Good question. I grafted on existing trees or sprouts. I guess you might say that I own a plum thicket. I have an area where, in the early 80s, I remember planting 3 Prunus besseyi plants and a Red Diamond Cherry Plum. (I was young and foolish back then.) The Red Diamond was never hardy enough. It only produced a few fruits on low branches that got covered by snow. The fruit was nothing special anyway. The P besseyi never fruited, in part because I changed my mind and grafted them over to 3 hardy plum cultivars. Toka, Pipestone, and I don't recall the 3rd one. They all fruited but besseyi is known to not live real long and they eventually failed. When the Red Diamond died, plum shoots started popping up here and there. I believe these sprouts are Prunus Americana. I have 4 or 5 mature trees that I grafted on these random sprouts. Some of my grafts last year were on these sprouts. I grafted Toka high in another tree in the group. Wanetta, I think. I still have one area where besseyi sprouts up. The biggest one did make 3 little fruits last summer. Little yelloish things. Not much flavor. I grafted one of the failed grafts on that one. I was doing a little pruning in the plums this evening and I came across a failed graft that I forgot about. Scion wood that seemed good when I grafted it. So I wasn't 100% with the good plum scion wood after all. Close, but not quite.
Steve