wildlifeman, If these are groves of suckering native plums, then the suckers are genetic clones of the original seedlings in these clumps, and the fruit will be identical to those you are so fond of. The native Chickasaw plums here in the Southeast sucker prolifically, and it's a simple thing to dig a few suckers and transplant them - Voila! you'll have your own plum thicket in virtually no time. Air layering might work, but digging suckers in the dormant season and transplanting will be much easier - and establishment will probably be faster as well. I'd be marking some likely prospects now, for digging this fall or late next winter. Some of these native plums root relatively easily from cuttings, so if you'd care to give that a try now, you don't have much to lose.

I'm not the best person to recommend varieties for NM- I know the northeast. What's your elevation? I'd think New Mexico would be a good site compared to TX (less humidity) but I'd grow something like Empress there. Better quality and it does equally well in the northeast as in CA so it probably is pretty good where you are. I don't know if it's self fertile though. I think Castleton is and is a perfect shaped home orchard tree as grown on Myro. It's naturally pretty dwarf and a consistent cropper. But as I say, my experience is in the northeast. Maybe you should post another message asking about best E plums for New Mexico.

The sucker is probably the rootstock. Original graft was probably where the bend is a couple inches above the tag. So anything below the bend is rootstock variety not Santa Rosa.

Ironically, I have the opposite challenge: a semi-desperate quest to propagate an heroic purple plum before it is completely dead. No luck, so far, in germinating pits or in rooting cuttings. The fruit comprises the prettiest and tastiest plums I've experienced in half a century; foliage is lovely, and the tree plows on through the searing, arid climate. Borers love it to death, too, though, and I gather that fruit trees can be rather short-lived, anyway. Maybe the edge-of-the-desert conditions in north Texas suppress "suckers" here: I've never seen a single one in a decade-and-a-half or so.