garden culture of North American Cyp species

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Abgeschickt von Charles J. Sheviak am 16 Juni, 2000 um 15:27:04

I have been growing North American Cypripedium species as garden plants for over thirty years. Because it appears that they may require somewhat different conditions than many of the Eurasian species that are now available, it may be useful to summarize my experience with American plants.

My early experience with these plants in the wild shaped my understanding of their requirements and dictated my techniques for their cultivation. I am originally from the western portion of the Great Lakes region and began growing them there; I now live and cultivate these plants near its eastern edge.

Throughout the region Cypripedium species are abundant, all growing in the fine quartz sands derived from the larger lakes and glacial deposits. The soils are predominantly sand with only small amounts of organic matter; they are not organic-rich. This is an important point. All of the species present in these sands also occur in the countless fens that occur across this glacial landscape. With the exception only of Cypripedium acaule, which requires extremely acidic soils and which occurs in sphagnum bogs, the region's species occur in calcareous fens in finely decomposed, calcareous muck.

Given my early experience with the species in these two soil types, it was reasonable to attempt to prepare garden soils from their two basic components. I mix fine dune sand, which I easily collect as needed, with a small amount of muck, which I can purchase. The amounts of each are not precise, but rather are determined by eye. The resulting soil should be mostly sand, with sufficient muck merely to color it gray: 'dirty sand' I call it. It must not be an organic-rich mix. Because the organic matter is provided solely by the muck, which is already wholly decomposed, further decomposition and corresponding change of the soil structure does not occur. This is my standard mix. It is suitable for all of the region's species except C. acaule.

Cultivation of the species of this region and the rest of the continent should be attempted only with an understanding of their habitat requirements. This information will be briefly mentioned for each species.

Cypripedium acaule occurs from central Canada east and southeast through the taiga, and southward through the deciduous forest region of the eastern U.S. Across this wide range it occurs in a great diversity of habitats from moist forest to the summits of sand dunes in full sun, swamp forests, and sphagnum bogs. It grows in sand, peat, and clay. Its only requirement is for extremely acidic soils; reactions are no higher than pH 4.5, commonly around 4.0, and down through the 3's.

Cypripedium arietinum is a rather rare species distributed from the taiga of central Canada across the Great Lakes region and eastward to the Coast. Only in the Great Lakes region is it abundant, and then only rarely. It most commonly occurs in the immediate vicinity of the larger lakes, growing on bluffs and sand dunes under open, scrubby forest of conifers and deciduous species. Soils are often moderately acidic but over calcareous substrates, very well-drained and often moderately dry. The species also occurs in calcareous fens; rare in such habitat, the plants grow atop well-drained but moist hummocks in calcareous muck.

Cypripedium reginae occurs across the eastern taiga and southward through the northeastern U.S. and locally into the southeast. It is characteristically a plant of calcareous fens, growing in the light shade of conifers and deciduous trees and especially in openings between them. It sometimes grows in a surface layer of sphagnum, but the roots penetrate into calcareous muck below. The species also occurs on moist dune slopes and sand flats in light shade or sun.

Cypripedium parviflorum is the most widely-ranged North American species. It is transcontinental in the North, ranging south through most the southeastern U.S and in the mountains of the West nearly to Mexico. It is also the most variable and taxonomically complex of our species. At present three varieties are recognizable, but this taxonomy does not accommodate all variation and some plants cannot be assigned to an infraspecific taxon.

The most widespread variety is var. pubescens, which occurs virtually throughout the range of the species. This is also the most variable variety. In the eastern deciduous forests the plants are large with large flowers, and similar plants occur in the north and west. Commonly the sepals and petals are rather pale with highly variable darker markings. From the Great Lakes northward and westward, however, the plant is much more variable, with a tremendous range of size and shape of flowers and floral parts. Small northern plants were described as variety planipetalum, because their small flowers commonly have mostly unspiralled petals. These plants are merely extreme forms of var. pubescens, and diverse populations of var. pubescens include a great range of forms. Many plants can be phenotypically manipulated by growing them under different conditions; cool conditions induce small growths bearing small flowers with flat petals. The point here is that many such plants will develop into larger var. pubescens in cultivation; var. planipetalum does not exist.

