Thank you for directing me to that site on hybrid plum pollinization!
Excellent article!
This year was the first year that my hybrid plums, have produced super
abundantly, like the pics you see in Gurneys, branches drooping from crop
load, large plums every three inches or so.
All plum trees have P americana frames topgrafted to the hybrid plums. I let
some P americana branch out lower on the tree so that I could test the fruit
and also get extra pollinization.
The P americana, in the first year or two seems to flower out of synch with
the hybrid plums. I attribute the abundant crop this year to the blooms of
the P amer and hybrids having overlaped for the first time, becoming more in
synch as they have grown together. This is just a guess though, I have not
formally recorded bloom times. (I neglected to thin any of the plums, but,
this was not necessary with Superior which was 50% shaded, overbearing, and
still produced large deliciouis, well flavored plums. Monitor doesn't seem
to suffer from lack of thinning either).
I also have P besseyi seedlings nearby but they didn't seem to be
pollinizeing the hybrids.
The only plum in that area that didn't produce well this year was S Dakota,
if I recall it blooms much earlier, but it did crop very well last year,
perhaps more in synch with the P amer. than this year)
About 150 ft south of that plum thicket are two large plum trees with very
little cropping, there are no native plums down there only another hybrid
plum. I suppose they weren't pollinized because they were too far from the
very few P americana branches.
A friend of mine up northhas a Pipestone with no pollinizer nearby, it still
set fruit, I suspect due to a native thicket about 100 yards away.
Kevin B
zones 3 and 4/5 Wisconsin, north and south