Pink Lady’s-slipper orchids: a species study — 2020-04-22

Recently, Walter Ezell and I visited a local heritage preserve in upstate, Greenville County, South Carolina. I had been seeing evidence on FaceBook of Cypripedium acaule or Pink Lady’s-slipper orchids in flower. Although a bit early for them in our region, I thought it would be prudent to check on our local ones, especially since many of our Spring ephemerals had shown early blooming.

On the trip up to the preserve, I began to toy with the idea of dedicating an entire blog report to this wonderful, mostly East Coast species. Here is a range map of the species, compliments of Biota of North America Program’s (BONAP) North American Plant Atlas. This map is somewhat out-of-date as all range maps are, due to lack of current information. For instance, I know of at least two additional counties in South Carolina where this species is found which are not indicated on the map. I have also photographed it in several Canadian provinces as well, but BONAP includes only entire province data (dark green color) for Canada in its range maps:

Pink Lady’s-slipper orchid distribution range

I figure that most of you will know, already, what this species looks like in bloom, but just in case it’s new to you, here is what a typical Pink Lady’s-slipper orchid (PLS) looks like:

Pink Lady’s-slipper orchid

BTW, for this blog report, I’m mixing recent images with images that I’ve taken over the years while looking for photographic opportunities with this species

Disclaimer: This is in no way intended to be a rigorous academic discussion of the species. I am a self-taught naturalist and photographer, not a trained botanist (and I don’t even play one on TV), so please keep this in mind as you peruse the pictures and discussion. What I am presenting to you are my observations and some online research. Having said that, if you find inaccuracies, PLEASE point them out in the comment section, below, so that I might correct them for the other readers.

SPECIES DESCRIPTION

OK, let’s look at some of the descriptive characteristics of the PLS. From eFloras.org‘s Flora of North America, here is the text portion of their key entry:

5. Cypripedium acaule Aiton, Hort. Kew. 3: 303. 1789.
Small pink lady’s-slipper, pink moccasin-flower, stemless lady’s-slipper, cypripède acaule

Fissipes acaulis (Aiton) Small

Plants erect, 15–61 cm; stems leafless scapes. Leaves 2, radical, arising directly from rhizome, ascending to spreading; blade broadly elliptic to oblong-, ovate-, or obovate-elliptic, 9–30 × 2.5–15 cm. Flowers solitary; sepals reddish brown to green; dorsal sepal lanceolate to lance-ovate or elliptic, 19–52 × 5–22 mm; lateral sepals connate, synsepal 17–49 × 6–25 mm; petals deflexed to somewhat spreading, somewhat spirally twisted, same color as sepals, linear- to ovate-lanceolate, 24–60 × 4–17 mm; lip magenta to white, obovoid to oblance-ovoid, 30–67 mm, orifice a longitudinal fissure, length of lip; staminode quadrangular to suborbicular-rhomboid or ovoid-deltoid. 2n = 20.

Flowering Apr–Jul. Dry to wet forests, bogs, brushy barrens, heath, and roadsides on highly acidic soil; mostly 0–1200 m; Alta., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask.; Ala., Conn., Del., D.C., Ga., Ill., Ind., Ky., Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., S.C., Tenn., Vt., Va., W.Va., Wis.

This is eFlora’s distribution map for Cypripedium acaule:

As seen in the above description, one of the facts about this species is its preference for “highly acidic soils”. In our neck of the woods, it is often found growing under Pinus strobus or Eastern White pine, whose needles produce very acidic soils. Incidentally, the botanical epithet, “strobus” comes from the Greek word, strobos, meaning “whirling around”, referring to the whirled aspect of the tree limbs. Here is a shot of the trunks of White pines in a nearby pine grove:

This preference of acidic soils, along with the presence of very thin, shallow roots, makes it very difficult to transplant it from its natural setting to a domestic or garden setting. The plant may live for a year or so, existing on stored energy in the roots, but the chances of long-term survival are quite small. In any case, most states list all orchid species as protected, and it’s not only illegal to dig them, but also disrespectful for others who might wish to see them in their natural habitat. But, I highly suspect that I’m preaching to the crowd, here. ‘Nuf said about that…

POLLINATION MECHANISM

As with most of our North American native Slipper orchids, the pollination mechanism that PLS employs is quite interesting:

I’ve not been able to discern a fragrance emitted by the flower, but some have said that it has a light, floral or sweet fragrance. Apparently, this is one attractant for its main pollinator, a Bombus or Bumblebee species. A few years ago, I managed to photograph a sequence of images which shows this pollination mechanism in action:

1. Note the upper red circle representing the pollinium or pollen sac containing consolidated, waxy pollen grains. This is one of a pair of pollinia — the remaining one is in a similar position on the other side of the flower. The lower red ellipse represents the slit or opening in the orchid’s lip where the pollinator enters the flower. This pollinator is considered a “naïve pollinator” because it receives no reward for its considerable efforts. It soon learns this and will eventually learn to avoid this orchid species and choose a different flower species as a source for collecting nectar or pollen.

