Answer: All sweet cherry varieties, except Stella and some Stella crosses, are self-incompatible and must be cross-pollinated with another sweet cherry. Any sweet cherry, except Lambert, can be used as a pollinator for your Royal Ann. Sour cherries are self-fertile and do not need cross pollination so perhaps you were reading the labels on sour cherries and that's why they did not mention cross pollination. In any event, your Royal Ann will need a companion in order to produce fruit.
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