Amateur Rhododendron Propagation Techniques
John G. Lofthouse, Vancouver, B.C.
My leisure time interest and hobby, over the past eight years, has been the propagation and growing of a select group of plants with the aid of artificial light. For those who have not used this comparatively new technique, I would certainly advise you to do so. Plants grown from seed, show phenomenal growth (often three years growth in one year), cuttings root easier, grafting can be accomplished in ideal conditions resulting in almost 100% success. The artificial illumination, applied for 16 up to a maximum of 24 hours a day (I use 18) can he compared to the short summers near the Arctic circle where the long days result in phenomenal plant growth in a comparatively short time.
My experience with artificial lighting started in 1956, when I installed a four lamp fluorescent unit in a 6 foot plywood container in a finished heated room in my basement. Results were so satisfactory, the following year I constructed a much larger eight tube fluorescent unit in the same room. I now have an eight foot dual fluorescent unit in the greenhouse for extra illumination during the short days of winter. This is automatically turned on at sundown and gives additional light until 11:00 or 12:00 at night.
Young plants respond as if by magic to this treatment. The usual waiting time for rhododendrons, from seedling to bloom, can be cut from ½ to ⅓ of the usual time. For example; a small cutting of the sweet scented 'Fragrantissimum', taken May 15, 1963, has flower buds forming at the time of writing, March 13, 1964. This plant has been pinched back several times and is bushy and healthy.
A fluorescent lamp installation need not be elaborate or expensive. One or two dual, 2 feet to 8 feet units, hung over a basement bench would be sufficient to start. I prefer a temperature of 60 to 70 degrees. Use Sylvania Gro-Lux tubes, or a combination of either cool white or daylight (for the blue light spectrum) evenly mixed with either deluxe warm white or Sylvania Natural (for the red light spectrum).
Using cool white, or daylight alone, produces a stubby plant with few flower buds; Natural or deluxe warm white alone, a tall spindly plant. A mixture of both is about right. Use a white enamel reflector to reduce light loss.
For those using fluorescent light, and those interested in doing so, I would like to discuss some methods I have found most satisfactory in producing rhododendrons from seed, cuttings, and grafting. These methods are for amateurs who wish to propagate a few plants for their own use. If instructions are carefully followed, you will have few failures.
Fig. 13. 'Naomi' seedlings, to be used for
understock, 4½ months after sowing. They
were grown continuously under artificial
light and are ready for second transplanting
into individual plant bands.
Fig. 14. The seedlings shown in the picture
adjacent, transplanted into plant bands, and
still under lights.
Picture taken March 31, 1964.
Propagation from Seed
For containers I use 2½ or 3 inch plant bands. Bend and staple these to form a small square. Cut a band in half, bend at partially cut lines, and fit in bottom. Position and staple to sides. Fill to ¼ inch of top with packaged, pre-dampened, granulated peat moss. Take to sink and gently flush with hot water (160 to 180 degrees) for about 2 minutes. Drain and cool in clean atmosphere for about 30 minutes. Sow seeds thinly and evenly on top. Do not cover seeds or press down. Cover plant band completely with thin polyethylene, (dry cleaning bags are ideal), but leave covering loose. Allow about 1 inch over top of plant band.
Put under lights about 6 inches from tubes. Do not water. Enough moisture will remain for several months if totally enclosed. Plants can stay in bands until transplanted into sterilized peaty soil when third leaf emerges in four to six weeks. Polyethylene can be removed at transplanting or at stage above. Before removing, punch small holes in covering to admit air gradually over a period of about 10 days to 2 weeks. When transplanting, put into clean flats one inch apart, or singly into two or two and one-half ins. plant bands or pots. Use straight granulated bagged peat or a mixture of ½ sterilized John Innes, or a similar compost and ½ peat. If transplanting is delayed and after polyethylene is removed, plants can be watered from below with weak fish fertilizer. I have found rhododendron seedlings do better if not potted on until the roots fill the pots fairly well, and then placed in a pot only slightly larger. Over potting results in a soggy soil lacking air. This inhibits roots, the leaves yellow and drop, and the plant can even die.
If the above method is followed, you will have no "damp off." This method, with slight modification of compost, can be applied to most hard to propagate small seeds. Cleanliness of hands and utensils is important at all times. I use the same general method in sowing tuberous begonia seeds, which I also hybridize, and have unvarying success.
At this point, I would like to add some further points on composts, fertilizers, and "growing on." I do not add sand to the seed compost for several reasons. Sand adds weight to the compost, makes for a harder surface for the initial tender roots to penetrate, and restricts the root growth. Sphagnum as a medium is light and fluffy, and as no additional water is given from the top, sand for additional drainage and aeration is unnecessary. Coarse sand in the compost tears away roots when the plants are moved when they are quite small. In experiments conducted to establish the quickest growing conditions, I have grown rhododendrons to the ¾ in. to 1 in. size before moving. The transplanting at the 3rd leaf stage results in a better plant over a comparable period of time.
Experiments were also conducted several years ago to find the best compost for "growing on" rhododendrons under fluorescent lights. From the same batch of seedlings, which by the way, were from the Naomi group, seven composts were made from straight sphagnum to soils and peat combinations varying from PH 3.5 to neutral. Growing in straight peat, and adding fish fertilizer to the water at 3 week intervals, gave good results. The best one, and the one I now use, consists of the following:
7 parts sterilized loam (170 to 180 degrees for ½ hour)
9 parts of Western Canadian sphagnum peat, or equivalent.
8 parts of perlite (coarse sand can be used but makes a much heavier compost)
To each bushel (8 gallons) add ¼ lb. John Innes fertilizer, plus ¾ oz. of ground chalk or whiting. John Innes fertilizer is made up of 2 parts Hoof and Horn, 2 parts super phosphates (18%) and 1 part sulphate of potash (48% potash). The addition of chalk or whiting, seems to benefit the mixture, but could be omitted if a more acid soil is required.
Excellent results can be obtained by using the above compost at time of first transplanting. As plants grow quickly under fluorescent lights additional fish fertilizer can be given in water, at monthly intervals, with good results. If plants are to be "grown on" for garden use, pinch out tip to induce branching when 2 ins. high. If grown to be used for understock for grafting, do not pinch, but grow to single stem.
In the next article, which will appear in the July, 1964 Bulletin, I will discuss grafting, and the striking of cuttings, as applied to artificial light. A generally unknown technique to get even most difficult rhododendrons to strike on their own roots, will be discussed. If you would like to try this, get busy and start your seedlings now, as this requires under stock in the initial stages. On the west coast I have found the Naomi group, the native R. macrophyllum , along with several others, quite satisfactory. In the east, R. ponticum , R. catawbiense , R. maximum, or similar seedlings might be your choice. Growth under artificial light is fast. Your understock will be ready for you to proceed, in about 8 months. I guarantee you will not only find it interesting, but also a lot of fun.