Cherry trees require another variety flowering at the same time to ensure fertilisation. They are henceforth put into flowering groups from 1 (very early) to 5 (very late). A cherry tree can be partnered with another in a group that is plus or minus 1 (+-1) its own group. To make matters more complex, even if some varieties have their flowers open at the same time they may not pollinate. They are henceforth put into groups (from A-O). Cherry trees thus require another tree in the same group and within plus or minus 1 flowering groups.
Thankfully, this is where universal donors come in that will pollinate any cherry tree within plus or minus 1 flowering group. We henceforth recommend, you pair one of these fertile varieties with another. As previously mentioned, some cherry trees are self-fertile and do not need to be paired with another. However, they do benefit from cross-fertilisation, so for heavier crops we recommend pairing.
It is important to note that cherry blossom trees will not fertilise edible cherry trees, although acid cherries will. As cherry trees are somewhat common, it is possible that another tree could fertilise your own tree. As bees forage widely, a tree within 30m could fertilise one of your own, although it is simpler to plant another within the immediate vicinity. If you believe pollination has been disrupted, you can pollinate yourself by using a paintbrush and transferring pollen from one plant to another.