Orchid composts are often the most misunderstood factor in orchid culture; they are frequently misused and are a continual source of failures. Yet the rule for their use can be summed up as a simple corollary to the rules in Chapter 2: Orchid composts must be exceptionally well drained and exceptionally well aerated.

That's all there is to it, but in practice no man-made compost can duplicate the soils, dead vegetation, mosses, and other physical conditions in which orchids naturally grow. We can offer only artificial measures to satisfy the habits of growth and the unusual root systems of orchids.

Orchid composts are of two general kinds, corresponding to the two classifications of orchids—terrestrial and epiphytic. Terrestrial orchids never have been much of a problem. They have been grown in all types of composts ranging from plain loam to pure leafmold, including such unusual materials as excelsior and coke. The most successful composts, though, are those which closely approach the physical properties of the soil in which terrestrial orchids naturally grow.

During the early days of orchiid culture, terrestrials were handled in the same manner as tree orchids. They were wired to bare blocks of wood and soon died of starvation. After plant explorers finally pounded home the fact that many orchids were terrestrial, orchidists began to devise composts of turfy loam, peat, sphagnum moss, broken crocks, and coarse sand in every conceivable combination. Orchids responded by staying alive. Somewhat later several experimental growers added a bit of oak leafmold and cow dung to their composts, and terrestrial orchids started to thrive. Other growers then tried guano, ground fish meal, and weak applications of manure water. To all these supplemental fertilizers terrestrial orchids responded happily.

As more reports from plant hunters trickled back to Europe it suddenly dawned on orchidists that, in one sense, terrestrial orchids did not live in the soil. Published reports indicated that plant roots stayed in the loose, open, not quite decayed top litter of the forest floor. Some plant hunters carefully brushed aside the accumulated vegetative litter around terrestrial orchids and learned that most orchid roots spread out horizontally six or eight inches below the surface of the litter, rarely entering the true soil.

Accordingly, orchidists simplified their elaborate composts, limiting them to loose, porous mixtures of partly decayed vegetation. Today, simplification has finally resulted in a combination of fine oak leafmold and a suitable proportion of decomposed granite or small gravel. The most successful mixture is two parts by volume of leafmold to one of small gravel or decomposed granite. Once in a while, peat moss may be added, but only to the compost of those orchids whose water requirement is continuous; but even then it is not a good practice. Some growers add a small handful of carefully aged manure to their composts. Others prefer a tablespoon of bone meal or cottonseed meal thoroughly mixed with each six-inch pot of compost. These organic fertilizers more nearly simulate the food in the natural litter and are not injurious to terrestrial orchids. Later, applications of weak liquid fertilizers are made, if necessary.

On the other hand, epiphytic orchids have been and continue to be a problem. The compost in which they are grown at present, while theoretically satisfactory, is far from perfect. They too were once starved on bare blocks of wood in greenhouses. They too were once grown in the most unlikely composts of decayed wood, sawdust, tanner's bark, loam and manures—and failed, except for the brief success of Belgian orchidists. The Belgians called their compost "tierre bruyere"—a. coarse leafmold raked up from about Belgian oaks. Orchids were loosely potted in the leafmold, given a quarter-inch top dressing of sphagnum moss, occasionally and lightly watered, and exposed to full sunlight. The technique was successful because sunlight helped the plants utilize the extra food in leafmold. Watering was reduced to a minimum since leafmold tends to pack and stay wet as it progressively decays. Although no healthier orchids have been grown since, the practice was finally abandoned because the moisture content of the compost in the pots could not be controlled. It would be an interesting experiment to retry the Belgian tierre bruyere, adding to leafmold an equal or greater volume of gravel. The drainage and aeration of the compost might be improved enough to assure continuously successful growth. The trouble in growing tree orchids lies more in their roots than in the compost. Over a period of many thousands of years the roots of tree orchids have adapted themselves to a mode of growth unsuitable to most plants. Their roots are freely exposed to sun, wind, and rain. Similarly expose the roots of day lilies or carnations, and they will die. Orchid roots are protected from the sun by thin layers of sponge cells capable of absorbing in a few moments many times their own weight in water. Later, the water is slowly released to the plants. Orchids have been called the camels of the plant world.
 
Although, in their native habitat, epiphyte orchid roots are frequently soaked, they are quickly dried out by the sun and wind. Whether in nature or in culture, they rot if kept too wet over too long a period. That is the reason a compost must be aerated and well drained, and even then, if the compost is overwatered continuously, tree orchids will die.

Orchid roots usually are large in diameter, fleshy and brittle, breaking easily. They do not have the rock-crushing force of soil-bound plant roots. A simple obstruction may stop their growth; side shoots aren't often developed. That is the reason many orchid composts must be soft and loose; epiphytes, in particular, must have open composts that permit easy root development.

There you have the basis of epiphytic orchid composts, which must be quick drying, open, porous, and easily drained of excess water. There is another requirement: they cannot stand rich diets. Growing on trees, where they were at first supposed to live on air, they are adapted to thriving on a minimum of food. That is one of the reasons why most soil and leafmold composts don't agree with them.

