Propagation and Hybridizing in the Piedmont Area
Robert L. Means
Winston-Salem, North Carolina
As a youth my family spent a summer vacation at the Taylor House in Valle Crucis in the Blue Ridge Mountains near Boone in North Carolina. The lush growth of rhododendrons, kalmia, azaleas, and pieris along rocky mountain streams and paths left an impression that never faded.
In the late 1960's I decided to landscape a newly acquired property in Winston-Salem with ericaceous plants - primarily rhododendrons. When it became evident that recreating a mountain landscape in the Piedmont would require many plants and great expense, I began exploring ways of propagating on my own.
The first seedlings were raised under Gro-Lights in the office of the anesthesiologist in the operating suite of the hospital. Dr. Leonard Nanzetta was also a rhododendron enthusiast and had purchased many plants from the late Warren Baldsiefen in New York. The convenient birthing area allowed us ample time to observe the small growing seedlings all during the days and nights of those busy years. It provided a pleasant diversion from the stresses of our workdays. Initially species seed of Rhododendron minus Carolinianum Group, R. catawbiense, and R. maximum were collected from native populations. Later species seed from China, Europe, and Japan were obtained through the ARS Seed Exchange. As proficiency and the collection of mature plants increased, seeds of personal crosses became a greater part of the annual crop. Also seeds of many other shrubs, trees, annuals, and perennials were germinated in a similar manner. The inexpensive seedlings that were not used in my garden were used as gifts to my family, friends, neighbors, and in plant auctions of our local chapter of the ARS.
In the early 1970s I joined the newly established Piedmont Chapter of the American Rhododendron Society and began a never ending education. There was a generous exchange of knowledge from such outstanding members as Herbert Heckenbleckner, Larry Mellichamp, Don Kellam, Charles Dewey, Marshall Stillwell, Tracy Lounsberry, Dallas Chappell and many, many others. Frequent field trips to the mountains, private gardens, and regional meetings provided affordable plants and other propagation material.
Most of the foreign species plants did not survive the hot, humid summers in the Southeast, but many flourished in their new habitat and have been excellent parents. They include: R. minus var. chapmanii, dauricum, decorum, degronianum, fastigiatum, fortunei, griersonianum, hyperythrum, keiskei, makinoi, degronianum ssp. heptamerum, ponticum, pubescens, racemosum, smirnowii, souliei, vernicosum, and yakushimanum.
When the rhododendrons begin blooming in early April, pollen is collected by using thumb forceps to remove stamens from mature but unopened buds before contamination of pollen brought by bees from other plants. It is then stored in small envelopes used by stamp collectors. After labeling and dating they are then placed in sealed containers, usually glass with screw top lids and kept in the refrigerator for use in current season crosses. If a later blooming plant is to be the pollen parent of an early blooming one, the pollen must be dehydrated in the refrigerator for several weeks before being held over in the freezer for use the next year. Modern frost free refrigerators allow enough dehydration although a layer of silica gel granules used by florists may be placed on the bottom of the glass jar as well. If not first dehydrated, the freezing and thawing of the water in the pollen may damage it. If properly collected and stored, pollen may be used for many years.
An unopened bud just beginning to show color is selected on the seed parent plant and is emasculated using small, pointed scissors to remove all petals and filaments being very careful to avoid touching the anther to the stigma and produce inadvertent self pollination. The remaining exposed style and stigma will not attract bees or butterflies and thus will avoid contamination by unwanted pollen.
Usually only two to three flowers on each truss are prepared in this manner inasmuch as each resulting seed pod may contain several hundred seeds. The remaining buds and individual flowers are cut away to reduce the attraction of bees to the immediate area and to eliminate nutritional competition from other developing seed pods on the same truss. It is then labeled and dated and entered into a current index which clearly defines the location of the plant. Very often the stigma is not receptive to pollen in the unopened bud but becomes so in a few days (1-5). The surface of the stigma becomes "sticky" when it is receptive, and pollen should be applied at this time and the name of the pollen parent is added to the label and to your desk-top index.
The novice should remember that the small leafed, scaly lepidotes rarely cross with the large non-scaly elepidotes. Every cross should have a well defined objective as haphazard ones are usually a waste of time and rarely produce offspring worthy of either parent. For example, I have been interested in producing a compact, hardy but heat tolerant cultivar with nice flowers. The dwarf R. yakushimanum has been crossed with the heat tolerant R. hyperythrum and back again with the large flowered, fragrant R. fortunei. Sounds like the perfect scenario, but so far after raising dozens upon dozens of plants, the trials go on. Hopefully, my son or perhaps a grandson may discover the one worthy of the effort.
The seed pods are collected in the fall from September through October when pedicels turn yellow and the capsule begins to change from green to brown. This usually occurs after the first frost. Frequent observations are required to collect them when mature but before the valves open and scatter the seeds. The capsules are placed in labeled envelopes and stored at room temperatures. Occasionally, after drying out for a month, the capsule splits on its own and seeds can be shaken out onto a large piece of white paper used for collection. More often it is necessary to assist nature to obtain clean seed. My wife's needlepoint magnifying lens with attached light and a small, sharp instrument such as a scalpel is used to anatomically split open each seam in the capsule to expose and extract the seed with the least amount of chaff. Crushing the capsule and using various sieves to separate seed from chaff may be used as well. The seed is stored in small, labeled stamp collectors envelopes and planted as soon as possible, usually in November, although seed viability persists for six to 12 months when stored at room temperature and reportedly up to five years if frozen. My experience indicates that the fresher the seed, the higher is the percentage of germination.
