JARS v44n4 - Rhododendrons from Seed: A Novice's Perspective

Rhododendrons from Seed: A Novice's Perspective
Howard Roberts
Rosemont, Pennsylvania

Adapted from article published in Green Scene , the magazine of the Pennsylvania Horticultural Society

The reason for my writing this brief report is to encourage other amateurs to try their hand at what can easily prove to be a rewarding experience. Frankly, if we can raise a rhododendron from seed, anyone can do it, and probably do it faster and better.
Ten years ago, when freshly planted young rhododendrons filled an area in front of our house, a visiting friend was quick to remind my wife and me that these shrubs don't stay two feet tall forever. The remark was obviously not referring to mature dwarf and semi-dwarf rhododendrons, but to the selection of vigorous, tall-growing elepidotes that greeted his eyes. We knew full well at the time he was right, but like the majority of beginners, we had to learn for ourselves; by the time new growth began to block daylight from the living room windows, we had discovered that there were more appropriate rhododendron varieties for dooryard planting. Instead of chopping down or discarding, we transplanted and added. Today it is no surprise that space seems at a premium, and the lawn, which we happen to value, is losing a battle to the encroaching rhododendron population on this three quarter acre lot.
Some of the original dooryard plants, such as Pink Pearl, 'Maryke', 'Old Copper', 'Autumn Gold' and 'Scintillation', have been moved to distant corners of the grounds and have been replaced with dwarf and lower growing varieties - both species and hybrids ( Rhododendron williamsianum , R. hanceanum var. nanum , R. insigne , R. recurvoides , R. roxieanum , R. pachysanthum , R. yakushimanum , R. kotschyi [syn: R. myrtifolium ], 'Ginny Gee', 'Patty Bee', 'Rik', 'Golfer', 'Ming Toy', 'Hotei', 'Mother Greer', 'Percy Wiseman', and many others).
The real space problem, however, still lies ahead, for after joining the local chapter of the American Rhododendron Society, our enthusiasm for these broad-leafed evergreens shifted into high gear. When I mentioned to the chapter president that we were contemplating growing our own rhododendrons from seed, the response was: "Then you'll never have to buy another plant!" It just doesn't work that way. Each year nursery catalogues persuasively offer new varieties, and equally tempting is the stock displayed at the mid-May rhododendron and azalea sale at Tyler Arboretum near Media, Pennsylvania. Random purchases, when added to one's own crop of seedlings, only exacerbate the problem. Even on our very small scale, fifty or sixty seedlings are certain to become quasi-permanent parts of the landscape. However, after the weird winter of 1989-1990 when temperatures hit record lows in December followed by record highs in January and early February, culminating in a drop from 71° to 8° in 48 hours, we have more space than originally anticipated for the next generation of seedlings.
The greatest enjoyment in our hobby of raising rhododendrons so far has been the anticipation of what a plant grown from seed will develop in the way of flowers and foliage. Since we are working under the constraint of very rudimentary conditions, without the benefit of grow-lights, greenhouse or cold frame, it has usually taken us at least six years to see a new flower. Beginners shouldn't necessarily be put off by hearing this. I have spoken with people who have been raising rhododendrons for many years longer than we and who are unwilling to wait that long for a bloom. With the proper equipment and environment they have managed to produce blooming size plants in much less time; in fact, there are commercial growers who can obtain flowers within eighteen months of germination. Perhaps fortunately, we are limited here by a more primitive setup with two long window sills facing east for starting the seedlings and an area before a south-facing window that will accommodate a table of three flats in the winter time. If we had a greenhouse, propagation could really get out of hand.

Rhododendron seedlings
Stages of propagation showing plastic "greenhouse" glasses of
vermiculite at right (one is uncovered to expose seedlings);
pots with seedlings at various stages of development in the
center; and a flat with rows of transplanted seedlings at the left.
Photo by Howard Roberts

The ARS Seed Exchange makes available to society members at a nominal charge some 1,500 varieties of species and hybrid rhododendrons each year from donors around the world. The seeds, from our experience, generally take from two to four weeks to germinate. Each grower seems to have a preferred method of sowing seeds. One that has served us well is to set aside a 3 clear plastic drinking glass for each variety, filling it about two thirds with plaster grade (fine) vermiculite, and dampening the medium without allowing any water to accumulate in the bottom of the cup (in other words, damp but not saturated). Since the seeds occasionally encounter mold, we have sometimes added a weak solution of fungicide, such as Benomyl, to the water at the outset. We have not made a study to test its effectiveness. We then sprinkle the seeds on the surface of the dampened medium and cover the top of the container with clear plastic wrap, holding it in place with a rubber band. The result is a miniature "greenhouse" ready to be placed on a window sill where it will get light but not strong, direct sunlight. Usually within two weeks after sprouting, we transfer the seedlings into 3 pots filled with potting soil. Since the seedlings are extremely small at this stage, there is danger in allowing the soil to dry out, as we have learned the hard way. From the small pots, the next stage is on to larger pots or flats and finally to a protected area of the garden. We have had unexpected success in wintering first-year seedlings outdoors in Pennsylvania under a pine needle mulch, although we acknowledge losing a percentage to surface heaving from frost and from leaving the blanket of needles on too long. Tender and slow developing varieties spend their first winter indoors in flats. There has been a degree of attrition indoors as well as outdoors, but a reasonable number of young plants survive to face their second season, after which we don't expect to bring them indoors again.