The pollinator has already entered the flower’s lip, at this point, and has discovered that there is no reward and is looking for a way out. I only noticed this pollination was in progress, because I heard a “buzzing” and saw the flower’s lip shaking back and forth vigorously.

2. The pollinator has located the openings in the upper portion of the lip, and it has used the upward-pointing hairs on the inside of the lip to crawl up toward the light. At the end point in this process, the pollinator is forced to brush its back against the large pollinium, removing its thin cover, thereby allowing the waxy pollen grains to adhere to the hairs on its back. Here is a close-up of the exit hole with exposed pollinia:

3., 4., 5., and 6. The pollinator struggles mightily to extricate itself from the relatively small hole in the orchid’s lip.

7. The pollinator has just flown off, probably with a negative memory of this experience. If it had previously been to another PLS, it would have left pollen grains attached to the female stigma just below the upper opening it used to escape. The passage from the inside of the lip to one of the upper opening holes is so narrow, that the pollinator’s back is forced against the stigma, hopefully depositing any pollen it acquired from a previous visit to another orchid flower. The pollination rate for PLS is relatively low because the pollinators learn very quickly that there is no point in visiting this species again. For pollination to occur, a single pollinator must be tricked into entering and correctly exiting at least two flowers.

As an aside, one might ask if there are any natural hybrids with PLS and other orchid species. I know of none, at least natural ones. Apparently there is some sort of DNA barrier that prevents this from happening. I do know of a nearby location were both PLS and Cypripedium parviflorum var. pubescens or Large Yellow Lady’s-slipper orchids (YLS) are in bloom at the same time within just feet (meters) of each other. They employ the same pollination mechanism. If hybridizing between these two slipper orchids could occur, I’m sure there would be evidence of it at this location. Perhaps lack of hybrids is due to having differing pollinators. I have read that YLS can have up to ten species of hymenopteran visitors in the families Andrenidae, Apidae, Halictidae and Megachilidae act as pollinators. No report I’ve see records Bombus (also in the family Apidae) species pollinating YLS.

FLOWER COLOR FORMS

This species exhibits quite a wide variability in color form from very light pink to a deep, almost maroon color and, rarely (at least in the southernmost extent of its range) white. I will attempt to identify the location for the following color form images.

Typical pink to rose colors :

Upstate South Carolina:

Typical pink to rose colors with white reticulation in the center opening of the lip:

Upstate South Carolina:

Deep rose to maroon colors:

Manitoba, Canada:

Newfoundland, Canada:

Light pink colors:

South Carolina:

A couple of notes about the above images are in order. Due to fact that PLS only get color as the flowers mature, those flowers which are just maturing may seem to be very light in color — even white, so don’t be confused with the light pink color on flowers just opening. Also, note the Tetracis cachexiata or White Slant-line moth on the left image. It has been reported on PLS orchid lips by several observers in the eastern states. I have seen this on three instances in South Carolina. This moth is not a pollinator, but the reason for its presence on the lip opening of PLS is little known — an interesting puzzle…

White forms:

North Carolina:

Note that the petals on the white form show no sign of red or brown — they are green. Many flowers which appear white are actually a very light pink, and these are all distinguished by petals with a color other than green.

New Hampshire:

Ground zero for the white form seems to be in New England. In 2015, the Native Orchid Conference held its annual symposium in New Hampshire, and one of our field trips was to an area where the white form of PLS predominated! It was so unusual for this blogger to see so many white forms when it was so rare in the Southeast. Here is a selection of these beauties:

Also on this field trip, we found a curious color form of light pink with light petals:

As you have seen, this orchid species is capable of forming rather large groups of blooming plants. Here are a few groups that I photographed in several different areas in the East Coast states:

LIFE CYCLE

The life cycle begins with the plant beginning to show itself at the beginning of Spring. I photographed the following image the first week in April, about 4 weeks before the plants would be in flower:

Note how hairy this plant is. All parts of the plant, including the flowers are covered with tiny hairs. It has been reported that the plants of this orchid species can cause severe dermatitis in some individuals, as do the other Cypripedium species, upon contact with the glandular hairs on the leaves and stem. I have not noticed such a reaction, myself. I have heard that another Cypripedium species, Cypripedium reginae or Showy Lady’s-slipper orchid is more commonly associated with causing this reaction.

