JARS v50n1 - The Live Sphagnum Method for Germinating Rhododendrons

The Live Sphagnum Method for Germinating Rhododendrons
Bryan Johnson
Scarborough, Ontario, Canada

I have been growing rhododendrons from seed here in Toronto, Ontario, for the past nine years with relative success, or so I thought. The potting soil mix with milled sphagnum germination surface that I had been using gave mixed results, since potting mixes vary from year to year, even from the same supplier. After initial germination, seedling vigour would vary based on variations in nutrients. The milled sphagnum that was sprinkled on top as a germination medium was still dead sphagnum and must be dosed with fungicide lest outbreaks of mold kill entire seed lots. But fungicides tend to retard the growth rates of seedlings. Finally, germination of some seed lots (especially yaks) was unpredictable. Even if I used the same batch of seeds in two consecutive years, the second seeding might give far heavier germination. Yet the treatment was the same during both years! I was perplexed and ready for change.

In August of 1992 a friend and fellow member of the Rhododendron Society of Canada, Heinz Ruckdaeschel, mentioned that he had heard of a German method of sowing rhododendron seeds directly onto live sphagnum moss. I recalled seeing self-sown seedlings of Rhododendron 'English Roseum' under the plant in Edwards Gardens, Toronto, but only where the seed landed in mosses. I was immediately interested and asked to see his germination system. Heinz showed me a Styrofoam cup filled to just below the brim with healthy green sphagnum moss studded with nice looking rhododendron seedlings. Although he had covered the cup with a clear plastic bag to conserve moisture, and given 75°F bottom heat and light, Heinz emphasized that at no time did he fertilize the contents of the cup. To water, he periodically placed the cup in a shallow pan of water so that the moss could draw all the water it needed up through the drainage holes punched in the bottom of the cup. I was intrigued. Heinz brought me a bag of live sphagnum down from the cottage and I tried the method myself that fall.

I sowed two extra seed lots on the live sphagnum just to test the method, but I continued to use the standard system for the bulk of the seed crop. The germination on the live sphagnum was heavy. I had some difficulty pricking out these seedlings from the sphagnum since root penetration was unusually deep and extensive. No fungus problems were noted on the live sphagnum despite skipping a fungicide treatment. Next season I was eager to expand the use of the new system to all the seed lots (I generally sow 20 to 30 seed lots each year).

Last fall I refined the system. In the bottom of each four-inch germination pot, I placed two inches of water-saturated peat moss with one-fourth strength Miracid 20-10-10 as the wetting agent. I felt that this layer might act as a reservoir of nutrient rich moisture for the live sphagnum and reduce the necessity for bottom watering. On top of the dead sphagnum, I placed a "bunch" of live sphagnum with growing terminals facing up and terminating just below the lip of the pot. Next, I labeled each pot and sprinkled the appropriate seed onto the surface of the live sphagnum. Having covered each pot with a plastic sandwich bag to retain moisture, I placed the pots on a bottom heat source at 70°F to 80° F.

Germination commenced in six days and all but R. hirsutum showed some germination by the end of the second week. Even the R. yakushimanum hybrids germinated heavily. No fungus problems were seen even though this was a perennial problem with the standard system.

Whatever works may need no further interpretations, but I have drawn some conclusions about the live sphagnum method:

1.  The low pH of the live sphagnum protects the seedlings from fungus diseases.

2.  The sphagnum provides the humidity and root aeration ideal for germination and root penetration.

3.  Live sphagnum provides nutrients that sustain the initial growth of the seedlings.

There are some problems with the new system, however:

1.  One needs a source of live sphagnum (a bog or a fen).

2.  The sphagnum must be kept alive (moist) until time for sowing.

3.  The sphagnum may grow faster than your seedlings, requiring a search through a sphagnum "jungle" to retrieve the seedlings for pricking out.

All of the above problems pale into insignificance when compared with the great advantage of virtual 100 percent germination rates.

Sphagnum Moss

Sphagnum is a large genus of mosses that inhabits the cool and cold temperate wetlands called "fens" and "bogs." Canada has about 20 percent of the world's supply of these habitats, much of it concentrated in the James-Hudson Bay Lowlands. But you don't have to live in the James Bay Lowlands to acquire sphagnum! There are sphagnum wetlands in many of the cooler areas of the American Northeast, such as Pennsylvania northwards through Maine and westward to Minnesota, where the great patterned bog of the Red River basin is world famous. Sphagnum peat lands should exist in the higher parts of the Appalachians as far south as Georgia, and the same pattern should exist in the Western Cordillera south from Canada.

The sphagnum that I used was collected near Massey, Ontario, on the north shore of Lake Huron, at a site where rain collected in small catchments underlain by impervious Precambrian quartzite. Quartzite and granitic bedrocks are non-buffering acidic surfaces common to the southern Canadian Shield, the Adirondacks, and the Piedmont of the Appalachians. Sphagnum has low nutrient requirements beyond water and has the capacity to absorb two and a half times its own dry body weight in excess water and excrete hydrogen ions in exchange for metal ions such as calcium and magnesium that it absorbs with water. The excreted hydrogen ions make the moist outer surface of the moss highly acidic, with a pH as low as 2.8. A pH level this low will kill most bacteria or impede the spore germination of fungal pathogens. This explains sphagnum's use in World War I as dressings for wounds. (Bears have been observed in peat bogs plastering live sphagnum onto wounds.)

To grow sphagnum, a persistently moist area should be prepared using a sand/peat mixture and fragments of live sphagnum sprinkled lightly across the surface and pressed down to facilitate contact with the rooting medium. Provide semi-shade and above all, keep moist ! Once established, very light applications of liquid fertilizer may be used. (When happy, sphagnum will grow at the rate of about 1 cm per month.) A bog garden so-created may lead to the cultivation of other acid-tolerant plants such as pitcher plants, sundew, caribou moss and orchids.

With a month of further observations on the germination pots, I can elaborate on one weakness of the live sphagnum method. At 1 cm per month, the sphagnum grows at least twice as fast as rhododendron seedlings, and the low nutrient levels further retard seedling growth. Six weeks after sowing, one must search a sphagnum "jungle" for your rhododendron seedlings. To off set this problem, I now prick out seedlings immediately after they have full petiole development but before their true leaves fully open. I keep the growing on flats very moist under plastic even if this means misting with a ultrasonic humidifier. The live sphagnum method is not a good growing on environment, but it is an excellent germinating environment.