Viburnum are deciduous or evergreen shrubs (sometimes small trees) in the moschatel family (Adoxaceae) that originate in temperate and subtropical areas of the world. There are more than 120 species of viburnum (authorities vary on the exact number) and numerous cultivars have been developed. While some plants bloom on bare wood in winter, most Viburnum that we grow in the northeast produce white or pink flowers in the spring, followed by bright red (or sometimes blue or black) fruit (drupes) in the fall. In some species the flowers are very fragrant. The flower heads may be flat clusters of florets, flat umbels with larger flowers or snowball shaped. Their leaves are arranged opposite and many deciduous species also have brilliant fall color.
Viburnum are very versatile plants with varieties available for many garden locations and landscaping projects. Not only are they worthy of a place in the garden, there are many that are native to the United States. Choosing a species native to your area implies a relatively easy to grow plant that offers benefits to local birds, mammals, caterpillars and pollinating insects. Most viburnum experience little damage from deer.
Many viburnum are easy to grow in the New York City area. Most types are not very particular as to soil conditions but prefer moderately fertile, lightly acid, moist but well-drained soil with a pH between 5.5 and 6.5. They are healthiest and produce the most fruit and flower in full sun to partial shade, but some popular plants, including the native Viburnum acerifolium (mapleleaf viburnum) are habituated to the woodland understory and will tolerate greater shade. Most species grow well in USDA Zone 5 - 9 but some species, like Viburnum trilobum (the native American cranberrybush), are tolerant to Zone 3.
If you select an early flowering type, choose a site with some wind protection and mid-day (rather than morning) sun to reduce the chance of bud and flower damage from spring frost and wind. Viburnum grown for decorative and wildlife-sustaining fruit should be planted in multiples. Follow the selection process described in the Select identically diverse plants section below. Evergreen species and cultivars will sustain less leaf damage in winter if used in a wind-protected location.
Viburnum are most effective when allowed to grow in a natural shape. Select compatible plants for the size of your space so that pruning is kept to a minimum and their naturally tidy appearance can be featured. Space plants so that air circulation is maintained between plants and buildings to reduce the incidence of mildew and fungal diseases.
Fruit produced by viburnum are a source of food for many native and migrating birds, but without selecting plants carefully, yours may not fruit well. Most viburnum grown for garden interest are not generally found to be self-fertile. You will get better fruit production by mixing plants of the same species but purchased from different sources. Plants sourced from a single nursery, unless grown from seed, are possibly clones of each other and tend not to pollinate each other well. Sourcing from multiple nurseries enhances the chance of genetic diversity and excellent fruit set in viburnum. Plants do not need to be side-by-side to cross-pollinate but can be placed within about 50 feet of each other.
For most species, a plant and its cultivars bloom at compatible times and work as pollinators. Viburnum nudum and its cultivars can vary too much in bloom timing and only a genetically diverse V. nudum should be selected as a pairing with another V. nudum.
Smaller plants (preferably container-grown) are notably easier to transplant than larger ones. Small, container plants can be successfully planted in spring or fall and should be planted as soon as purchased. Larger plants should be moved "balled and burlapped" and deciduous plants should go in the ground when dormant (leafless) but when the earth is unfrozen, in late fall or early spring. Evergreen species should be planted in early to mid-fall or mid to late-spring.
Dig a hole at least 2 to 3 times as wide as the plant's root ball and the same depth. If the plant is "balled and burlapped," place the root ball in the hole and cut the twine wrapped around the trunk. Roll the burlap off the root ball so that it lies flat in the hole. If you can remove the burlap without damaging the root ball, then do so. Otherwise, let it lie on the bottom of the hole. Natural burlap will decay but synthetic burlap needs to be removed.
The top of the root ball should be slightly higher than the ground level, particularly for larger plants. Heavy shrubs usually settle once they are planted and watered so it's better to plant your viburnum slightly too high than too low.
Shrubs purchased in containers should be removed from their pots. Roots should be inspected. If the plant is pot-bound, take a knife and cut vertically on several sides of the root ball to loosen the roots. Plant container shrubs at the same height they have been planted in their pot.
It is important to water well after planting and mulch with several inches of organic mulch, applied with a margin of six inches from the trunk of the plant.
With some exceptions, Viburnum are at home in slightly moist and well-drained soil. Keep the soil uncompacted by adding some organic compost each spring. Mulch to hold in moisture in summer and winter. In weeks without significant rain, especially in the hottest months, irrigate soil, avoiding splashing on plant leaves. Many common problems can be avoided by preventing the plant from becoming drought stressed.
Viburnum need little pruning. If pruning seems necessary, some tidying of the plant should be done immediately after flowering (within a month). Before you prune, make sure you have comfortable, sharp, clean, pruning tools to work with.
All viburnum flower on old wood, that is to say the flower buds are formed in the previous year. Pruning too late in the year will result in the loss of next year's flowers. To shape a viburnum, prune it lightly immediately after flowering. Remove the flower heads and cut back any branches that spoil the shape of the shrub to a pair of newly sprouted leaves. Remember that removing the flower heads will preclude the formation of fruit, which can be very attractive in many viburnum.
More extensive pruning should be left until late winter and early spring. Rather than cutting back the whole shrub, it is preferable to prune to remove about a third of the branches each year. Branches can be cut back to within a few inches of the ground. The plant will be encouraged to replace these with new healthy branches annually, the overall size of the shrub will be limited and you will continue to have flowers on the remaining branches. At the same time, remove any dead or damaged branches. For winter flowering viburnum (e.g. Viburnum x bodnantense and V. farreri), even this more extensive type of pruning should be done immediately after flowering.
Finally, any suckers (new shoots emerging from underground roots causing the plant to spread) that appear during the growing season should be removed.
Viburnum need little fertilizing, but the addition of compost or well-rotted manure is helpful. An application of a balanced organic fertilizer in late winter and another after flowering is adequate and often unneeded if the soil is managed well.
Viburnum bear little or no fruit if too genetically similar to other viburnum they are planted with. Viburnum self-pollinate poorly and you will get better fruits by mixing plants of the same species but from different sources.
Leaves may lose their depth of color as nutrients are devoted to the plant fruiting heavily. For shrubs that provide generous fruit, feed the shrub once after flowering to help support fruit.
Bacterial leaf spot occurs in cool, wet spring weather. Leaves will have angular spots with a yellow corolla and tender new growth may die back. Remove affected leaves and clean under shrub to reduce reinfection in successive years. Improved air circulation reduces incidence.
Cracks at the base of the plant and stem die-back may indicate the presence of viburnum crown borer. The caterpillars of two types of clearwing moth attack the lower section of the plant stems, leaving holes, sawdust and mangled wood. The caterpillars are deposited in June and July and tunnel into the bark. Plants in good health and free from mechanical injury are the best defense.
Twisted and curled leaves on your viburnum can indicate the presence of aphids. Black bean aphids and snowball aphids hatch in the spring after overwintering on the plant. Their feeding causes leaf distortion that may remain after the insects depart for the summer and sticky honeydew coating leaves. Dark, sooty mold may grow on the honeydew. Damage resembles herbicide exposure. In many cases, natural predators including, ladybugs and lacewings, will control this problem. Viburnum dentatum, Viburnum opulus, Viburnum trilobum (syn. V. opulus var. americanum) are considered the most susceptible to these aphids. Viburnum x bodnantense, Viburnum x juddii, Viburnum plicatum, Viburnum plicatum f. tomentosum and Viburnum sieboldii have demonstrated resistance to the pests.
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