Beeone, I know that most of Wyoming is colder than here in western Montana, but FWIW there are several carpathian, and I think a few black, walnuts growing here, doing quite well for years. They don't seem to forgive drought, though.

Good question BeeOne. It certainly helps to have multiple nut trees. All different kinds to be exact. See my site link below. I have a nut orchard, however, in early stages. I've got two black walnuts, one grew from the others seed and is about 1' tall. I didn't see anything important about pollination of the black walnut tree, however, do be aware that they give off a retardant chemical to surrounding trees. Other trees will grow much slower due to this. I would suggest looking into an English Carpathian Walnut as a prospect pollinator. They are good in your zone 4-5. I get mine from WillisOrchards.com . Phone in your order, not online. Tell them you wish to get ample roots with it and go with the 2-5' tree range. The largest ones come with the ever so needed tap root severed.

Here is a link that might be useful: Green Anything !

beeone,
gpm really likes to push Willis - and I don't know but that they're a good nursery - but if I were located in z4 Wyoming, I'd be more than a little bit hesitant about ordering some things from a z8 nursery in south GA.
I'd want to make sure I was planting a selection that could withstand z4 winters - on rootstock that can also handle that situation. Case in point - a number of years back, a fellow planted 1000 grafted pecans here in KY - all were northern/midwestern selections, but came from a TX nursery, grafted onto Southern pecan rootstock; a particularly harsh winter killed the rootstock back to the ground - or outright, on a signficant number of trees - and although the grafted variety would have otherwise been relatively unharmed, the loss of the understock doomed them.

Some of the Carpathian strains of the "English/Persian" walnut, J.regia, may be reliably hardy for you, and they can cross-pollenize with black walnut. For whatever reason, hybridization between black walnut and butternut doesn't happen with any significant frequency, so I don't know if they can't readily pollenize one another, or if most of the resulting nuts just don't have viable embryos to produce seedlings. Hickories aren't compatible as pollenizers for walnuts, so far as I'm aware.

Seems like I recall some hardy walnut varieties trialed & released from the Morden Research Center, in Manitoba, that might be worthy of a look. John H.Gordon, Jr.(Amherst, NY) and St. Lawrence Nurseries(Potsdam, NY) both offer nut tree varieties proven to perform in cold, northern zones(even z2-3) - I think JHG has some of the Morden selections. SLN offers only seedling nut trees, but JHG offers both seedlings and graftlings; both have websites, with good info, if you'd care to search 'em out.

Thank you all for your comments. Mark, you are probably the closest to me in climate, although I think you are wetter overall and average a little warmer in winter than I am. I live in a giant rain shadow with generally low humidity, and I'm east of the Divide and have noticed that many (though by no means all) of the real arctic blasts seem to plunge down east of the Divide, leaving me in the frigid air while western Montana and Idaho stay relatively balmy. But, then the beauty of these valleys is that they get a batch of cold air and it just hangs around while the chinook zones start bouncing between +50 and -20, so that is a benefit even if eternal cold gets old.

Seems the wisdom in my area is that if you aren't growing apples, it won't survive the climate. But, I've also found so many things that won't grow here that someone is growing successfully. They may get winter killed every 15 years and have to replant, or maybe in 25 or 50, but if you aren't growing commercially, I think it is worth trying and am prepared to occasionally replace trees after a bad winter if they are otherwise productive.

I've seen St. Lawrence's site and have actually bought some pear trees from them several years ago. One of the trees has had fruit a couple times now and it is doing well. The others have bloomed, but never set fruit, so I grafted another variety to it this past spring to hopefully get a good nearby pollen source for cross pollination.

My black walnut was a seedling I dug up from a grove in a little valley at the base of the mountains about 45 miles away. It was a few hundred feet in elevation lower than me, but I'm out in the flats and still at a low elevation--only 4,500 feet. I've noticed with native trees that only a few hundred feet in elevation can make a real difference in which ones grow wild and which ones don't, although the ones that don't can still usually be successfully grown at a bit higher elevation if transplanted, so I always wonder if I might be crossing an elevational line in trying things that tend to be marginal anyway.

Anyway, I had gotten the impression that black walnut was the hardiest, so started with it. Now I'd like to experiment with some of the others, but since they grow to be pretty big trees, I can't do too many and still avoid underground drains or power lines above or interfere with areas outside the yard.

Anybody know whether they are compatible to graft onto each other? That would be less space intensive, and I certainly would like to find out if I can get a carpathian type walnut to survive since I like them better than the black walnuts for eating.

Beeone,
Actually, butternut is considered the most cold-hardy of the walnut family - there are some that hang in there all the way into zone 2. But the butternut blight fungus is decimating it in large expanses of its native range - you WY folks may be isolated enough that you'd escape it.
To some degree, the future is already well established among us, as there are a number of 'butternut' cultivars that have been propagated for decades that have, with the advent of DNA analysis, been shown to actually be hybrids with the Japanese walnut, J.ailantifolia, or its budsport the heartnut, J.a. var cordiformis. These hybrids resemble butternut in virtually all respects, but have varying degrees of resistance to the blight fungus.

Graft-compatibility is quite friendly across the genus, though black-line disease seems to be a concern for some folks doing Persian-types commercially. Most eastern nurseries use J.nigra as a rootstock for all walnut species - Paradox hybrid rootstock seems to be preferred in some areas on the west coast. I have butternut, Carpathian, heartnut, and black walnut here - all grafted onto seedling J.nigra rootstocks. I've not tried to build a multi-variety nut tree, but it could be done.

Thanks, Lucky. I will play!

B1 - email me off list, when you get a chance.