JARS v57n4 - Tips for Beginners: Propagation by Cuttings

Tips for Beginners: Propagation by Cuttings
Dr. Mark G. Konrad
Sewickley, Pennsylvania

The following elements are necessary for successful propagation by cuttings.
1. High humidity around the cutting. This can be supplied either by misting, fogging or closed container techniques.
2. Porous rooting medium. Canadian peat and perlite are most often used.
3. Proper hormone treatment. 1

Notes and Commentary
Evergreen azaleas and small-leaf rhododendrons are the easiest to root. The growth is generally indeterminate meaning that the plants will continue to grow as long as the environmental conditions are ideal. That is probably why they are relatively easy to root. The deciduous azaleas have a strong dormancy factor and can be difficult to root. Allow the first flush of growth to pass into dormancy only slightly. It is too late when there is too much hardening or maturity of the stem. The proper concentration of a rooting hormone treatment is also needed. In addition one should have a method of growing on with extended light periods after rooting occurs.
Large-leaf rhododendrons have a strong dormancy cycle. Most cuttings are best taken in late summer and early fall after the plant has passed into early dormancy. Wounding is essential with a strong concentration of a rooting hormone.
Commercial propagation has been successful with the use of greenhouse facilities. Misting, fogging or polytents can be used in conjunction with bottom heat, which is a key factor. Keeping the environmental temperature as cool as possible may be important since water evaporation possibly plays a key role. Under these conditions cellular growth is stimulated along the stem by the bottom heat while the vegetative response is held at bay. Holding the plant in a normal dormancy cycle is an important consideration. Premature stimulation of the vegetative component under fluorescent lighting has been a negative in my experience.
After rooting (approximately 3 months) a period of after-chilling is considered beneficial. The temperature is lowered and held around 40°F (4°C) for about twenty days after which the night temperature is raised to a minimum of 65°F (18°C). Supplementary light can also be added at this point for growing on.
Generally, the most difficult to root deciduous azaleas and large-leaf rhododendrons may have missing cofactors (auxin synergists) and other inhibiting factors necessary for rooting. It has been postulated that the failure of cuttings to initiate adventitious root formation, even after the use of auxin, may be related to one or more of the following: lack of necessary enzymes, enzyme activators or the presence of enzyme inhibitors. Other factors could be separation of enzyme reactants or the lack of substrate phenolics (Plant Propagation, Principles and Practices, Fifth Edition, 1990, p. 217).
That is why highly specialized techniques are necessary and why amateurs may have difficulty with the rooting process. Using the Nearing frame might be the best approach by the majority of amateurs. The issue of getting the plant out of a normal dormancy cycle is sidetracked; however, the downside is that it may take several years for some of the cuttings to root.

Table 1. A Guide to Rhododendron Propagation by Cuttings.  Conditions for: Pittsburgh, Western PA Region
Plant Type Timing Hormone
Stem Wounding Tip removal
Evergreen Azaleas Late June, early July Not necessary Not necessary Yes
Small-leaf Rhododendrons Late June, early July Proper hormone concentration Not necessary Yes
Deciduous Azaleas Mid June or when stem shows signs of early
maturity (hardening)
Proper hormone concentration Not necessary Yes
Large-leaf Rhododendrons Late summer, early fall for most some midsummer Proper hormone concentration Yes Leave apical vegetative bud intact.
Flower buds should be removed if present.

Baldsiefen, Warren. 1965 & 1971. Nursery Catalogs.
Davies, Hartman ester. 1990. Plant Propagation, Principles and Practices, Fifth Edition . Simon and Shuster Company.
Leach, David G. 1961. Rhododendrons of the World . New York: Charles Scribner's Sons.
Wells, James S. 1982. The propagation of hybrid rhododendrons, an historical review. J. Amer. Rhod. Soc. 36:4.

Dr. Konrad is a member of the Great Lakes Chapter and a frequent contributor to the Tips for Beginners column.

1 Proper hormone treatment is as follows:
FIRST CHOICE: 3-indole-butyric acid, which can vary in strength from 35 ppm (parts per million) to 600 ppm. The cuttings are pre-soaked approximately 18 hours. Use the following strength, depending upon the type of rhododendron:
• Small-leaf rhododendrons: usual 75 ppm
• Deciduous azaleas: usual 75 ppm
• Large-leaf rhododendrons: usual 150 ppm
• More difficult to root cuttings: up to 600 ppm
SECOND CHOICE: Liquid commercial hormone dips. The label should be read for proper concentration.
THIRD CHOICE: Hormone powders.