The other varieties of C. parviflorum have small flowers. In the forests of the eastern U.S. occurs var. parviflorum, often growing on rather high, drier slopes. The sepals and petals appear dark due to closely spaced, tiny spots. This plant is rather rare and not generally known in cultivation. The plant that is commonly known and cultivated as var. parviflorum is actually var. makasin. This is a more northern plant, occurring from the Great Lakes region northward into the taiga. It is typically a plant of calcareous fens, growing in open forests and openings. It also occurs in thickets in moist sands. The flowers bear uniformly dark sepals and petals, rather like C. calceolus. Typical plants of this variety are very rare west of central Canada; in the western mountains the variety seems to be represented by plants without the uniform dark sepals and petals.

Cypripedium candidum is a related species of the prairie grasslands of the central U.S. It has the highest light requirements of any of our species, and it will not bloom without full sun for much of the day. It occurs typically in mesic prairie, but most of this habitat has been converted to cropland. This has left mostly the populations occupying the wet extreme of the species tolerance, because these areas are often too wet to cultivate. As a result, the literature commonly refers to this species as a plant of wet prairies and fens. Such occurrences are actually very rare. The plant typically grows in sites that are wet only briefly as the soil thaws in the spring; rapidly it becomes moderately moist, and by mid-summer it is becoming dry. The wet sites occupied by the plant are unusual, and excessive moisture in cultivation is usually lethal.

Where the two species grow together at the prairie margin or rarely in fens, C. candidum sometimes hybridizes with C. parviflorum forming the natural hybrid C. x andrewsii. These hybrids are often quite beautiful and horticulturally desirable.

These wide-ranging and more eastern species are those that I first learned to grow; fortunately, some of them are the easiest of our species to cultivate. Except for C. acaule (which will be discussed below), all these species I grow in my 'standard mix'. In this soil, some grow simply as open border plants under ordinary garden conditions. These include C. parviflorum var. makasin and var. pubescens, and C. candidum, and their natural hybrids. Especially C. parviflorum var. pubescens is very adaptable to such culture. However, it may be important to determine the origin of plants of this variety before attempting open-sun garden culture. Plants from the eastern deciduous forest of the U.S. may not be so adaptable as the plants from elsewhere.

Cypripedium reginae also responds well to ordinary garden conditions if appropriate light and some shelter from wind is provided. The plant will grow in full sun, but it is especially successful on the north side of a building where it receives sun only early in the morning and in the evening, and is shaded from all sun during much of the day. However, full sky light is available all day. With moderate garden moisture the plant may grow into a large group.

The extraordinary C. kentuckiense will grow well with light and moisture levels similar to those for C. reginae. This more southern species is adequately winter-hardy for growth in the northeastern U.S.; it should prove hardy across a reasonably wide area, as in most of its range soils freeze deeply at least in some winters. In the western portion of its range it occupies unremarkable habitat: mesic slopes in deciduous forest. In the eastern and southern areas, however, it occurs under conditions unique for a North American Cypripedium: rich bottomland deciduous forest in alluvial soils along creeks and small rivers. This remarkable habitat has given rise to erroneous literature reports of the species growing in "swamp forest" and for recommendations that in cultivation it be grown wet. In fact its bottomland habitat is not wet. The soils of these sites have built up through a cycle of leaf fall in the autumn, followed by deposition of fine sandy alluvium during spring flooding. The resulting layered soils are very open and full of air; one sinks into them when walking on them. Although the sites commonly flood, even after the plants have begun growth, the soils drain quickly and within days become merely moderately moist. In cultivation, then, excessive moisture must be avoided; I grow the plants easily in pure, fine dune sand at normal garden moisture levels.

