To add to the discussion here, I have not yet had any problem with my
hybrid plums getting pollinated and I have no wild plums. Superior I
had grafted the previous spring and it produced an abundance of fruits
only one year later. I am in Maryland. Maybe Alan is on to something
with cold climates having more difficulty. I have had more problems
with the more pure salicina varieties such as Satsuma, and even
greater problem with pluots which are very unreliable here. Prunus
simonii hybrids also do poorly for me, e.g. Wickson. All of the
varieties that do poorly for me do well in even more mild climates
such as California.
Scott
[NAFEX] Asian/American Plums,
Scott Smith, 11/18/2008