Rooted in Mystery
How Does The Pink Lady's-slipper Grow?
Written by Bill Cullina, this article was originally printed in two installments in the Spring/ Summer and Fall/ Winter 1997 Journal & Programs/ Events Catalog of the New England Wild Flower Society.
FEW PLANTS ARE AT ONCE as beloved and as enigmatic as the pink lady's-slipper, Cypripedium acaule. Its rare beauty belies the fact that it is actually one of our most common orchids, found growing in open, oak-pine forest in the company of lowbush blueberry and huckleberry.
Much has been written about the difficulties of transplanting pink lady's-slippers. They are indeed very difficult to transplant, and are best left to enjoy in their native haunts. Untold thousands of wild plants have been dug and sold with little or no chance of surviving for more than a year or two; therefore, NEWFS does not recommend any collection of these plants in the wild. In the past ten years, however, great strides have been made in research aimed at raising the plants to maturity from seed, and it is only a matter of time before nursery-propagated plants become commercially available.
The Root of the Problem
To understand why transplants usually fail, one must first look at the roots. Lady's-slippers store their vital food reserves in their root system. These roots can live for five years or more, but if the growing tip is damaged, a new tip will not be produced, and the root will stop growing. A mature, single-growth plant will only produce 4-10 new roots per year. These spread horizontally from the crown of the plant and, if undamaged, weave 12-18 inches through the thin layer of spongy humus and tree roots that lies above the mineral soil. The roots rarely leave this well-aerated, organic zone for the oxygen-poor soils below. In fact, Pink Lady's-slippers (unlike most of their cousins) are very drought-tolerant plants. Even in a dry year, like the summer of 1995, their leaves will persist well into the fall.
When Cypripedium acaule is dug from the wild, many of its long, shallow roots may be cut or broken, stopping their growth and allowing root rots to enter the wounds. Further, unless the root system is spread out horizontally and covered with nothing more than a thin layer of leaf mold, the roots will suffocate and die. With damaged roots, the plant loses much of its energy reserves, as well as its capacity to accumulate new reserves. As a result, the plant will grow progressively smaller and weaker over several seasons and eventually die.
A Special Relationship
The slow death of transplanted pink lady's-slippers has been commonly attributed to the absence of certain fungi the plants may depend on for survival; however there are still many unanswered questions about orchid fungi and their importance to mature plants. Orchid seeds are unique in that they have evolved a symbiotic (meaning "to live together") relationship with certain groups of soil fungi. The seeds are adapted to wind dispersal, which means they are very light in weight and produced in huge numbers. The typical Lady's-slipper seed pod contains between 10,000 and 20,000 seeds! Orchid seeds are light because they lack the endosperm or food reserves that most seeds rely on for initial growth, much as the egg yolk nurtures developing bird embryos. Without endosperm, orchid seeds cannot germinate unless they become "infected" by certain soil fungi, which the seedlings actually digest to obtain the sugars, hormones, and other nutrients necessary for growth. Once a seedling is old enough to have leaves and roots, it can begin providing these substances for itself, and gradually becomes less dependent on the fungus for survival. Research on mature yellow and showy lady's-slipper roots reveals very little fungus infection, so it appears that, for these species at least, the fungus becomes much less important as the plants grow older. It seems likely that root damage, not a lack of fungal partners, is primarily responsible for the decline of Pink Lady's-slippers under cultivation.
This summer we began an experiment along the lines of one described by Don Jacobs in The Rock Garden Quarterly, the bulletin of the North American Rock Garden Society. C. acaule plants rescued from a development site were carefully excavated-roots intact-and moved to several locations at Garden in the Woods. We receive many requests to rescue Pink Lady's-slippers from construction sites, but usually decline due to the expense and high failure rate. This time, however, volunteers actually removed the plants bare-rooted and carefully replanted them by laying them out on a bed of rotted leaves and lightly covering them with more leaves.
We are always hesitant to publicize such a project because we do not wish to encourage wild-collecting. For this reason, and because it will be five years before we will know for sure if this method was successful, we have avoided going into too much detail, but we will keep you posted. Once we are more confident that it will work, we may be more detailed about our methods. We hope that this knowledge will make long-term cultivation of pink lady's-slippers feasible when seed grown, nursery-raised plants start to become available in a few years.
When you hear the phrase "old growth forest," do you picture towering emerald cathedrals soaring hundreds of feet above a dimly lit forest floor? This type of forest is found in the Pacific Northwest, where winters are mild and wet and summers are dry and calm. Fires sweep through occasionally during the dry months, and trees can live 1000 years or more. Here in the Northeast, moisture is more evenly distributed. Winters are too cold to allow much growth, but summers are warm and wet. Fire is much less common, but hurricanes, ice storms, and tornadoes cause massive canopy disturbance every century or so and most trees live only 100-200 years.
For plants like the pink lady's-slipper (Cypripedium acaule), such a pattern provides many opportunities. Cypripedium acaule is a forest plant, with large, flat, deep green leaves that are perfect for collecting the dim light that penetrates the canopy. Though this lowslung growth habit makes it a poor competitor against dense shrubs, grasses and forbs, that is not a problem in the uplands where it typically grows. Here shade and dry soils limit undergrowth to sparse shrubs like blueberries and huckleberries.
The amount of light the plant can collect and convert into energy determines how fast it will reach maturity and bloom. If the tree canopy is light and broken, the lady's-slipper will accumulate reserves that it can squander on flowering and seed production. If the canopy is more dense, the plant may accumulate only enough energy to maintain itself through the dormant months. If the canopy becomes too dense, the plants will lose ground from year to year and eventually die. In this way, lady's-slippers are dependent on occasional disturbances like the severe ice storm that breaks up the canopy but does not destroy it. This is important, as the plants cannot compete with sun-loving grasses and shrubs; too much disturbance, such as clearing for agriculture or construction, will eliminate them. After a disturbance, lady's-slipper reproduction rates climb and the next generation can become well established enough to persist for the next 50-100 year cycle.
