Rubrum plum is a cultivar of the edible Japanese plum species. Its official name is Au-Rubrum, as it was developed by the University of Auburn in Texas. Au-Rubrum flowers beginning in January to early March depending on your climate, and produces dark skinned fruits with red interior flesh ready for harvest starting in the middle of June. Japanese plums are not self-fruitful cultivars, meaning that trees of the same cultivar cannot pollinate each other. Hence Au-Rubrums require a partner fruiting Japanese plum tree of a different cultivar to cross pollinate them.
Choose a partner cultivar to cross-pollinate your Au-Rubrum plums. It must be a Japanese plum cultivar or a Japanese-American hybrid, as European or American plums are different species that cannot pollinate your Au-Rubrums. There are many candidates, including Waneta, Santa Rosa, Burban, Redheart, Methley, Au-Rosa, Au-Gold and Ozark Premier. You can use more than one of these cultivars if you so choose.
Plant one cross-pollinating plum tree for every five to eight Au-Rubrum trees in your yard or orchard. The cross-pollinating trees can be widely dispersed--trees within a quarter mile radius can be effective pollinators.
Provide hive habitat for honeybees if wild bees are lacking in your area. Bees are the key pollinator for plum trees, with one bee visiting and pollinating up 5,000 flowers in a single day.