'Donna Hardgrove' x 'Weston's Hardy 
'Donna Hardgrove' x 'Weston's Hardy Yellow',
seed from ARS Seed Exchange.
Photo by Howard Roberts

In conferring with other members of the Greater Philadelphia Chapter, I have found that a more common method of seed germination is through sowing in containers filled with an inch or more of evenly mixed peat moss and perlite, and topped with a thin layer ( inch or so) of milled sphagnum moss. The top layer of sterile sphagnum moss is apparently essential to satisfactory germination. If grown on a window sill, the container is placed in a clear plastic bag or covered with clear plastic to create a humid atmosphere and keep the medium moist at all times. I learned that although most seeds germinate within four weeks, some varieties have required a number of months. By waiting until the seedlings are a half inch tall, it is possible to transfer them directly into flats of the peat moss and perlite mixture, thus eliminating the small pot stage. We are considering trying this approach to see how well it succeeds in our environment; it is possible, however, that without a greenhouse or controlled atmosphere, our present method may prove safer for us.
Of the more than 70 varieties we are presently growing from seed, eleven are mature enough to have flowered for us. The results have not been uniformly pleasing. For example, last spring the large, healthy buds on one plant turned into severely distorted blossoms in contrast to the flowers of a sister seedling which were well formed and gratifyingly beautiful. If the same grotesque flowers appear next spring, we will at least have room for one more variety. We have also had the interesting surprise of a white flower whose parents ('Odee Wright' and 'Letty Edwards') are both essentially yellow. A sister seedling is cream-colored, with identical purple markings in the early stage.

'Vulcan' x ('Golden Star' x 'Whitney's 
'Vulcan' x ('Golden Star' x 'Whitney's Orange'),
grown from ARS Seed Exchange seed.
Photo by Howard Roberts

Our own hybridizing efforts, which began over five years ago, have only just started to produce their eagerly awaited results. Our first cross to bloom was R. wardii x 'Cheyenne', the species parent being a rather compact semi-dwarf specimen whose flower buds are more often than not winter-killed in this climate. It was a happy surprise to see the leaves of the hybrid take on the size of 'Cheyenne' foliage but with a roundness reminiscent of R. wardii . The new growth, as an added bonus, is shiny bronze. When buds appeared for the first time this year, the outer petals showed promise of a yellow blossom, but as the flowers opened, they became somewhat pendulous bells of ivory with a red blotch far within the throat. Although undistinguished in size, form and color, they are rather appealing, and the foliage is superior; and so there is no likelihood of our jettisoning this first hybrid. Three other young plants of the same seed lot did not develop flower buds last fall, so we will have to await next spring to compare their trusses.

R. wardii x 'Cheyenne'
R. wardii x 'Cheyenne', our own hybrid;
the first of our attempts at hybridizing to bloom.
Flower is ivory with maroon blotch deep in the throat.
Photo by Howard Roberts

The colors we have primarily concentrated on are the warm tones - oranges, yellows, apricots, ambers - colors not usually associated with rhododendrons in this region. Unfortunately they are not generally as hardy as the purples and whites, but with adequate protection and a modicum of good luck, a number of them can be grown successfully in this area. Commercial growers are striving to increase the hardiness of warm colored rhododendrons, so that in the future it should be easier to raise them in the eastern states. Hybrids that have done well for us so far are: 'Autumn Gold', 'Whitney's Orange' (orange tones); 'Evening Glow', 'Odee Wright', 'Hotei', 'Skipper', 'Cheyenne', and 'Ming Toy' (yellows); 'Old Copper' (orange-pink); 'Amber Gem' (peach); 'Moonwax' (pale yellow, pink and white); 'Percy Wiseman' and 'Mary Belle' (peach, yellow and pink tones). Most of these represent parents of crosses that we are presently hybridizing.
Our three primary sources of seed have been the ARS Seed Exchange, fellow members of the local ARS chapter, and our own plants. Recently Morris Arboretum in Philadelphia participated in a seed collecting trip to South Korea, the rewards of which have been shared with several of our local chapter's seed enthusiasts. Since the chapter is in the process of forming its own experimental group to grow rhododendrons from seed, this comes at an ideal time; and it presents an opportunity to learn from one another and from the arboretum itself.

Mr. Roberts is a Greater Philadelphia Chapter member whose avocations include gardening and writing.