Growing Rhododendron Seedlings in Glass Containers
Johannes Hedegaard, Lyngby, Denmark
An easy and nearly infallible method to grow rhododendrons from seed, is to sow in preserving glasses, or in small aquariums with a piece of glass as a lid.
The sowing medium is fine cut, living, green sphagnum moss, which has the advantage of being nearly free from injurious fungi: further it is tremendous in respect to holding water, and with a pH about 3.5.
The process begins by placing a layer, say 2 inches of coarsely cut green sphagnum on the bottom, saturated with rainwater, or other water free of lime. This is covered, ½ to 1 inch with very finely cut green sphagnum. In this last operation use only the green parts, but avoid the tops themselves, because those can overgrow the seeds before the seedlings have developed.
Now, pack the sphagnum down fairly tightly and sow on the surface; do not cover the seeds, but put on the glass lid for closing the container. Put it in a place where the temperature will reach 70° F. inside the glass. Never leave it in the sun, where the temperature will increase disastrously, but in very good light.
The conditions needed for germination are: heat, water, and oxygen. In the closed container, with the wet moss, the humidity will soon increase, and soften the seed shells making them permeable. The oxygen, released by the photosynthesis in the green leaves of the moss, will speed up hormone and enzymatic activity, inside the embryo.
The germination commonly takes place in 10 to 14 days, during which time the lid must not be opened. When the germination is well advanced, not more than room temperature is needed, and one can now open the lid daily, beginning with about 10 minutes a day. It is, on the other hand, possible to leave the seedlings, under these conditions, without any attention for a long time, because the moss contains a lot of feeding materials, such as: Mg, K, N, Ca, Phosphate and Nitrate. I have held R. schlippenbachii myself in this way for several months, and without opening the lid at all.
Pricking-off time is when 3 to 4 pairs of leaves have developed, not earlier in this technique. Pricking-off medium is, 1 part Dutch peat, 1 part coarse sand, (best is fine granulated granite) and 1 part of shredded green sphagnum. The easiest way to mix these materials is under dry conditions, and afterwards water in as necessary.
The seedlings, now pricked-off in boxes, are placed in an atmosphere of medium high humidity, under a plastic tent or something like that, and hardened off the following 14 to 20 days.
I normally sow in the middle of October, and am ready for the first pricking-off in January. During the dark wintertime the plants must have artificial light, not very much, about 40 watts approximately 18 inches above for 16 to 18 hours. I use, for this purpose, the American product, "Gro-Lux" fluorescent lamps, which in my opinion is the very best I can get hold of here in Denmark.