Asexual, or vegetative, propagation of viburnum shrubs remains the most common and economical way to reproduce the plants, especially cultivars or hybrids with specific desired traits. Sowing seeds is economical, but takes more time and resources. Plus, the resulting seedlings may mature to produce flowers, fruits or other features you may not like, while those produced from cuttings will be exactly like their parent plant. Bear in mind, however, that not all viburnum shrubs root from cuttings as easily as the next.
Ease of Cutting Success
Cuttings taken from the shrubs vary in their success of sprouting roots and surviving their first winter, according to plant expert Michael Dirr, author of "Viburnums." Species, cultivar, age of the plant, seasonal timing, use of a rooting hormone and the handling and maintenance of the cuttings in potting soil mix all determine success rates. In fact, success of rooting cuttings from the same plant may vary year to year, on the season, or other small variables. A first attempt at rooting cuttings may fail, but a second attempt could prove successful.
Fill a tray or 4-inch pots with a soilless rooting medium in which to insert viburnum cuttings. A mix of 3 parts perlite to 1 part peat makes a prime substrate for cuttings to develop roots. An automated mist system is ideal, according to Dirr, as it keeps the substrate evenly moist until cuttings root well. He also recommends an artificial light source to promote strong leafy growth on cuttings once the roots form. For example, a 60-watt incandescent bulb shining 24 hours per day suspended three feet above the cuttings.
When to Take Cuttings
Take cuttings of viburnums in mid to late spring, depending on climate and growing season. The cutting, about 4 to 6 inches long, needs to include semi-hard new growth. Do not use thick, old stem tissues or the green, flimsy stem tissues that are at branch tips. In the southern U.S., such as in U.S. Department of Agriculture plant hardiness zone 8, make cuttings around May 1 to 15. Farther north, in USDA zone 4, the same species or cultivar needs to have cuttings taken around June 15; in USDA zone 5, around June 1.
The use of a growth regulator, also called a rooting hormone, improves chances for viburnum cuttings to quickly sprout roots in the substrate. Typically the hormone is indolebutyric acid (IBA) in liquid or powder form. Varying concentrations of the hormone yield different results on different viburnum species or cultivar cuttings. Experiment. Some viburnums root with diluted hormone treatment, while others need significantly stronger concentrations to yield roots.