The specific bloom times for "snowball bushes"—a name that most often refers to viburnums—vary by species, ranging from late April to early June. If a snowball bush isn’t covered with blooms during its appointed time, one factor or more may be to blame.
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Three varieties of viburnum—European viburnum (Viburnum opulus roseum), Japanese viburnum (V. plicatum) and Chinese viburnum (V. macrocephalum)— display snowball-like flowers. They produce white, globular flowers for a week to 10 days each spring.
Viburnum buds form in the fall and survive cold weather wrapped in thick sepals. A few early warm days or a late freeze confuses the shrub’s natural sense of timing. Depending on the timing and duration, the variation may either reduce or eliminate blooming. Too much shade will also limit blooming. Viburnum needs at least six hours of sun each day.
Viburnum, like other spring bloomers, must be pruned immediately following blooming. Fall and spring pruning remove the wood and buds necessary for the plant’s spring blooms. They may also not bloom the first year after they have been planted. Renewal pruning in spring will limit blooms for several years. Too much nitrogen may also limit blooms by forcing foliar growth. Garden fertilizer with little or no nitrogen is recommended.