As a general rule of thumb, viburnums are not self-fertile. This means that you need two compatible plants to cross-pollinate to receive the maximum fruit production. This does not mean that you can plant two of the same clone/cultivar and expect fruit. For example, two V. dentatum 'Christom' Blue Muffin® planted in close proximity will not pollinate each other because they are genetically identical and therefore incompatible. A Blue Muffin® will however fruit if planted with another dentatum cultivar (i.e. 'Little Joe'), the species dentatum or another dentatum seedling. Another critical requirement is that for pollination to occur, both plants have to be blooming at the same time. Most cultivars within a species, i.e. cultivars of V. nudum and V. plicatum f. tomentosum, will all flower at the same time so specific plant selection is not an issue, but V. dentatum (Arrowwood Viburnum) is an exception. Arrowwod cultivars do not all bloom at the same time so it is important that the correct cultivars be planted in tandem. We can assist you in selecting the proper compatable plants. Two different viburnums are not required, however, to produce flowers. Flowering will occur whether or not pollination occurs.
Pollination can also occur between two different compatable species which will result in a hybrid. These can be of a natural occurance as would be the case in most homeowner situations or as a controlled cross; when two specific plants are isolated and pollinated in an attempt to produce an offspring with specific desired characteristics. An example of a controlled cross would be V. x burkwoodii (V. utile x V. carlesii). This does not mean that a particular species will cross with all other species. The key word here is compatable. We can also help you with selecting plants that meet this criteria.
Pollination occurs thanks to certain insects (bees, flies and moths) and wind currents. There is no set rule as to how close your plants should be planted to each other, but, the closer the better. If you live on an acreage, pollination chances are obviously much less if the plants are placed on opposite ends of the property. Keep in mind that pollination is a two way street. When we talk about planting a pollinator, both plants now become a pollinator to each other, and both will then set fruit.