Snowball bushes produce white hermaphrodite flowers, which are organized in clusters of 2 to 4 inches in diameter. Each cluster has a ring of sterile large flowers, which can reach 1 inch in diameter. These flowers surround the center of the cluster, which is made of diminutive fertile flowers, measuring about 0.2 inch in diameter. Each fertile flower contains both male and female reproductive organs, respectively pollen-producing stamens and an ovule-producing carpal. Although not directly involved in the process of pollination, infertile flowers play a important role attracting pollinators.
The pollination occurs only in the small central flowers that form the snowball bush cluster. Insects, such as bees, butterflies, flies and beetles are the main pollinators. Pollen grains germinate in less than one hour to 20 hours after pollination. Once germination occurs, petals and stamens start to wither, and the ovary, located at the basal part of the carpal, starts to grow and eventually becomes a fruit. As the snowball bush cannot self-pollinate, seed production depends on pollinators visits.
Cetonia is a genus of beetle from the Scarabaeidae family. These beetles play an key role in the pollination of Viburnum opulus, preferring this species to other plants flowering at the same time, according to a study from the University of Uppsala Sweden and published in "Oecologia." Cetonia beetles can travel about 60 feet to feed on the pollen and nectar of V. opulus, even when other flowering plants are four times nearer. This behavior is not observed in other pollinators of the snowball bush.
Some factors can interfere in the pollination success of the snowball bush, including frost during the flowering period and the lack of nearby species with compatible pollen for cross-pollination. Poor weather conditions also affect pollinators such as beetles, butterflies and bees, which travel shorter distances when temperatures are lower than 50 degrees Fahrenheit. The increasing use of insecticides to kill agricultural plagues can also reduce pollinators' population numbers.