Rhododendron Seedlings as Grown by an Amateur
Merle Cisney, Camas, Washington
I have been growing rhododendron seedlings in my house, on window sills, and so forth, for the past six years. Usually, I have been able to grow more seedlings from a planting of seed than I have been able to use, so my approach seems to be satisfactory. I usually have enough seedlings to supply several other people.
Seed treatment is simple. I usually break the capsule into several valves and clean these with a toothpick. In this way I am able to recover clean seed. I usually do not dust the seed with any of the seed treating agents and have found that when clean seed is planted there is usually little mold or other spoilage. The seed is stored until used in clean coin envelopes, at room temperature or in the cool basement.
Plant in Plastic Boxes
The containers that I have used almost exclusively have been polystyrene boxes measuring about 3½ x 9½ x 2¼ inches deep with a clear, rather tight fitting cover. These are filled about half full of a mixture of ½ sand and ½ peat and the mixture firmed down. The mixture is moistened well but not soggy. I have used straight sphagnum with less success.
The sand and peat mixture may be over laid with 1¼" sphagnum, and this seems to be good. An advantage of the sand and peat mixture is that the seedlings may be shaken loose from each other with little damage to the root systems. The boxes have no drainage, so watering must be done carefully. In one container, I usually plant four to six batches of seed, separating each seed area by a toothpick, wire, or other means.
When planted the lid is fixed in place rather tightly. As time goes by, the moisture condenses on the lid and, if the box is tipped, will run down the side, either inside or outside. I have found that usually no more moisture is needed until after the seedlings are up and have one or two sets of true leaves. I do not sterilize the medium or the seed and have had little trouble with damp-off. I usually watch closely and when damp-off or mold becomes apparent, dust the seed and seedlings with a 50% Captan powder. If too much trash or seed capsule residue is planted with the seed they tend to promote mold and require the early use of Captan. Since there is some indication of inhibition of germination by Captan, it would seem wise to restrict its use.
Control of Light and Air
These boxes are usually stored at room temperature in a dark area until the seedlings have germinated, although I have germinated seed in rather good light. As germination proceeds I move the boxes to full north light on a window sill. The covers are then moved slightly to one side to allow air to reach the seedlings until, by the time the seedlings have two to three true leaves, the covers have been entirely removed. The removal of the cover and admission of air does not seem to be very critical. The small seedlings should not be exposed to direct sunlight either before or after the cover is off, unless they have been properly hardened off.
When the plants require moisture, watering is done by pouring, in a thin stream, water down the inside of one side of the box. If the box is tilted, the water will run down the wall and out across the bottom. The seed medium should never be soggy and I never water the seed surface itself. In these containers, I try to keep water off the plants at all times and even use a dry Captan powder, as mentioned above, rather than liquid preparations for the suppression of damp-off or molds. When the seedlings have at least two or three pairs of true leaves, they are large enough for transplanting. The longer the plants are held in the seed flat the sturdier they are and the more easily handled, but larger plants are apt to suffer root damage because of entanglement with neighboring plants.
I transplant the seedlings at this point into the 4x6 plastic flats used by nursery men for bedding plants. The seedlings are planted on 1" centers in rows one inch apart using a mixture of fine leaf mold and peat. This mixture is porous and the small seedlings seem to start out fast. The little plants are watered in when they are planted and the flats are covered for one or two days to avoid shock. Again, Captan is used if needed. After one to two months, the seedlings are transplanted again into regular cedar flats on two or 2½" centers in rows 2 to 2½" apart. I use the same mixture of leaf mold and peat moss, watering the plants in and at this time using Miller's Booster Powder either added to the soil mixture or in the water used to water the plants in. I try to get the flats into a lath house quite early, to get the largest plants possible by fall as the larger the plants the better the chances of them surviving their first winter. During the winter, one should watch the drainage in these cedar flats. I usually store the flats under a sloping plastic frame and keep the plants a little on the dry side.
Some Good Growers
I have found in general, that seedlings of the Fortunei series and Fortunei series hybrids are good growers. Loderi hybrids are also strong growers. Seedlings grow easily from the Triflorum, Heliolepis, Scabrifolium and Lapponicum series. In addition to being easy to grow, these seedlings will usually bloom quite early. I have bloomed R. impeditum seedlings at 18 months, although this is usually about half the time required.
In conclusion, I would say that the success of growing seedlings by the method that I have described depends strongly on the amount of care that one wishes to take with them. One must check the seed often, watching for damp-off and mold dryness. While a commercial grower might not be able to afford the amount of time that these require, the hobbyist or amateur will usually succeed.