Closely related species (eg. V. nudum and V. cassinoides) can cross pollinate if they bloom at the same time. Most of the others need two plants of the same species, but not clones. For example, two V. nudum 'Winterthur' will not pollinate each other. They'd need a V. nudum 'Earthshades' or a straight species V. nudum. Same with V. dentatums and dilatatums. And of course, they must bloom at the same time.

Thanks for such a well-written explanation! I purchased viburnum trilobum compactum plants from Forestfarm several years ago-- and although they survived for ten years or longer, they never did anything. Now I understand why they never produced fruit . . . but actually, they barely even bloomed! I finally gave up and pulled them out this spring. Very disappointing. As to why plants of different provenance bloom at different times even if they are of the same species, maybe you could think of it this way: every species has some genetic variability within the species. A group of specimens growing in a certain area would tend to produce seedlings that bloomed at several different times. However, the seedlings that bloomed at the best time for that location would have a natural advantage over the seedlings that bloomed a little too early, or a little too late. Therefore, by natural selection, the bloom time that is best for the area would eventually become the most commonly found genetically-programmed bloom time amongst plants in that area. In other words, the gene pool of seedlings in that location would have been narrowed. Similarly, seedlings from a different part of the country would slowly narrow their genetic pool to reflect whatever bloom time is best for their location. When you then order plants from the two different locations, even though you are now growing them in identical conditions, their bloom times will continue to be influenced by their genetic programming as well as their current growing conditions-- and this will be true for as long as those two plants live, even if they are growing side-by-side. Does that make sense? I realize that I'm not explaining it very clearly. :( (Maybe another way to understand it is to think about humans: we are all one species, but we look a lot different according to our ethnic backgrounds because when groups of people were isolated geographically from other groups, an unusual or recessive trait-- red hair, for example-- had the opportunity of becoming very common within that specific isolated population.)

Are you concerned with berry production? If so, you need a different clone of the same (or a closely related) species that blooms at the same time. For example, V. dentatum 'Blue Muffin' will need a V. dentatum 'Red Feather' or a V. dilatatum 'Cardinal Candy' will need another dilatum clone. They can be some distance apart--50 feet maybe or more.

I have a Viburnum dentatum across the lawn from my mapleleaf viburnum and I am absolutely convinced they pollinate each other. The key is overlapping bloom time. They have to bloom together. And I know this is true for mine because when some of the mapleleaf viburnums flowers precede the blooms of the V. dentatum, they don't make fruit. The blooms that open in sync with the other always make fruit.