Question: I have 15-year-old viburnum that is showing signs of decline, and 80% of the plant looks like it’s dead or dying. What could be happening? When and how should I prune it?
Many gardeners choose viburnums for their attractive flowers, berries and the wide variety of leaf sizes, textures and fall color. Flowers are white or pink, berries can be yellow, orange, red, pink or black and fall foliage ranges from red to scarlet to purple. With more than 150 species, the viburnum genus includes both deciduous and evergreen shrubs. Some species grow into trees up to 30 feet tall.
Most viburnum prefer a sunny location, but some do well in shade and are adaptable to a range of soil types and moisture conditions. Most types of viburnums are native to North America or Asia and thrive in plant hardiness zone 7, which covers Central and Southeast Virginia.
Many are considered as problem-free shrubs in Virginia, but there are a number of reasons that they can decline.
Diagnosing Plant Disease
Diagnosing the plant disease or pest affecting a viburnum starts when you identify the specific type of viburnum, learn its key characteristics, and proper plant care. Then compare the plant’s damaged appearance with a healthy plant of that species. Look for both the symptoms (changes in the plants physical appearance) and any signs (evidence of the disease or insects).
In this case, the primary damage to the plant is obvious: large areas of the viburnum are dead or dying. However, careful observation of the plant and its surroundings may reveal other signs of damage. Ask the following questions to narrow the possible causes.
- Is it a problem likely caused by nonliving factors (environmental or human-caused)? Damage can be caused by:
- mechanical factors like girdled roots or stems, branches broken by a mower or weed trimmer.
- physical factors, like extreme cold or heat, too much sun or shade, drought or too much rain, or insufficient air circulation, or wind exposure.
- chemical factors due to inappropriate use of pesticides.
- cultural practices, including soil pH too high or low, improper planting, too much or too little fertilizer, water, or bad soil drainage.
- Is the damage due to living factors (disease-causing organism or insect)? Do you see evidence of:
- a fungal, bacterial or viral disease?
- an insect infestation or wildlfe damage?
Virginia Cooperative Extension provides useful guidelines for diagnosing plant disease and describes common symptoms and signs of damage that fungal, bacterial and viral diseases have on infected plants.
What Could Be Wrong With This Viburnum?
For the most part, viburnums are not seriously affected by diseases or insects, unless the plant is stressed or in poor growing conditions. However, this virburnum seems to be the exception.
First, consider whether your plant has been damaged by one or more of the nonliving factors noted above. Perhaps, after 15 years of growth, the viburnum is being crowded by other plantings, receives too much shade, has suffered from a prolonged drought or accidental exposure to pesticides, or general lack of plant care. Keep in mind that any of these factors can weaken the plant and make it more susceptible to disease and pests.
If the nonliving factors are unlikely to be the source of the damage, look for symptoms and signs of disease or insect pests. A number of diseases and insects can cause this type of severe decline in viburnums. These include:
- Botryosphaeria Dieback and Canker: a fungal disease that causes cankers (sunken dead areas) that eventually girdle the branch or trunk. Branches fail to produce leaves in spring and then die.
- Downy mildew: a fungal disease that appears as light green spots on upper leaf surfaces, spots enlarge into angular patches between the leaf veins and a downy white growth appears on the underside of leaves. Defoliation occurs with severe disease.
- Armillaria root rot: a fungus that attacks plant roots and results in stunted growth and yellowed leaves. In some cases, there is no evidence of a problem and then the shrub dies. If the entire root system is infected, the plant cannot be saved.
- Scale: these small armored insects can infect viburnum and cause branch dieback. If the infestation is serious, it may kill the shrub.
- Root weevils: The grubs inflict more damage than adults. They cause the plant growth to be stunted and leaves turn pale or yellow. Grubs feeding on the roots and crown may kill the shrub.
- Viburnum leaf beetle: Both the larvae and adults of this insect feed only on viburnum leaves. Consecutive years of infestation can cause substantial dieback and eventually kill the plant.
Virginia Cooperative Extension’s An Introduction To Integrated Pest Management provides helpful general information on both chemical and nonchemical controls and the 2021 Integrated Pest Management Guide:Home Grounds and Animals provides in-depth information on cause, prevention, management and control of specific plant diseases and pests.
Pruning Viburnum
Spring blooming shrubs like viburnum develop flower buds on the previous year’s wood and should be pruned in the late spring or early summer, shortly after flowering is complete. In Virginia:
- prune deciduous viburnum during the months of May to July and
- prune the evergreen variety in May or June.
Winter pruning should be avoided, because pruning at that time will reduce the number of flowers that bloom in the spring. Generally, viburnums do not require heavy pruning and it can be done with hand pruners. Pruning is normally done to open up the interior of the plant to air circulation, and keep the plant the desired size and shape. However, more extension pruning may be necessary if there is a lot of dead wood, storm or other damage, or to correct structural problems. For further information, see this pruning calendar, guide to successful pruning, and pruning basics and tools for directions on when and how to prune viburnums and other shrubs.
Can the Viburnum Be Saved?
The answer depends on the the diagnosis. In some cases, the disease or pest will have done too much damage. The VCE Horticultural Help Desk is a good resource if you need assistance with the diagnosis or treatment.
More on Viburnums
If you are looking to replace that viburnum or plant more in your home landscape, the best practice is to diagnose the problem with your viburnum, BEFORE choosing new ones. For example, if your viburnum failed to thrive because of too much shade, you can avoid future problems by planting in a sunnier location. If a soil borne fungus like downy mildew caused the viburnum’s decline, you will want to plant new ones in a different location. Think also about varieties that are native to your area. This article focuses on good native choices for Virginia and this link identifies 5 species of viburnum native to Albemarle County. Fall is the best time to plant them!
References
“A Guide to Successful Pruning: Pruning Basics and Tools,” French, Susan C., Appleton, Bonnie Lee, Virginia Cooperative Extension, Publication 430-455, 2009.
“A Guide to Successful Pruning: Shrub Pruning,” French, Susan C., Appleton, Bonnie Lee, Virginia Cooperative Extension, Publication 430-459, 2012.
“A Guide to Successful Pruning: Shrub Pruning Calendar” French, Susan C., Appleton, Bonnie Lee, Virginia Cooperative Extension, Publication 430-462, 12 Jun. 2021.
“A Pruning Primer: Tools, Techniques and Timing”, Ralph Morini, The Garden Shed, Piedmont Master Gardeners, Feb. 2020.
“Choosing Your Viburnums With Care”, Maryland Invasive Species Council, 01 Apr 2017.
“Guide to Identifying Viburnums,” Swaciak, David, Cornell University, Updated 13 Mar. 2018.
“Oh No, Somethings Wrong With My Plant!” Morini, Ralph, The Garden Shed, Piedmont master Gardeners, Vol 7, No. 6, Jun. 2021.
“Viburnums: Flowering Shrubs for Every Season”, Michael A. Dirt, Timber Press, 2007.
“Viburnum-A Shrub for Many Settings”, Martin, Susan, The Garden Shed, Piedmont Master Gardeners, Vol 5, No. 10, October 2019.
“Viburnum Diseases,” Moorman, Gary W., PennState Extension, Updated 31 July 2016.
“Viburnum Diseases & Insect Pests,”, Home and Garden Information Center, Clemson Cooperative Extension, Factsheet HGIC 2057, 10 Mar. 2021.
“Viburnum Fact Sheet,” Home and Garden Information Center, Clemson Cooperative Extension, Factsheet HGIC 1075, 25 Jan. 2021.
