Plums are kind of tricky but the chances of fruit are better than with the pears. Most European plums are self-pollinating. The Mount Royal plum stock came from France and the variety originated in the area of Quebec, Canada. It is zone 4 hardy (about 7000 feet in elevation), self-fertile and a heavy producer. Most Japanese plums, which actually originated in China, require a pollinator to produce fruit. That pollinator can be another Japanese Plum or an American Plum or a hybrid of Japanese and American plums. The Toka plum, AKA Bubblegum plum, is one of these hybrids. On its own it is a heavy producer of sweet medium sized plums and it will produce even more with another Toka nearby. It is one of best pollinators for Japanese plums, such as the Santa Rosa, and Japanese American hybrid plums. American and Canadian plums are usually self-pollinating large suckering shrubs that produce small (less than one inch) tasty fruit. They can be managed into more tree like forms and some varieties, such as the Princess Kay, are more of a defined tree form.