A successful and practical compost for tree orchids was not discovered until an Englishman, whose name is lost, stumbled on what was called "turfy peat." It is dead plant material consisting of dried roots and stems of bracken, a native fern of Devonshire. Season after season, as the fern builds up new layers several inches thick, it is stripped off, dried, and packed in bales. This dried fern is entirely different in texture and appearance from other composting materials in that it has a grain. The old roots and stems are clumped together, all possessing one direction of growth. The small pieces of turfy peat somewhat resemble the tied bristles of an old and well-used whisk broom. Placed in clay pots with the grain in a vertical position, water rapidly passes through it. The individual fibers do not absorb water, but adsorb it; that is, a small amount of moisture clings to the outside of the fibers, never soaking them completely. Thus, drying out is accomplished more rapidly. Decay is extraordinarily slow, and the minute amounts of plant food released are mildly in keeping with the thrifty food requirements of tree orchids.

American growers quickly picked up the use of turfy peat, but eventually substituted local products. The dead roots and stems of osmunda, commonly called "royal fern" or "cinnamon fern," have proved suitable and cheaply available. As a rule, the fibers of the cinnamon fern have a neutral-to-acid reaction which aids the growth of orchids. Most orchids require a compost definitely on the acid side although one terrestrial, Cypripedium bellatulum, has been grown very successfully in the alkaline type of osmunda. As osmunda, or even leaf mold composts, decay they become increasingly alkaline and less suitable for most orchids. Two to three years is the longest orchids should be left in pots without renewing the composts.

For the beginner in orchid culture it is the better part of wisdom to accept without question the composts commonly used. It is inadvisable to leave the tried way and set out on exploratory paths until you thoroughly understand the standard methods. Many growers and amateurs have worked out elaborate compost formulas which they consider to be superior. This easily may be true. When you become an expert try them, but not before.

In the meantime, oak leafmold and gravel for terrestrial orchids and osmunda for epiphytes remain the safest, most workable, and easiest combination. The only permissible variation is to do away with leafmold, growing all orchids in osmunda. Many English and American growers have done this, believing that osmunda is just as good as leaf· mold.

At any rate, osmunda reduces their composting materials to a single substance. The difficulty is, of course, that osmunda is not always readily available—and is sometimes quite expensive. Then, too, osmunda is nearly sterile. Terrestrial orchids grown in it must be fed supplemental liquid fertilizers. Yet, for the gardener who dislikes fuss and muss, osmunda fiber is the cleanest of all composts, the simplest to use, the easiest to keep on hand.

ORCHIDS IN GRAVEL POTS AND FLATS

There is a third material and method you can use in growing orchids. It is not a compost, nor does it supply orchids with food. The material is gravel. The method is gravel culture. Although it is a new technique, requiring further experimentation to perfect, its use is increasing.

An orchid is knocked out of its pot and its roots thoroughly cleaned of osmunda or leafmold. A soft toothbrush and a pair of tweezers help in cleaning. All old, decayed roots are cut off. A clean garden clay pot, of the correct size, is half filled with three-fourth-inch rock. The orchid is inserted into the pot and the "rhizome," the base of the plant, held at rim level with the roots spread out. The balance of the pot is filled to the rim with one-fourth-inch rock (pea gravel or roofing gravel), the roots carefully and well covered. The rock may be tamped firmly with a small stick, or the pot gently bounced several times to settle it. Never use fine crushed rock, Haydite, or cinders; they absorb too much water, hold it too long, and keep orchid roots too continuously wet. The beauty of gravel culture is that it permits rapid drying of orchid roots.

Some orchidists prefer to use garden flats, or smaller boxes, for gravel culture. Wood flats are painted with aluminum paint on the outside, asphalt paint on the inside, and then filled with gravel. The bottoms of flats may be knocked off and replaced by one-fourth-inch wire netting. Laths may be used, if set not more than a quarter of an inch apart so the gravel won't spill through. Flats are ideal for young plants and seedlings. Larger plants are established in larger boxes essentially similar to flats in construction. Usually they will be six to eight inches deep and about six inches wide. Length is not important. They may be square, or may run the full width of an orchid bench or table.

Naturally, in such an inert medium as gravel, orchids must be fed. Disputing this, one amateur orchidist grew several orchids in a pot filled with glass marbles and gave them nothing but tap water for three years. This is not proof that orchids don't require food, but evidence of their tenaciousness and their ability to get along on very little, since tap water does contain some minerals needed by plants. The best feeding solution you can use is one of the nutrient solutions; nearly any one of them will do. One of the most successful is given in Chapter 12. A nutrient solution is applied regularly, depending in part on the water requirements of orchids. Two to three times a week in summer and once or twice a week in winter is sufficient for most orchids. Liquid fertilizer concentrates are not desirable because they leave deposits of crystallized salts on orchid roots which soon are killed by the toxic residues.

In an interesting experiment some years ago a Southeast amateur removed the compost from the roots of twelve orchids. Each day the roots were dipped into a chemical nutrient solution for two minutes. Afterward the plants were hung on wire hooks from the top of a Wardian case. The orchids grew faster and larger and flowered more profusely than control plants grown in osmunda. The technique, however, is impractical on a large scale if it must be done by hand. It would be interesting to test it with automatic equipment.

A word of final warning. Gravel culture of orchids is in its infancy. It has been used successfully on only five kinds of orchids—all of them epiphytes; it has failed or given mediocre results on terrestrials. It is not something to play with unless you resign yourself to losing a few orchids at first and unless you make a study of gravel techniques. For study materials, write to the University of California at Berkeley for Circular No. 347 on water culture by Dr. D. R. Hoagland; to the Missouri Botanical Garden for the bulletin by Dr. H. C. Fairburn on orchids in gravel culture; or to Ohio State University for the several excellent bulletins by Dr. Alex. Laurie.

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