When it comes to Rhododendrons, amateur gardeners are eager to learn various techniques for growing these plants. Methods of increase are cuttings, air layering, grafting, layering, and raising plants from seed.
Except for grafting, these methods can work for the home gardener who wants to increase his stock of choice kinds. For the specialist, the thrill that comes from raising seedlings is even better.
Growing Plants From Cuttings
In growing plants from cuttings, it’s known that the lighter shades root with ease, while many of the red flowering kinds are difficult. Hence, a large number of these sought-after colors are grafted plants.
However, some growers obtain a high percentage of stem-rooted cuttings by using a high concentration of hormones. Yet, to advocate this procedure for all Rhododendrons would be unwise.
Some of the Catawba hybrids treated in such a manner rapidly put forth an unusual amount of root growth. Examination a month later showed that these roots had become dried and brown.
It would be hard to specify the optimum strength of the hormone powder required for best results. The concentration varies with the ripeness of the wood, differing with each variety.
However, a large proportion of the early maturing cuttings made in June and early July can be rooted successfully with Hormodin #3. In general, as the season advances, one may increase the amount of indolebutyric acid up to 2%.
The most reliable method of determining amounts of hormone treatment is to run a sample test and note the comparable results in 10 days to two weeks.
Here, great care is needed to check that there is no variability in the plants or another factor that may change results.
Depending on the variety, take cuttings in June and at the end of January. The ideal cuttings to root are thin shoots of the current year’s wood from three to six inches long.
These are often surrounding the elongated terminal shoot. Avoid terminal shoots with fully farmed flower buds. If you don’t, lesser results will come from these than front the lateral shoots.
“Heel” Not Essential
In taking rhododendron cuttings, some gardeners ask if it is essential to take a heel. Fairly comprehensive tests revealed that cuttings made with sharp pruning shears or a knife gave proportionately higher percentages of rooted cuttings than those left with a heel.
Wound each stem cutting by making an inch and one-half slice lengthwise with a sharp knife. Expose the cambium layer to allow a larger surface for the hormone treatment.
To reduce the evaporation through the foliage, remove a few leaves from the cuttings or half of each leaf. Both methods are used and found to be beneficial.
How to do it is a matter of individual preference. The majority of grower have reduced each cutting to three leaves.
Simple Propagation Kit
Simple propagation kits can be made with a standard flat and covered with a plastic tent. The constant humid atmosphere within this unit closely resembles nature’s conditions and becomes an ideal place to nurture the cuttings.
Use a mixture of sand, peat moss, and granular styrofoam for the rooting medium. This mixture has excellent moisture-retaining qualities. The addition of styrofoam ensures proper aeration for the cuttings and prevents the medium from packing and becoming stagnant.
In high humidity and a temperature of between 65°-85° degrees Fahrenheit, cuttings will rapidly develop callus tissue. In six to ten weeks, this will produce a mass of roots.
Fortunately, cuttings rooted in this medium can winter over in the same flat kept in a room of 60° degrees Fahrenheit. Harden off rooted cuttings by removing the plastic tent gradually.
With the coming of spring, plant rooted cuttings outside in the partial shade. Sawdust is an ideal mulch for young plants. To obtain compact and shapely plants, pinch out the terminal bud when planting.
Layering Method Of Propagation
Another method and one that is much more successful is layering. Difficult plants of Catawba and Fortune hybrids can be readily increased by this method during the early spring months.
When practicing this method, make a two-inch slice lengthwise along the stem with a sharp knife, taking last year’s growth. Treat the wound with Hormodin #3, then place moist peat moss over the treated cut shaping it into a ball.
Cover the moss with a piece of polyethylene plastic. Tie it at both ends and in the middle, using electrical scotch tape, overlapping the tape on the top to keep out excess moisture.
When roots appear after six to eight weeks, sever the rooted cutting beneath the ball of moss and remove the plastic. The rooted cutting is ready to be potted in a mixture of peat moss.
No special training in the field of propagation is necessary to root rhododendrons by layering.
Select shoot or even a large branch of an established rhododendron and bend it down into moist, fluffy, peaty loam. Early spring gives the best results. Then cut a slice from the wood that comes in contact with the moist loam.
Firmly peg down this section with a stone or a notched stick. Twist the free end (one to two feet) of the branch upward. To ensure a well-established root system, delay the removal of the rooted plant until the second year.
November to April is the time to plant seeds. Use a quarter-inch sieve and sprinkle sphagnum moss evenly over a flat containing peat moss. In a still room, sow the dust-like seeds evenly over the sphagnum.
Water the seed flat well with mist and cover the flat with a sheet of the plastic film until the seedlings appear, which should be within three weeks. These seedlings will develop with fewer chances of damping-off in a temperature between 60°-75° degrees Fahrenheit.
When it is time to transplant the seedlings (usually when they have their first two leaves), move them into a mixture of peat moss. Grow them in a partially shaded spot.
Because rhododendrons have fibrous roots close to the ground, they are sensitive to lack of moisture. Seedlings are much less hardy than mature specimens, so it is advisable to cover the seedlings with evergreen boughs during the first two winters and use a mulch of sawdust.
HCM-0356 – L Lewis, Ohio