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Unique Features of the Orchid Family
Orchids possess some interesting deviations from the norm for flowering plants. Coincidentally, their genetic predilections also encourage orchids to succeed in trying circumstances. Why do native orchids have a reputation for being difficult to cultivate, when tropical orchids have been cultivated successfully for decades?
Plant anatomy often determines culture requirements. For example, because the roots of tropical orchids are susceptible to rot if kept too moist, they require a sharply drained growing medium. Might this not also be the case with hardy orchids? One quickly learns that it is.
Many of the hardy orchids in which a gardener might be interested grow naturally in bogs. A bog is not a pond filled with moss. The sphagnum moss that dominates true bog habitats acts as a conduit for the slow diffusion of water from the underlying aquifer to the surface, allowing oxygen to reach the roots of the plants growing in the moss carpet. If the bog held water like a pond, the plant roots would be thrust into an oxygen-depleted environment and would be quickly replaced by species that can grow with their roots submerged, such as cattails. Draining the bog, on the other hand, leaves too little moisture behind, and plants such as ironweed, milkweed, and showy goldenrod appear, eventually to be followed by tree seedlings. Thus, merely mimicking the moisture conditions of the bog habitat in a garden bed enables gardeners to grow and flower a wide range of orchid species. Attempting to locate the orchids alongside other native plants, such as phlox or trilliums, in a typical woodland soil generally leads to failure, as does planting more rampantly growing moisture lovers, such as cattails, alongside the orchids.
I suspect many a frustrated novice simply fails to take into account that any given geographic area, even one as small as a building lot, may consist of a patchwork of microhabitats, each characterized by variations in parameters such as the amount of sun or shade, the moisture retentiveness of the soil or lack thereof, exposure to wind, and the presence or absence of competing species. Anyone doubting this need only compare the vegetation growing along the interstate ramp near my house with the vegetation growing in the swale 30 feet away. In late summer the dry-baked area of clay soil along the asphalt blooms with Rudbeckia, Bidens, and Coreopsis, while the low-lying area beyond boasts stands of Typha and Sagittaria. Among these vigorously invasive moisture lovers, the dusty-rose-colored plumes of Eupatorium maculatum and the scarlet spires of Lobelia cardinalis call out to swarms of butterflies. Even the grasses differ between the two locations. Three or four years ago the area was stripped down to the clay subsoil, compacted by the movements of construction equipment, and changed utterly from its former status as a cow pasture. Yet, seeds have blown in from undisturbed areas nearby, the activities of birds, small mammals, and humans have brought in still more, and once again the land blooms. Tree seedlings are already showing up, sorted according to their moisture preferences, just like the herbaceous plants. Nature's hallmark is this kind of resilience and exuberance.
Why then aren't the roadsides teeming with orchids? Actually, in some areas they are, when conditions favor them, although this is certainly not a commonplace occurrence. Orchids at least seem relatively rare. While some orchids are indeed extremely rare, they can be amazingly abundant in the right spot. This observation reinforces our strategy for developing successful culture methods: identify the parameters that appear to favor orchids in the wild, and then attempt to mimic these conditions at home.
Moisture Content of the Growing Medium
Typically, hardy orchids require a relatively constant level of soil moisture during the growing season. This may be naturally maintained in any of several ways. In the most common situation, the water table lies close to the surface, perhaps within a foot or less, and moisture permeates a loose, fluffy compost made up of plant debris and often mosses. Evaporation from the surface causes water to diffuse upward from the water table through the compost, but the compost itself never becomes waterlogged. Such conditions are typical of bogs. True bog habitat is now rare in the southern Appalachians, but in more northerly regions of the country, bogs are common and are often home to an abundant orchid flora.