Square, plastic containers 4 inches wide and 3½ inches deep with eight bottom drainage holes are used. They are filled with a ½-inch layer of pre-moistened composted pine bark and then 1 inch of vermiculite or perlite or a mixture of both. The bark prevents the perlite and vermiculite from being lost through the bottom holes, especially in later stages if bottom watering is used. It is best not to compress the latter ingredients, as this may tend to collapse the small air cells and overcome the desired porous but water retaining properties. Actually vermiculite is derived from a mica-like material which in nature has small amounts of water in its layers. When heated the resulting steam explodes the material into the small kernels as we know it.
The top and final layer is 1 to 1½ inches shredded, milled sphagnum moss which is soaked in a container of water for several hours and preferably overnight. Hand wringing of water from the moss is required before fluffing-up and lightly compressing the surface. The clean bottom of another square container works well to smooth the final sowing surface and the back of a spoon may be used as well. Sphagnum moss works very well because it contains some anti fungal properties and enough nutrients to sustain seedlings until their first set of true leaves. It could be used to the exclusion of perlite and vermiculite but is much more expensive.
About 1 inch from the top of the container remains for head room for the young seedlings. A 4½-inch stake label is then placed and acts as a central tent pole to support the clear plastic. Occasionally, if time constraints prevent transplanting and the seedlings need more headroom, a taller central tent pole is fashioned from the hook portion of a clothes hanger. A wire cutter is used to divide each shoulder support about 3 inches down from the vertical extension of hook. The divided shoulder extensions are adducted together just enough to fit into the 4-inch cup and provide a double spring-like stable support.
Seeds are lightly and evenly distributed on the surface and the container placed in a 1-gallon clear plastic food storage bag. The twist tie is not used but adequate closure is accomplished by simply twisting and folding the top of the bag to the bottom of the soil-filled cup which is allowed to rest on it. This permits rapid opening of the bag and removal of the seedling container for inspection and care.
Trays of seedling containers are placed 6 to 12 inches under 4-foot long fluorescent fixtures. One of these is cool white, the other a Gro-Light, and both are timed to provide 16 hours of light each day. The trays are placed on a propagating heat mat set to provide bottom heat of 70 degrees.
The seedlings are kept in a minimally heated garage and are apparently provided the ideal situation of "cool tops and warm bottoms."
If a gray mold appears it is easily controlled with a misting of Benlate solution, or in more recent years Bayleton. Occasional misting with water will be necessary, and ¼-strength liquid fertilizer with trace elements may be applied after the first set of true leaves appear. After a while, with experience, the weight of the container will be used to monitor the moisture status of the medium. If it feels too light it can be opened and watered or, if too heavy, it can be left exposed for a short time to permit some evaporation. The most common error is having the medium too moist.
Transplanting may begin anytime after the first true leaves (not the cotyledons or false leaves) have matured, as doing so at this early stage seems to cause less shock or slowing of the growth rate. Usually this begins in January and continues into spring. The seedlings are pricked out into individual 2- or 4-inch cups and moved into the greenhouse under lights. Normally humidity in the greenhouse is high and weaning from the plastic storage bags is not a problem.
The transplant medium consists of 1½ bushels of screened pine bark; 1 gallon peat moss, or vermiculite, or combination; ½ cup of dolomitic or hydrated limestone, or combination; and 2 tablespoons of trace elements. Limestone provides calcium and magnesium and overcomes the extremely low pH of pine bark and peat moss. After mixing in a small electric cement mixer the moistened medium is allowed to age for several weeks before use.
The seedlings are allowed to grow on in a lath house providing 47% shade. After a week or so to become established, fertilization may begin, and one must remember that the most common error is over fertilization. Chances of this are reduced if only light applications of a slow release, long acting material are made. Research has demonstrated that nitrogen is needed more than the other basic elements of phosphorus and potassium. Therefore a fertilizer containing a greater proportion of nitrogen as well as trace elements would be ideal. It is important to withhold fertilization, especially nitrogen, after mid August to avoid late season new growth that might be injured by an early frost.
Pinching out or removing the terminal growth buds is begun in the early summer to produce plants that are well-branched from the base and to overcome tendencies to become "leggy." Two or more of the auxiliary buds will be stimulated to replace the lost central one. Again, pinching should be discontinued in the late growing season to prevent late growth that will not have time to "harden off" before freezing weather occurs.
The young plants require protection during their first winter, usually in a shaded greenhouse, white poly covered house or cold frame.
The foregoing discussion has assumed the use of a heated greenhouse. However, the directions can be modified by those with less expensive amenities. For example, seedlings can be raised at room temperature under a lamp with timer in a bright window where direct sunlight can be avoided. The seed can be planted later so that at transplant time they can go directly under light shade outside where they can be protected from downpours, dogs, and rodents. A winter protection device can be fashioned from plastic on the north end of the house which doesn't receive direct sunlight. Remember that personal innovations may be worthy of sharing with others.
Dr. Robert L. Means is a member of the Piedmont Chapter.