In an article in the Spring 2010 edition of the magazine Gardener for the Prairies, Rick Sawatzky outlines what kind of pollinators different plums need. In short, the main problem for many gardeners is that hybrid plums cannot be pollinated by other hybrids; they must be pollinated by wild plums, which are difficult to find. A city dweller's single apple tree will probably be well pollinated, as there is likely a compatible apple or crabapple tree nearby, but it is not as likely that their neighbour will have a plum tree suitable for pollination, and plums also need to be nearer than apples or pears for adequate pollination.
So what kind of pollinator do you need? There are three main types of plums: true wild varieties, salicina hybrid plums, and salicina plum cultivars (also sometimes called Asian plums). Wild plums can be difficult to find commercially, but you can look for the species names Prunus nigra, Canada plum, or Prunus americana, American plum. 'Dandy' and 'Bounty' are two cultivars sometimes sold as wild plums but which, according to researchers at the University of Saskatchewan, are not good pollinators and are probably actually hybrid varieties. Wild plums must be pollinated by other wild plums, but they will also pollinate all other varieties of plum, including hybrids, cultivars and even hybrid cherry plums.
Salicina plum cultivars (or Asian plums) are usually very hardy, with good tasting fruit that has a short shelf life. Some examples are the 'Ptitsin' series, 'Brookgold', 'Green Elf', 'Fofonoff' (also called 'Homesteader') and 'Ivanovka'. These plums will pollinate other plums of the same type only, and can be pollinated by either other salicina cultivars or wild plums.
The salicina hybrids were developed by crossing native wild plums with plum varieties from California that were not hardy, producing hardy trees with good quality fruit. These include 'Pembina' (sometimes called 'Prairie', 'Acme' or 'Elite'), 'Patterson Pride', 'Brookred', 'Geddes' and 'Perfection' (sometimes known as 'Superb'). These hybrids will only produce fruit if they are pollinated by a wild plum, and they do not provide pollination for any type of plum, including each other. This may explain why some growers have poor fruit production with these trees.
Hybrid cherry plums are crosses between the western sandcherry and Asian plums that grow into bushes three to six feet tall and produce smaller fruit. Any two of these varieties will pollinate each other, as well as pollinating sandcherries, and they can also be pollinated by wild plums.
Of all these varieties, 'Patterson Pride' is often considered to be a worthwhile cultivar, with sweet flesh and a thin skin, and a tree with an attractive weeping habit. But remember, it must be pollinated by a wild plum!
If you've been keeping track, you'll see that any planting of plums will require at least two trees. Two Asian plums will be sufficient, or you could choose a hybrid and a wild plum. But if you want fruit from the wild plum, you would need one hybrid and two wild plums (that's a lot of jam!). As long as you have one wild plum, however, you could plant any other type of plum or cherry plum and have adequate pollination. Some gardeners have success with grafting different varieties onto one tree, and some garden centres sell combination trees (but check to be sure the varieties are compatible). Of course, the most effective solution might be to convince your neighbour to plant a wild plum tree - good luck!