Plums with the pollinator on the left and the other
variety (in this case Red Beaut) on the right.  They are not synchronized and cross pollination will not occur.

That is why we applied supplemental pollen in  2003.   See the Pollen Blower mounted on the All Terrain Vehicle  between the double planted trees.

These are bee hives.   Bees are essential in pollinating some crops like almonds, plums and kiwis.   In warm sunny weather they fly and go from flower to flower.   On cold and rainy days they stay in the hive.    A spring with lots of warm sunny days means a high fruit set.  
A spring with few days like this means less of a fruit set.

Plum trees require cross pollination.   Note the pollinator limbs that are higher than the main variety have yet to bloom while the main variety is blooming.   This is a problem.  Supplemental pollen to achieve cross pollination is required for adequate fruit set.

Left: Plum Trees require cross pollination.   The pollinator limb grafted into the center of this Red Beaut plum is not blooming with the Red Beauts.  Supplemental pollen to achieve cross pollination is required for adequate fruit set.