birds a break. My stepping on a California Walnut isn't enough to crack them let me tell you.

They are also famous tree's for being allopathic. Something to worry about on the tree close to the fence.

Walnuts are weed trees in NorCal, they freely volunteer and are barely edible. I would pull it.
Carla in Sac