Cypripedium candidum is a charming plant that, once established, is hardy and resilient. The plants require abundant sun and good air movement, responding well to windy conditions. They are slow to become established, however, and during the first two or three years are often weak and susceptible to leaf and stem rots. Hot sun wilts them; the leaves then lie on the ground and cook or rot; if too much water is given in an effort to counteract this, the stems rot below the soil surface. Soil splashed on the leaves by rain may lead to leaf rots. Watering must therefore be done very carefully. Because of these difficulties, and because I like to grow plants under conditions that simulate their natural habitats, I grow this species in an artificial 'prairie' composed of several dozen species native to these communities. The C. candidum occupy an area of low-growing grass and sedge with other appropriate species. In most years the growth is open and does not significantly shade the cypripediums. If these other plants become too tall, they are clipped off just at the height of the C. candidum, exposing them to full sun. Now that my plants are well established, I almost never need to water them. Even in the driest summers, with only one or two useful rains during the course of the season, the plants have done well with only two or three supplemental waterings. Because my normal eastern climate is much more wet than the prairies where the plants normally grow, in the autumn I mow and carry-off the dead vegetation, and then burn the remaining stubble and grasses that are too tough to cut. This eliminates the rotting mass of dead material and also deprives rodents of winter habitat; I have had voles eat the root systems of C. candidum. All this effort has been well repaid: I have abundant seedlings appearing from self-sown seed, and many of these plants have bloomed.

In contrast to my success with these species, my experience with the curious little C. arietinum has been mixed. These plants will grow well in my standard mix in partial shade with moderate moisture levels. They are small and weak, however, and correspondingly difficult to establish. They grow in sites that often become quite dry in nature, but until established they require careful watering: too little and they may wilt fatally, but they rot easily if over-watered.

The most notoriously difficult American species is C. acaule. This has a well-deserved reputation for being essentially impossible to cultivate. Typically a collected plant (for until recently all plants of this species appearing in the horticultural trade were wild-collected) planted in the autumn will bloom the following season and appear healthy that summer. The following year it appears but does not bloom, and often it appears again as a smaller, non-blooming plant the third year. It is not seen again. The species can be successfully cultivated, however, if its requirement for intensely acidic soil is met. This can be done in the following manner, provided that absolutely no compromises be made. The site chosen should be partially shaded and with perfect drainage, so that all water goes straight down through the soil, as in sand or gravel. If there is any chance that water will flow across the surface or laterally through the soil, a raised bed must be constructed. A hole is dug one half meter deep (or a bed built one half meter high). This should be no less than about 3/4 meter across (larger is better). Such an area can be used for one to a dozen plants. The bottom of the hole or bed is lined with a layer of pine needles, fiberglass fabric or similar material (it must permit perfect drainage) and the sides lined with plastic sheets (to prevent lateral water movement). The hole or bed is then filled virtually to the top with quartz sand of pH 3.0-4.5. The rhizomes of the C. acaule are then laid on the surface of the sand, and about half of the roots are pushed down into the sand, the other half allowed to spread laterally. The roots and rhizomes are then buried in about 5 cm of partially decomposed conifer humus of appropriate acidity. All watering must be with distilled water or clean rainwater. Absolutely never use water with any mineral content. Fortunately, after the plants are established they are very drought-resistant. Even during a summer with only a single useful rainfall during June-August I did not water these plants and they still produced good growths and some flowers the following year.

These mostly eastern species are those with which I have the most experience. Plants from the western mountains I have grown much less extensively, and some species I have not attempted to cultivate. I will briefly summarize their habitats and what I know of their cultural requirements.

In general, the far north is, of course, cool and moist. These conditions predominate through most of montane western Canada and into the northwestern U.S. Southward in the U.S., however, the climate very rapidly becomes very dry and hot. Obviously, Cypripedium species do not occur in the widespread deserts and dry, hot forests, but a few species are locally distributed at higher elevations where conditions are cooler and more moist. Moist areas in the generally arid West, however, are very different than what most of us are familiar with in more humid regions. Here the air is dry, and moisture from the common montane thundershowers dries quickly. As a result, despite rather frequent rain, soils are very dry at the roots, and the plants grow almost like epiphytes, with rapid drying of the roots. This situation may be important for cultivation of some plants.

In the far northwest, in Alaska and Yukon, occurs Cypripedium guttatum. This species grows in moist forest and even in open tundra above the Arctic Circle. Frequently it grows in rich forest humus over limestone. I have found it to be easily cultivated. I grow it in two places. One is on the north side of my house, as for C. reginae. Here I grow it in pure pine humus. I also grow it in an open bed in a soil produced from mixing pine humus, shredded stems and leaves of coarse grasses, sand, perlite, and a little crushed oyster shell. In this bed the plants receive sun during the middle of the morning, and partial shade the rest of the day. Plants in both sites have grown well, but those in the sunnier bed have quickly multiplied and have begun to form a ground cover because of their long-creeping, branching rhizomes, whereas the plants in the shade have not increased.