In the southeastern mountains, a similar habitat for orchids often develops at the edge of a creek or stream, especially where the grade approaches level, and water easily percolates into the underlying substrate. This can even occur where a roadside drainage ditch is flanked by a layer of construction aggregate. At points along the Roaring Fork Motor Nature Trail in the Great Smoky Mountains National Park, the roadbed is only a few inches higher than the surface of its namesake stream, winding 10–25 feet from the road, down the narrow valley between Mount Winnesoka and Piney Mountain. Roaring Fork tumbles precipitously from near the summit of Mount LeConte, dropping in elevation nearly 2000 feet within its first 2 miles. On the valley floor the slope is far more gentle, and Roaring Fork becomes broader and correspondingly gentler. The motor trail alongside Roaring Fork consists of a layer of asphalt atop about 4 inches of crushed limestone. The limestone forms a broad shoulder along the roadside in places where it has been pushed away from the asphalt and scattered toward the creek by the pressure of the thousands of cars and RVs that circle the trail each year. A constant rain of plant matter falls from the forest canopy or is carried by Roaring Fork during floods to form a compost that fills the spaces in the limestone gravel layer. Such places are home to an abundance of showy orchids (Galearis spectabilis), an otherwise uncommon species. Road construction within forested areas appears to favor showy orchids, which may benefit not only from the extra lime in their growing medium but also from increased sun exposure at the road's edge. I have often observed this species within a few feet of a parking area or mountain road. Nevertheless, this observation should not be taken to suggest we need more road construction through our southeastern forests!
Another specialized moisture condition occurs in the areas favored by orchids that prefer a strongly acidic growing medium. Poor, impermeable clay subsoils form where silt once accumulated in low-lying, relatively flat areas, such as on the floor of the Tennessee Valley. Lying undisturbed by humans for millennia, these areas were cloaked in magnificent deciduous and mixed forests until relatively modern times. Unfortunately for the flora, level areas near water were among the most desirable locations for pioneering humans, and most of the original forest cover was quickly cut to make room for farms and towns. When the land was exhausted from agriculture, or further denuded by a second round of clear-cutting in the early twentieth century, it was often abandoned. Such sites are now frequently dominated by stands of fifty-to hundred-year-old pine trees. As a layer of pine needles accumulates on the surface of the clay, a growing medium for orchids (and certain other plants characteristic of this habitat type) slowly develops. Each time the region's abundant rainfall arrives, it quickly penetrates the layer of pine duff, but is held in place temporarily by the layer of clay, like coffee spilled on a flat tabletop. Before the water completely drains away, it can be carried up through the duff by capillary action. In this manner a relatively constant low level of moisture is maintained around the roots of the orchids growing in the layer of well-aerated organic matter.
Only one of the orchids mentioned in this book, Spiranthes cernua, grows well with submerged roots, a capability that has led to it being called the "aquatic orchid" in water-gardening circles. Nevertheless, this orchid grows and flowers quite well in the same bog garden conditions favored by Cypripedium and Platanthera.
From these examples it can be seen that most hardy orchids do best in a loose, fluffy, organic growing medium capable of holding moisture without becoming waterlogged; fine-textured media such as the peat-based potting mixes used for many indoor and outdoor plants smother the roots of hardy orchids and rapidly kill them. Orchidists often exhort us to use a growing mix that "retains moisture but is nevertheless well drained," or words to that effect, but what exactly is a well-drained growing medium?
In an attempt to resolve the question, I decided to compare a mix that works for Cypripedium acaule, a particularly finicky orchid, with media composed of other commonly used materials. I mixed up a fresh batch of four media: a basic woodland wildflower mix, tropical orchid bark, a bog orchid mix, and a C. acaule mix that includes PermaTill, a mineral product equivalent to very coarse sand.
I used these media to fill standard 6-inch plastic pots, as if repotting a plant, and left them sitting in the sun all day to dry out. I then poured 2 liters of water into each pot, allowed the pots to drain into plastic tubs for fifteen minutes, and measured the amount of water in the tubs. Subtracting this from 2 liters gave the amount of water retained by each medium: 560 ml for the woodland wildflower mix, 250 ml for the tropical orchid bark, 300 ml for the bog orchid mix, and 155 ml for the Cypripedium acaule mix.
