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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 32, Number 1
Winter 1978

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Growing Rhododendron Seedlings Indoors Under Artificial Light
Douglas W. Johnson, Issaquah, Washington

        About five years ago I began growing rhododendrons from seed. Since then several methods and probably over 100 different species have been tried. While no single method is optimum for every rhododendron, experience indicates that the procedure now used is certainly adequate and very simple. Before outlining this procedure, a brief survey of my experiences may be of help to those of you who have an urge to try still other methods.
        Once scattered over moist peat moss the seeds must be kept damp at all times. Trying to moisten the peat moss afterwards is a sure way to float the seeds down the river or, worse yet, if you resort to a sprayer, blow them right over the edge of their container. Pricking out the newly won seedlings from their first home and planting them in a suitable soil and container is critical. A soil composed largely of sand will dry out quickly and may require the frequent application of fertilizer. On the other hand, a soil made up of mostly peat moss will soon become overgrown with fresh moss while little rhododendron growth will be produced. As for containers, small plastic pots seem to remain overly damp too long after watering, often resulting in loss of plants. Shallow plastic flats are an improvement over pots because there is less difficulty maintaining proper soil moisture. Planting seedlings whose growth rates differ greatly into the same flat will soon allow the larger seedlings to crowd out the smaller ones. In a shallow flat, vigorous roots quickly reach the bottom and form a mat across the entire flat that is impossible to disentangle when the time comes to separate the seedlings for outdoor planting.
        With space at a premium indoors, seedlings were grown on lighted shelves. Fluorescent lights were mounted to the underside of adjustable metal shelves. Aside from the hazard of electric shock, this arrangement produced unwanted bottom heat on the upper shelves. Although fluorescent lights are much cooler in operation than incandescent light bulbs, the ballasts used in fluorescent fixtures waste too much heat. Experimentation with lighting configurations was limited because plant failures seemed due to other factors. The lighted period was varied between 15 and 18 hours without much significant effect. One daylight type and one deluxe warm white lamp were used to each fixture. Distances from lamp to leaf were varied from 10 to 28 inches, again without much effect in plant performance. Distance requirements were dictated more by height of plant and shelf spacing than by anything else.
        Watering was accomplished with the aid of a gallon size, long spout watering jug. Run-off was caught in aluminum cookie trays. Plastic film was used to separate the aluminum trays from the steel shelves. Aluminum corrosion was a problem, probably due to the minerals contained in the water run-off. A greatly diluted liquid fertilizer was applied as an additive to the water. Some species and no doubt all newly-germinated seedlings are quite sensitive to fertilizer. While I encountered no significant problems, during one period of over-fertilization some plants did develop a white dust on their foliage. The real problem in using a fertilizer is having the space available for the resulting abundant growth. For example, seed of R. diaprepes var. 'Gargantua' was germinated in June and moved outdoors the following May. During this eleven-month period indoors several of the plants attained 18 inches height with leaves 8 inches long.
        While rhododendron seeds vary in size, it is fair to say that the product of just one seed pod will yield more seed than you can reasonably use. Seed germinated in peat moss will keep a couple of years if left in the moss. The seedlings seem to reach a point of suspended growth about four to six months after germination. Little growth is produced thereafter if no fertilizer is applied. Therefore, it is possible to set aside seedlings while an overproduction glut is processed and moved outdoors. It should be noted that seed may be kept in a sealed, dry container placed in a refrigerator until needed. While all seed deteriorates, the rate of deterioration is reduced under cold temperatures. Most of last year's seed, started this way, will still germinate this year.
        Some words must be said about the choice of seed. Seed may be collected in the wild, or be hand-pollinated, or be open-pollinated. The latter is the least desirable for all but the true gambler who has free help and boundless land. While it may be said that if we are sure of the identity of the seed-bearing plant we can thereby eventually determine which seedlings came "true," the fact is a lot of unwanted plants will have to be raised to flowering size in the meantime. Hand pollinated seed, though better, sometimes turns out to be open-pollinated if the person doing the work fails to act soon enough and in a thorough manner. I have purchased "hand-pollinated" seed that was no better than open pollinated seed. However, when properly done, hand-pollinated seed is as desirable as seed collected in the wild. Therefore, if you raise a rhododendron from seed, the first label you affix to your seedling should indicate that it is a seedling. Later, if it can be confirmed that the plant is a true species, then the label should be updated.
        Based on my trials and tribulations, here is what I now recommend:
        LIGHTING: Four foot long, two-lamp fluorescent fixtures with reflectors suspended on adjustable chains should be hung above a table or the floor. Use one daylight type and one deluxe warm white lamp in each fixture. They will give you several years of service before re-lamping is required, they are less expensive than the special "grow" lamps now available, and the light is better balanced for color photography (if required). Incandescent light bulbs designed for plants produce unwanted` heat. Lights should be controlled by an electric timer set for 16 hours on and 8 hours off. Keep in mind that each fixture consumes 80 watts minimum. If you are planning to use very many fixtures, be sure that your timer has adequate capacity. Incidentally, if power costs are a problem, the 16 hour light duration may be shortened if you closely monitor the effect on your seedlings. Malaysian rhododendrons will require nearly a 12-hour on, 12-hour off light cycle. Set the distance between light and soil at around 12 inches to start with. This may be increased as the seedlings grow. The larger-leaved species can tolerate less light once they reach about four inches in height.
