Successful pollenation is all about TIMING and PROXIMITY.
There are a few different pollinators (some say 'pollenizers') for the Black Doris plum. They include the Billington, Sultan, Purple King or Elephant Heart varieties. When I originally planted my Black Doris plum I wasn’t too concerned about which pollenator to get so long as it did the job. I had a few to choose from at the Nursery at the time. Up until recently I couldn't remember which one I actually got. So after doing a bit of research here I've concluded that I must have the Billington variety.
The Billington's plum blossoms earlier than the Black Doris, you might say a little bit too early. As you can see below the Billington Tree (background) is almost finished blossoming before the Black Doris’ flowers (foreground) are ready to open.
There was a slight overlap between the two which this year only lasted a day or 2. Unfortunately, around that time it rained and all the pollen on the remaining blossoms got wet. Thankfully, before the rain arrived I’d plucked off a couple of nice looking blossoms and manually painted them onto the Black Doris blossoms that were open, of which there were only about 4 on the whole tree.
Needless to say, my hopes of having many fruit on the Black Doris this year look pretty slim. One thing I learned about the Billington and it may well be partly the reason why I chose it was that it itself is self-pollenating. The last 2 or 3 years it has fruited tremendously.
According to one of my neighbors, whose husband planted a Black Doris tree some 30 odd years ago, the tree had never fruited until I planted my Billington. Perhaps because of the size of the neighbors Black Doris tree which is much bigger than mine, there is a sufficient overlap of blossoms to enable pollenation, even though they're about 15 metres apart. You can see where they are in relation to one another in this pic...
I’m hoping the blossom-overlap between my 2 plum trees will get longer as my Black Doris gets bigger. I’m just glad the bees know what they’re doing.