There are more than 150 varieties of viburnum, a flowering shrug that includes semi-evergreen, evergreen and deciduous cultivars. Many cultivars bear ornamental fruits, but viburnums are best known for their spectacular blossoms that can range from cream and white to partly or wholly pink. Their ability to withstand cold varies with the species, but most can stand U.S. Department of Agriculture hardiness zones 4 through 8.
Experts at the Lady Bird Johnson Wildflower Center at the University of Texas, Austin, say that although many varieties of viburnum are native to the United States, others come from Asia and South America. Viburnums are known for being dependable bloomers. If your viburnum doesn't bloom, it could be because the species you have planted isn't appropriate for your growing zone. If that's the case, it might not ever bloom.
Too much fertilizer that is nitrogen-rich can discourage your viburnum from blooming. Plants need to bloom so they can reproduce. If they're getting too much nitrogen, a flowering plant can get "lazy" and not bother to bloom. If you fertilize, don't use lawn fertilizer, which ordinarily contains a large amount of nitrogen.
The first number on the label of a package of commercial fertilizer, N, stands for the percentage by weight of nitrogen. The second number, P, stands for amount of phosphorous. The third number, K, stands for potassium. Lawn fertilizer will have a large ratio of N. You don't want that for your viburnum.
Some plants need to achieve a certain level of maturity in order to produce blossoms. If your plant is too young, it might not be ready to bloom.
Those who grow viburnum say one of their charms is that you really don't have to prune them. If you do prune, don't do it early in the year so that you won't eliminate buds that will turn into blossoms. Don't prune until after the viburnum blooms. When you prune, eliminate branches that had blossoms. That will encourage the plant to produce more.
Sun and Soil
Viburnums like lots of sun and are more likely to bloom if they get full sun. They also like moist, acidic soil that is well drained.
Many varieties of viburnum only bloom for a couple of weeks. It's easy to go on vacation and miss the whole show.