        SEED: Use seed collected in the wild or seed that has been hand-pollinated by someone who knows how to do it. Seed may be started indoors anytime, but keep in mind that you will want to move the seedlings outdoors once they have developed good root systems. The length of time indoors will vary depending on the rate of growth, so do not start all the seed at one time. The R. diaprepes var. 'Gargantua' mentioned earlier should not have been started eleven months ahead of warm spring weather. Seed may be stored in a refrigerator for many months as long as the seed remains dry.
        GERMINATION: Germinating seed is so simple, it's like falling off of a log - if you just remember to keep the seed damp. The simplest way is to first thoroughly moisten the peat moss. Make a year's supply at one time because it can be stored in a sealed plastic bag until needed. Purchase several 4" x 4" x 2¼" deep plastic food containers with lids. These are often sold in supermarkets. Fill each container two-thirds full with moist peat moss. Remove from the moss any large chunks and twigs. Lightly tamp smooth the moss, then sprinkle over a pinch of seeds, being careful not to get too great a concentration in any one place. Replace the container lid, being sure it is on tight. Label the container and place it under the lights. At this point, check your fingers to be sure they are free of any seeds that may have stuck to them or become lodged under a fingernail. Once the seed has been planted, the room temperature should be maintained between 65° and 75°F. The seed will germinate sometime between ten days and three weeks if it is reasonably fresh. Be patient. After the seed has germinated, allow the tiny seedlings to develop some roots. This will take at least a month. After two pairs of leaves have formed, the seedlings are ready to be pricked out and planted in a larger container. Stamp collectors use large tweezers called tongs that also work very well for pricking out the tender seedlings. Each seedling should be moved with its roots holding some moist peat moss. This is the most tedious task you will have to perform and it cannot be rushed.
        S0IL AND THE FINAL CONTAINER: If flats are used, they should be at least four inches deep. I prefer one-gallon plastic bottles cut in half, with the bottoms punched with holes for good drainage. Distilled water bottles are ideal since no contaminants are involved. These are filled to within ¾inch of the rim with the same soil that will later be used outdoors. Sterilizing the soil is not necessary if you do not mind pulling a few weeds along the way. Good judgment must be used when moving the young seedlings to their larger containers. For example, R. macabeanum seedlings will require greater spacing than seedlings of R. megeratum. Perhaps nine of the latter can be accommodated in one bottle wherein at most only four of the former will fit. The use of plastic bottles allows a greater degree of flexibility than is possible with flats. If overcrowding begins to occur, it is easier to change the spacing of the bottles than it is to replant a whole flat containing perhaps two or more dozen plants.
        FERTILIZER: Once the seedlings have been planted in their final containers, a greatly diluted liquid fertilizer may be applied as frequently as with each watering. A small amount will go a long way. I prepare a common brand of fertilizer according to the manufacturer's directions for house plants. I then mix about one cup of this with water to fill a one-quart sprayer. Each time I fill my watering jug I spray in two or three tablespoons of the diluted fertilizer to produce a very diluted application indeed. About once a month I moisten the foliage with the sprayer.
        In the beginning I feared problems with excessive humidity, mold, insects, and so on. None of these fears proved justified; however, caution is advised. Your situation is bound to differ from mine. Until you have gone through the process at least once, there will be little things that crop up which only you can resolve. For example, just how much fertilizer and water to use for your soil and seedlings is pretty much determined by trial and error. Keep in mind that rhododendron seedlings do not wilt until it is often too late for first aid. This is especially true in cases of over-watering. During the summer, indoor spaces not air-conditioned may be subjected to temperatures about 80 degrees. Good ventilation will help, but it may not save some plants particularly sensitive to heat. Use good judgment in selecting an indoor location for your set-up. Avoid nearby windows revealing harsh direct sunlight. They are not only a problem in the summer. In the winter they can cause cold drafts.
        With all this having been said, one may well ask: Why grow rhododendrons from seed when one can root a cutting and soon have a liner size plant whose character is just like the mother bush? Propagation by rooted cutting is to be preferred when enough wood is produced on the stock plant and nearly exact replica is required. On the other hand, nature instills its seeds with a degree of variation not always revealed in the rooted cutting. Seeds collected in the wild or hand-pollinated ones will produce seedlings of varying vigor and character. Some may even display variegated leaves as does one of my R. griersonianum seedlings. Such possibilities make raising seedlings more rewarding for me. Perhaps the most satisfaction has come from watching a tiny speck of growth appear out of a seed and, under my daily observation and care, become a yearling as large or larger than a rooted cutting of similar age.


Volume 32, Number 1
Winter 1978

DLA Ejournal Home | QBARS Home | Table of Contents for this issue | Search JARS and other ejournals