Tropical orchid bark would have garnered my bet for minimal water retention, but the Cypripedium acaule mix proved to dry out most quickly, probably owing to the absorbency of the bark versus the complete nonabsorbency of the PermaTill. Based on this simple experiment, I propose we regard a mix that retains about 10 percent of the volume of water applied as well drained but moisture retentive.
Any tropical orchid enthusiast who has ever allowed a prized plant to get a sunburn understands another critical factor for hardy orchid cultivation: providing the correct amount of sun exposure. In the Sunbelt, where summer temperatures may reach 100°F, an orchid that is located in too much sun will cook, literally, in one afternoon. Through painful experience I have learned to err on the side of too much shade. If the plant does not bloom after a year or two but otherwise grows well, perhaps it needs more sun, but wait until you have good reason before relocating it. For a mixed planting of hardy orchids, dappled shade such as that provided by high deciduous tree branches gives the best results. If the plants receive any direct sun, morning sun is best, while afternoon sun is less preferable. Avoid midday sun, which is the most intense. Tropical orchid enthusiasts will note that this is also true for the species they grow. Trees with spreading branches near the ground — dogwoods, for example — may provide excellent cover for shade-loving species, such as some platantheras. For species that need a bit more sun, such as cypripediums, some lower limbs may need to be pruned to open up the growing area to more light. Pruning should be done carefully and conservatively, as lopping off the wrong branch may be a mistake impossible to correct.
Altering the sun exposure may be difficult to accomplish in an outdoor garden. Properly siting the hardy orchid bed or beds right from the start will prevent you from having to relocate them later, or resorting to other costly or time-consuming measures such as pruning or planting additional shade trees. You may thus find it helpful to grow the orchid plants in pots for the first year or two. Place the pots where you want the plants eventually to reside, and see how they fare. This approach provides a "dress rehearsal," allowing you to evaluate the design of the bed and permitting a quick rearrangement if mistakes have been made. Inevitably, of course, mistakes will be made. Therefore, I recommend this method for any rare or valuable plant in your collection, even though you may be an experienced gardener.
Not long ago I moved to a new house on the other side of town, which involved relocating my entire plant collection of twenty-five years. As anyone who has ever moved will understand, many decisions about accommodating the new plants were arrived at on the spur of the moment. That fall, much transplanting needed to be accomplished, mostly to give each species its proper sun exposure. Orchids are inherently tough, however, and only two plants out of more than two hundred suffered from the move.
Artificial shade, in the form of a lath house, pergola, or similar structure, may be one alternative if the otherwise perfect spot for hardy orchids receives too much sun. The most shading will be needed on the south side of the bed, but do not make the mistake, as I once did, of failing to take into account the potential effects of exposure when the sun's rays arrive at a low angle, early or late in the day. I have damaged Cypripedium plants by neglecting to shade them from the western sun. The fail-safe approach to designing artificial shade for hardy orchids is to enclose the growing area completely. A screened porch or deck with a lath roof shade will provide the dappled, constantly shifting patterns of light the plants seem to prefer. When considering this approach, remember that the design and construction of the shade structure must always conform to local building codes. Overhead shade structures in particular must be well made and properly installed to avoid any possibility that they might come crashing down on you or your guests. Always seek professional advice if you have any doubts about the design.
Half-hardy terrestrial orchids are those able to tolerate a minimum temperature of about 40°F. Even in my zone 7 garden, sheltered spots can nurture plants that otherwise would not survive the average winter low. A frost-free but cold greenhouse or "alpine house" offers the best protection for a collection of half-hardy plants, and becomes essential as you move north out of the southeastern and southwestern corners of the country. Many native orchids of Europe, the Middle East, Asia, and Australia belong to the half-hardy group.
Another unique anatomical feature of the orchid famil is the absence of guard cells around the tiny leaf openings, or stomata, that allow the leaves to "breathe." In most plants the guard cells can shrink or swell depending upon the plant's water needs of the moment. Not so with orchids, which therefore are at the mercy of their environment. Hardy orchids, like their tropical cousins, thrive best when the relative humidity is above 40 percent.