Pink lady's-slippers can live more than 100 years, but may flower only 10-20 times during that period and set seed a mere 2-5 times - an extremely low reproductive rate. It's as if these plants mark the passage of time in decades rather than years, waiting for the right combination of factors to allow reproduction to proceed.
Many people have remarked that lady's-slippers were once common on their land but, over the years, have dwindled or ceased to bloom. This is probably because the forest has become older and shadier. If the canopy is thinned, the plants often make a speedy recovery.
Bees Bumble In
Research by Douglas E. Gill of the University of Maryland has further clarified Cypripedium acaule's requirement for occasional canopy disturbance. The strange shape of the flower has evolved as a way of assuring cross-pollination (thus preventing inbreeding). For reasons that are unclear, queen bumblebees who have just emerged from their winter dens are attracted to the pouch of the flower and crawl in through the opening. The flared opening is designed like that of a lobster trap - easy to crawl into but hard to escape. In order to leave, the bee must crawl up the back of the pouch and squeeze first by the sticky female stigma and then under one of two globs of pollen (pollenia), which sticks to its back, just in front of the wings. If the bee then visits another flower, it will brush this pollen onto that flower's sticky stigma before picking up a new glob of pollen.
This elegant system has one major flaw, however: there is no nectar reward to lure the bee to new flowers, and the placement of the pollen on its back prevents the bee from harvesting it. After as few as one or two visits, the bee learns to avoid the deception and looks elsewhere for nectar. Thus the orchids must have a large population of naive bumblebees to insure good pollination rates (most other nectar feeding insects, including honeybees, are too small to fit through the opening tightly enough to pick up the pollen). This is most likely to happen in the years following a canopy disturbance, when the increased sunlight both attracts bees and allows nectar-rich companions like lowbush blueberry to flower heavily. Fortunately, these same conditions trigger the heaviest orchid flowering, and in these years pollination rates can soar to 30 percent. The resulting seed will also find conditions optimal for germination and rapid growth. Gill has found that, in his study populations, mature plants may lie dormant for many years until triggered (possibly by the warmth of sun reaching the ground) to emerge, bloom and set seed.
A Subtle Symbiosis
Orchid seeds cannot germinate unless they become infected by certain soil fungi, which the seedlings actually digest to obtain the sugars, hormones, and nutrients necessary for growth. Orchid mychorrizae, as these symbiotic fungi are called, are grouped mostly into the genus Rhizoctonia. It does not appear, as was once thought, that every species of orchid has evolved with a specific fungus, but rather that an orchid can rely on at least a couple of different fungi interchangeably. In fact, research has shown that a species of fungus isolated from a tree-dwelling tropical orchid is capable of successful symbiosis with the completely unrelated temperate orchid, Goodyera repens, our Rattlesnake Plantain. Some species of fungus have worldwide distribution, while others are limited to local areas. What I find very interesting is that at least some species of Rhizoctonia are pathogenic, or disease-causing, when they infect non-orchidaceous species. It may be that in the distant past these fungi were parasites on orchid seeds, but eventually the seeds developed ways to resist and control this parasitism to their own benefit. In effect, the seeds have reversed roles, now becoming parasites of the fungus. It is not clear what benefit the fungi get from this relationship, although they may receive certain enzymes or nutrients from the orchid. Rhizoctonia species are part of the soil flora, and most, if not all, are able to grow readily as saprophytes (living by breaking down non-living organic matter, such as rotting wood). Thus you should normally find at least a few species in any soil that contains decomposing organic matter - whether orchids are present or not. Some species seem to be generalists, capable of growing in a variety of soils and organic materials, while others are specific to certain habitats.
Lady's-slippers in particular have been associated with at least six species of Rhizoctonia. These fungi are present in soils where the orchids grow, and are thus available to infect and aid in the germination of seedlings. Cypripedium seedlings have also been germinated in a lab using one of the most common and widespread species of orchid fungi, but it is not clear whether this particular species aids germination in the wild. Thus, even though orchids may not be growing on a particular site, suitable fungi are probably already present to allow germination if seeds are introduced.
Mature plants (those at or near flowering size) of some, if not all, of the species do not seem to be dependent on mychorrizae to any great degree. In fact, even small propagated seedlings that we have received in sterile bags (i.e growing without mychorrizae) grow on very well in a sterile, hydroponic mix. It is clear that once these seedlings have passed out of the critical germination stage, they can grow well without mychorrizae under cultivated conditions.
Better Left Wild
I hope this brief article has shed some light on the complex and highly developed lifecycle of the pink lady's-slipper and the specialized conditions that allow them to thrive. Some wild things are better left wild, and these striking natives reward our respect with beauty and an intriguing touch of mystery that is a rarity in itself. Please enjoy them growing, as they are so well-adapted to do, in the wild places of New England where others can enjoy them as well.
- Douglas E. Gill, "The Natural Population Ecology of Temperate Terrestrials: Pink
Lady's-Slippers, Cypripedium acaule," in North American Native Terrestrial Orchids, Propagation and Production, Conference Proceedings, March 1996, edited by Carol Allen, published by The North American Terrestrial Native Orchid Conference, Germantown, MD, 1996. Don L. Jacobs, "M'Lady's Slippers:
- Transplanting the 'Impossible'," Rock Garden Quarterly, Vol. 54, No.2, Spring 1996,