As the plant continues to grow, the leaves become more noticeable:

Over the next week to 10 days, those plants that will be producing the single flower stem for the season, will begin showing this green stem and flower bud:

As the days progress, the stem lengthens, and the flower begins to take on some color, although appearing mostly white:

SEED PRODUCTION

As was mentioned earlier, many PLS populations have very little successful pollination events. In a good year, maybe 50%-60% of the flowers will have produced seed capsules. My experience in the Carolinas shows seed capsule production of 10%-20%.

The seed capsule is actually the ovary of the flower. Here are some images of different stages of the seed capsule I have photographed:

This first image was taken about a week to 10 days after the flower opened and had been successfully pollinated. Although withered, the flower can still be identified:

By late fall or early winter, the seed capsule will dehisce and lose its seeds. Here is a PLS with last year’s seed capsule still standing next to this year’s flower:

I have read that it takes 7 years for a PLS orchid seed to grow and mature enough to produce a flower. I can state from my experience that they are fairly long lived. I have photographed a single PLS orchid plant for at least 10 years, and I suspect that under prime conditions, they may live even longer. But orchids are like the “canary in the coal mine”, meaning that they are an indicator of the health of their environment. I’ve noticed lately that many of the larger populations I’ve photographed over the years have been in decline. This is not due to man-made causes such as hydrology changes, timbering, trampling, or other causes. My fear is that it is due to the warming effect of climate change. After looking at the range distribution map, it is evident that the orchid populations in our southern region are at the southern extent of their range. Currently, they are healthy, but I’m not sure how long this will last.

SIMILAR SPECIES

Although there are about 60 species of Cypripedium that are widespread throughout boreal, temperate, and tropical regions of the European, Asian, and North American continents, more than 30 species are distributed in the Northern Hemisphere. 12 Cypripedium species (more or less) occur in the US.

Here is a sample of those that you are likely to see if you search carefully. Keep in mind that each of these uses the same pollination mechanism that was described above:

(Left to right: Cypripedium parviflorum var. pubescens or Large Yellow Lady’s-slipper orchid; Cypripedium parviflorum var. parviflorum or Small Yellow Lady’s-slipper orchid)

(Above Cypripedium parviflorum var. makasin or Northern Small Yellow Lady’s-slipper orchid)

(Above left and right: Cypripedium reginae or Showy Lady’s-slipper orchid)

(Above: Cypripedium reginae or Showy Lady’s-slipper orchid)

(Above left and right: Cypripedium californicum or California Lady’s-slipper orchid)

(Above left and right: Cypripedium candidum or Small White Lady’s-slipper orchid)

(Above top and bottom: Cypripedium fasciculatum or Clustered Lady’s-slipper orchid)

(Above left and right: Cypripedium montanum or Mountain Lady’s-slipper orchid)

(Above Cypripedium passerinum or Sparrow’s-egg Lady’s-slipper orchid)

(Above left and right: Cypripedium passerinum or Sparrow’s-egg Lady’s-slipper orchid)

(Above left and right: Cypripedium arietinum or Ram’s-head Lady’s-slipper orchid)

I’ve just shown you 10 different Cypripedium species (not including varieties), and I’m sure I left out the one(s) you like the most. There are a couple of them in Alaska which I have not had the fortune to photograph — yet. One of these days, I’ll empty my bucket list.

I hope you have gotten something out of this post. It was fun going through my archives to pull some of these images. The color rendition and focus are not so great in some instances, but a number of these images are more than a decade old, and my image editing skills have changed over time.

Who knows when I’ll have another chance to get out to photograph anything. I just heard that the Blue Ridge Parkway in North Carolina is closed to traffic. That is one of my favorite places to botanize, so I hope by fall that those parts will be reopened. I do have one more set of images from a visit Walter Ezell and I made a week or so ago, so maybe I’ll post it next week.

Until then, be safe, and I wish you and your families all the best…

–Jim