The most northern of the American species is Cypripedium passerinum. This is a diminutive version of C. reginae with wholly white or sometimes faintly pinkish flowers. This species occurs from near the Arctic Ocean in Alaska and western Canada, southward only to the U.S. border region. It occurs in open tundra and in thickets and forests bordering streams. It grows in a great variety of soils, in wet to rather dry conditions. Despite this, I have found it to be extremely difficult to cultivate, yet growers in western Canada report that it is easily grown in any soil, under common garden conditions. The plants do not seem to be sensitive to heat in my gardens, and I suspect that the problem may involve day length at my southern site. I have been experimenting with Alaskan material which has responded much more favorably than the more southern Canadian plants. I think the Alaskan plants, coming from a region with perpetual light during the growing season, do not respond to day length.

A well-known challenge among the western species is Cypripedium montanum. This species with its white lips and very long, dark sepals and petals is beautiful relative of C. parviflorum. It grows in the mountains of western Canada and the northwestern U.S. under a variety of conditions, from deep moist coniferous forest in the north to dry mediterranean oak forest and the open scrub called chaparral. Its soils range from acidic conifer humus to calcareous clay. Despite this great variation in habitat, the species is perplexingly difficult to cultivate. Plants from dry oak forest I have grown successfully in my standard mix in partial shade. Kept on the dry side, they increased in size steadily for years, only to be killed by an unusually dry year that more closely approximated their natural habitat. I have experimented with plants from wetter coniferous forest, only to have them rot immediately from excessive rain, or decrease in size over several years, apparently from insufficient moisture. Even growers in western Canada, who move the plants a short distance from the adjacent forest, report great difficulty, and often the plant does not appear the first year in the garden.

Perhaps the most reliable recipe for its culture is similar to that for C. acaule: Dig a large hole and place a layer of conifer humus on the bottom. Fill the hole with coarse sand and bury the root-system in the sand. Water as needed. The strong drainage seems to be important, and it permits normal garden moisture levels. I think also the sterility (low fertility) of the sand reduces problems from pathogenic fungi.

Cypripedium montanum hybridizes with C. parviflorum producing C. x columbianum. These plants have variably creamy or white lips with long sepals and petals that may be dark or pale. They intergrade with small-flowered C. parviflorum. These western C. parviflorum can be grown much like their eastern and northern relatives, and C. x columbianum can be grown with them. Interestingly, some plants of the hybrid that appear more like C. montanum than average are sometimes difficult to cultivate. I have grown a number of these hybrids and western C. parviflorum on a steep hillside in a soil prepared from coarse sand, conifer humus, and crushed oyster shell. They have grown very well in this mix, designed to simulate the commonly coarse mountain soils, but I don’t know that this soil is important to them.

The remaining two species I have never attempted to cultivate.

Cypripedium fasciculatum occurs in widely scattered areas in the western U.S. It is a curious species with a nodding cluster of small purplish-brown flowers held below the pair of leaves. The small plants apparently demand rather cool conditions, growing at rather high elevations, or lower in the cool coniferous forest along the Pacific Coast. The habitats range from open, windswept subalpine forest in nearly pure gravel, to sheltered sites in finely decomposed humus. Throughout, however, the soils are acutely drained and rather dry during the growing season.

Cypripedium californicum is the last of the North American cypripediums, and it is a remarkable species. Plants reach a meter in height, with a raceme of several small white flowers virtually identical to those of C. passerinum. It is the most restricted of our species, both in geography and habitat, as it occurs only in portions of northern California and adjacent Oregon, where it grows in wet, seeping hillsides, frequently with the unusual carnivorous pitcher plant Darlingtonia californica. The plants grow in the moist soils of hummocks, in saturated soils of spring courses, and even emerge from shallow, running water. The waters emanate from rocks rich in the mineral serpentine, which provides an unusual chemistry to the soils that greatly limits the range of species present; whether the cypripediums require such conditions or only take advantage of the reduced competition is unknown. The species has a reputation in the U.S. for being very difficult to cultivate, but it is known to be very winter-hardy, despite its geographic origin.