English Walnuts scions are often grafted onto a black walnut root stock. You need to find the graft union and remove all sprouts beneath it. Sprouts from the root stock will be black walnut, while the grafted stock will be all English.

English are also grown on their own (other English Walnut) root stocks, too.

I had to re-read this and rhizo's response a couple of times.

Nuts produced by Persian(they're not English) walnut, grafted onto black walnut rootstock will be a combination of the genetics of the Persian 'mother' and whatever pollenized the nutlet flowers.
If the Persian nutlet flowers were pollenized by another Persian walnut, then the resulting seedlings will be pure Persian. But if your Persian was pollenized by a black walnut, then the resulting seedlings, grown from those seednuts will be hybrids of the two parent species.

I should clarify, these are not sprouts from under the graft union, these are buried nuts sprouting around the property. Lucky, what you're saying makes sense: the young plants have serrated leaves, like the black walnut, (and unlike the parent English or Persian) but they have a terminal leaf, unlike the black walnut. I thought, though, that walnuts were self pollinating? This is the only tree, though I imagine there could be others around, though not in the immediate vicinity. Thanks for your info.

If they are sprouted seeds of an English walnut and if they are pollinated by an English walnut (or self-pollinated), then the offspring will be an English walnut regardless of what rootstock the tree was grafted on. There is no mixing of genetics from the rootstock to the top.

It's a matter of semantics; I grew up calling them 'English' walnuts, and it's deeply ingrained in our culture and language, but they're not 'English' - J.regia is of Eurasian origin, and is more correctly called Persian or Carpathian walnut.

Agreed, bob. The rootstock has no influence on the genetics of the seeds produced by the grafted scion variety.
But, if there are black walnuts(or even hybrid seedlings) growing in the area, which may provide pollen to cross-pollenate the nutlet flowers on marystem's J.regia, then the resulting seedlings may be hybrids.
In some areas of the country, 'Paradox' (J.regia X J.hindsii) seedlings are preferred as understocks for J.regia varieties.

My question is, if you have a bunch of black walnuts around and one carpathian walnut, would the black walnuts pollinate the carpathian walnut?

if you cut the top off roots .. and then graft a tree on top ...

the nuts from the top are from the tree/graft on top ...

the roots can not send genetic material up the trunk to form nuts different from the material grafted on top ..

is that the root [pun intended] of what you are asking??

ken

ps: the best example.. is in pines.. you can use a number of different understocks ... but the cones.. will not be variable.. they will be the top grafted tree ...

Here I have a lot of seedlings of both walnuts and it is impossible not to tell the leaves of one from the other. If the leaves are serrated as the back walnut, that is what you are growing. If you are using the wood for furniture, the black walnut is better. The black walnut nut meats are also highly regarded for their better flavor, but few have the patience to dig them out of the shells. Al