Source: JARS V48:No.1:p10:y1994
How to Grow Rhododendrons from Seed
Allan and Shirley Anderson
Growing rhododendrons and azaleas from seed is not difficult if one remains conscious of their physical needs, e.g., light, warmth, fertilizer and moisture. As long as these are provided at the proper time and in the right amount, many methods have been successful. Rhododendron seedlings are adaptable and can succeed with less than ideal environmental factors, but it is important to remember that whatever method issued the planting medium must never be allowed to dry out. Germinating seeds or small seedlings cannot survive even one such episode.
We grow our seeds on damp, slightly firmed milled sphagnum moss in small plastic "deli" containers. Screened peat moss and perlite mixtures have been equally successful. We drop the seeds on the surface and enclose the container in a polyethylene sandwich bag with the top folded under the container. Such an enclosure will usually keep the medium moist until the seeds germinate at which time the bags are gradually opened and medium watered carefully to be sure it doesn't dry out. Each container is labeled and contains seed of a single variety.
While seeds do not require light to germinate they do need light to photosynthesize and grow immediately after. We therefore put the containers on a bed of peat moss over the heating cable and suspend a light source over the top. A fluorescent shop light about 8 inches over the containers works well. A time switch is used to provide about 16 hours of light each day. In the past years we handled fewer containers by placing them on a serving tray on top of the refrigerator for warmth. A desk lamp with a time switch provided light. We have also used old aquariums using an automatic heating cable in the bottom with a layer of damp peat moss under the deli containers.
While some seeds, especially yakushimanum hybrids, may germinate unpredictably, most other hybrids and species seed germinates in 10 days to 3 weeks time. In 4 to 8 weeks more the small seedlings will have two or four true leaves in addition to the original cotyledons. Now we transplant them into flats containing a screened peat-perlite mixture and fertilize every other watering with one-third strength soluble acid type fertilizer (Miracid® or equivalent). About 50 seedlings are planted in each flat. We still maintain the 16-hour days with overhead light.
We place the flats on the bench in a small greenhouse, but other hybridizers have used basement tables under suspended fluorescent lights, sun porches or various types of light stands. Just remember the basic requirements which are moisture, weak fertilizer, light for 16 hours a day and warmth, about 70°F if you can provide it.
When weather permits and natural days become longer the flats are placed outdoors in a moderately shaded area. Supply with one-half strength soluble fertilizer and water as needed to prevent drying. The seedlings will grow through the summer. We stop fertilizing about the middle of July, and in the fall we transfer the 3 to 6-inch plants into individual containers for winter protection under plastic. In earlier years we left the seedlings in the original flats and wintered them over in cold frames for planting out in the spring.
At this point seedlings are much like other small plants such as mericlones* or rooted cuttings. They will still need some protection outdoors from wind and too much sun. Windbreaks and partial shade from high trees or snow fencing, etc., are described in many books.
Whether you want to try a few or many, the growing of rhododendron seeds is great fun and an absorbing pastime. We recommend it!
*At this stage of growth the seedlings may be handled as if they were rooted cuttings or like the mericlone propagules sometimes sold at plant sales and supplied by tissue culture labs.