Nature Can Protect Small Seedlings
By G. G. Nearing
Anyone who has wandered through wild stands of Rhododendron maximum and Kalmia latifolia must have noticed how thousands of seedlings spring up, not among the fallen forest leaves, not yet on bare ground, but chiefly along moss-covered mounds and hanks. A moment's reflection will suggest that this is because the tiny air-borne seeds happen to lodge in the moss, and are held there until they germinate. Further thought may lead to the conviction that there is more than just chance in the relation between mosses and rhododendrons.
In the protection of a greenhouse, watched over by skillful propagators, doctored with the latest and most potent chemicals, rhododendron seedlings are subject to sudden onslaughts of damping - of diseases, fungi like Pythium debaryanum which travel rapidly through the soil, destroying thousand of little plants overnight. Then out in the open hills and bogs where no one can interfere, why do not these same fungi wipe out the wild plants?
Years ago a moss specialist roomed with me, and when we returned from field trips, he would throw his bag of damp moss specimens on a shelf, leaving it there perhaps two or three days until convenient to look them over. This puzzled me. My specialties were lichens and fungi. Immediately on arriving home, I had to spread my specimens out to dry, else they were quickly ruined by molds or other fungous or bacterial growths.
He explained to me that mosses contain antibiotic substances which protect them against most fungus attacks. I had known that Sphagnum moss has protective properties made use of in various ways, and lately recommended as a medium on which to grow rhododendrons from seed. However, the rubbed Sphagnum customarily employed for this purpose has proved unsatisfactory in my tests, because I like to have my seedlings where they germinate, until it is convenient for me to handle them, and that may mean two or even three years later. The protection of the dead Sphagnum does not last. The texture is too soggy to maintain healthy seedlings for any length of time.
I never set out consciously to give my seedlings the same conditions as a moss-hank in a mountain hog, but one thing has led to another, until I arrived at the following method. My seeds are sown in 6-inch pots, which are first filled about two-thirds with broken soft brick, then leveled with a compost of three parts Michigan sedge peat, one part sand and one part top soil. This is then pressed down hard with the thumbs, making a space of half an inch or so below the rim of the pot, which is again filled with a different compost, 1 part Michigan sedge peat, 4 parts sand (the grade known as builder's sand or coarse concrete sand). This is pressed down vigorously with a piece of lath, and scraped off exactly level with the top of the pot. Pots are then placed in a soaking tank of shallow water, until moisture shows on the surface of the sand compost. Seeds are scattered thinly over this surface, then partly covered with a very thin sprinkling of sand.
These seed pots are placed in shallow pans in the bottom of what has become known as a Nearing frame, a frame completely shielded from direct sunlight but well lighted from the north sky and reflected light. The frame is closed rather tightly, the glass never lifted for ventilation. A little water is run into the pans, then allowed to dry out, but a little more water added every few days. so that the pots never suffer from complete drying out, but are not kept too wet. No water is given overhead.
In this way a fairly exact balance of soil moisture and air humidity is maintained with a minimum of attention. After a few days most of the pots show a greenish tint on the surface of the sand. This is moss protonema. Moss spores are everywhere floating in the air, and when they alight on a suitable surface, they germinate-not directly into a moss but forming a film of interwoven green strands, the protonema, from which, weeks later, the moss proper springs up in a characteristic cushion, according to species.
As soon as the protonema forms, it protects the rhododendron seeds or seedlings against most of the fungi which cause the dread damping off. The most common of these, Pythium debaryanum, can quickly kill off any number of unprotected seedlings. Chemicals recommended for the control of damping off may be effective if employed with sufficient skill and persistence, but are more likely to kill off any seedlings which the fungi spare. In any case, I like to leave my seedlings unwatched for days at a time, and when I do look at them, I like to find them in good health.
If in the early stages the pots are kept too wet, a pale gray fungus which I have not identified, may spread over the protonema, killing it, and exposing the seedlings to danger of damping off.
This invasion usually occurs on a few pots and results in some slight losses. However, once the moss has begun to grow up from the protonema, the seedlings are fairly well protected from fungus attack. When fungi do invade, their progress is slow, and most of the seedlings can be saved by transplanting.
There are at least half a dozen species of moss which will develop on the pots under my conditions. Most of them protect the seedlings, but the larger kinds grow too vigorously, and tend to shade out or smother young plants.
Polytrichum commune and Catharinea undulata (Atrichum undulatum) I pull out as soon as they appear, for they are much too large for the purpose. Until two or three years ago, my pots were variously covered with Funaria hygronetrica, Physcomitrium tubinatum, Bryum argenteum, and some species of Dicranella. These are more or less satisfactory, but I prefer Leptobryum pyriforme because of its small size and hair-like leaves, casting little shadow. This year virtually all my pots (about 350) are covered with the Leptohryum. To accomplish this I placed here and there among the newly sown pots, a few old pots with seedlings left over from the previous year or the year before, among which Leptobryum pyriforme had put up its spore capsules. These, though rather old, still contained spores enough to start my preferred moss on all the new pots.
Of course I never sterilize my soil nor wash my pots nor use any recommended chemicals. There are pots now in my frames, each containing over a hundred seedlings in good condition, from seed sown in March 1955. I always sow about the end of March, but it may be many months or years before I get around to transplanting all the seedlings. They are transplanted into 2-inch pots, never into flats, for fiats are disease traps which must have been invented by a very special devil.