Once upon a time, long ago in a place called Lewisburg, Pennsylvania, I was a graduate student studying plant physiology. One night, in the wee hours of the morning, after checking a growth chamber on my latest experiment, I wandered into the department’s completely overgrown greenhouse. In a state of semistupor, I happened to notice a group of plants. I do not know if someone had arranged them deliberately, if I was so tired that I imagined it or if their placement was serendipity. From a central plant, if I looked along a line in one direction, the leaf shape seemed to change slightly from one plant to the next in a definite progression. In another direction, the leaves of the center plant were entirely green, and each succeeding plant had a little more variegation than the previous. In another direction, it was how hairy the stems and leaves were. Every direction away from that central plant seemed to have a progression of some characteristic, and there seemed to be links between each of these progressions. This seemed to produce a web of morphological changes away from the central plant and in circles around it. Ah ha, I thought. Here is evolution and ecology and anatomy and probably physiology … all wrapped up in one place — a botanical web of everything. Did I mention I was exhausted to the point of near befuddlement?
But, as soon as I tried to organize what I saw and put it into words, it dissolved. It all fell apart, and I had the epiphany that it was impossible for me to fully grasp the complexity of it all. Plant life was absolutely too convoluted to be comprehensively understood all at once, at least by me. Since then, I have more or less lived by the motto “I believe in disbelief.” I compulsively research, read about, analyze and evaluate everything. I often find myself arriving at conclusions that disagree with conventional wisdom or at least offer a different perspective.
One such area is advice about the fertilization of epiphytic orchids. Conventional wisdom is that you should water thoroughly, then fertilize, and then water thoroughly again. It is also widely advised to spray the foliage with fertilizer. Another well-ingrained bit of advice is to never water toward evening or ever let any water remain on the plants after dark, especially Phalaenopsis. It is often said that you should grow in clear plastic pots so that the roots can photosynthesize. Finally, there is the rule of thumb that the leaves should be very light green. If they are not, you must provide a higher light intensity. Let me explain my somewhat unorthodox reasoning on these bits of advice.
First, let us take a look at fertilization. To do that, I have to start with the properties of orchid roots. First is the anatomy (Fig. 1). The root is essentially circular in cross-section. Though epiphytic orchid roots growing against a substrate may have roots that are somewhat flattened, they have pretty much the same anatomical organization. To simplify, just inside the epidermis, the next outermost layer is velamen. It is a thick layer composed of dead cells that function to absorb water and nutrients, limit evaporation, and provide mechanical protection to the living part of the root (Siegel, 2015).
Inside the velamen are a number of layers; most of the cells here have thick walls and do not pass water or nutrients. Also in these layers are thin-walled passage cells that allow water and nutrients to pass from the velamen, through the layers, to the innermost cells, the cortex and stele. Some of the vascular cells are responsible for water and nutrient transport to the upper parts of the plant. Others are responsible for transporting food made during photosynthesis to the roots and throughout the plant.
This is an extremely simplistic description of an epiphytic root. There are actually five or more layers, each composed of a number of different types of cells and in growing roots a mass of embryonic (meristematic) cells at the end. But the crux is that there is an outer layer of velamen that acts like a sponge to capture water and nutrients. Inside that are layers that regulate the passage of water and nutrients to the inner root, which transports water, nutrients and food throughout the rest of the orchid plant.
Fossils of orchid flowers have been dated to 45–55 million years ago (Poinar & Rasmussen, 2017) and even much farther back to 80 million years ago (Bradt, 2007). Even before that, they must have been evolving the sophisticated functions we see in epiphytic orchid roots today. Let us take a look at what happens when today’s dry orchid roots in nature are exposed to rain.
The very first raindrops collect dust and nutrients on the surface of whatever the orchid is attached to and growing on. As the first of this rainwater runs down the surfaces, it contains very dilute concentrations of these available nutrients. The very first of the water that reaches the orchid roots contains the most nutrients. Orchids have evolved to capture this initial flush. Research (Zotz & Winkler, 2013) shows that the epiphytic orchid roots tested became nearly saturated within 15 seconds and were fully saturated at one minute. That means they can grab this initial flush of very dilute nutrients extremely efficiently.
As rain continues, it quickly contains less and less nutrients. The concentration of nutrients falls off very quickly, and soon the water reaching the roots is pure rainwater. Because the velamen is saturated and has already captured the initial flush of nutrients, this is of little to no consequence. The rain stops, the roots dry and the velamen prevents the captured water from evaporating and releases it with the nutrients to the interior of the root to be transported and used.
How does this relate to watering and fertilizing our captive orchids? The conventional wisdom has us saturating the velamen with clear water, then flushing with fertilizer after the velamen is completely saturated and finally washing away almost all that fertilizer with more clear water. Nothing at all like what happens in nature.
To be sure, there is likely some small residue of fertilizer retained by the medium when fertilization is done this way. The next time the dry roots are flushed with plain water, this will probably provide the very dilute concentration the orchid roots have evolved to deal with. That does, more or less, approximate the natural course of events.
[1] Anatomy of an epiphytic orchid root.
But, and it is a big but, even though it might nearly accomplish something similar, it seems to me to be at least moderately imprecise and a rather large waste of fertilizer. In the scheme of things, the amount of fertilizer flushed down the drain into the environment is probably nominal. Still, with things as they are, the less excess nutrients flushed into the environment, the better.
I try to mimic natural conditions by fertilizing with an extremely dilute concentration of fertilizer onto dry roots. I grow all my plants exclusively in fern root (which I grow and harvest, but that is
another story). The fern root breaks down extremely slowly and probably provides some nutrients directly. Because of that, I fertilize every watering at a concentration of ¼ teaspoon (1.25 ml) of 15–30–15 per 1.75 gallon (6.6 l) — slightly more than 1/8 teaspoon per gallon (~0.165 ml/l), and I do not follow with a flush of clear water. Some of this very dilute fertilizer is absorbed into the fern root (more, I think, than in hard types of media — bark, for example). The rest is available to the roots until they are saturated. Watering when blooming and the plants are on display away from the growing area, is done only with clear water, which flushes the medium so there is not a buildup of salts.
My estimation of growth and flowering using this procedure is that both are quite good. For example, I can rely on my standard-size cattleyas to bloom twice a year. Most of my plants flower more than once a year. Roots become thicker than when first purchased (Fig. 2), and growths of mature plants become larger under my conditions than when first acquired. For example, a mature Cattlianthe Jupiter Drops “Another Tequila Sunrise” had a pseudobulb length that had leveled out at 5½ inches (14 cm) when purchased. Under my conditions, the pseudobulb length consistently increased to 7½ inches (19 cm). By those metrics, I think I am probably not going too far wrong. To be fair, I attribute some of my success to growing in fern root. But again, that is another story.
Next up is the popular idea that fertilizing the foliage is effective. The leaves and pseudobulbs of epiphytic orchids are covered by a thick, waxy cuticle. This prevents air exchange and, thus, water loss during the day when the stomata on the underside of the leaf are closed. (De & Biswas, 2022). Because the cuticle prevents air and, thus, water from leaving (or entering) the leaf, it is almost certain that larger molecules (fertilizer) cannot pass through this barrier.
[2] Roots at purchase and after the author’s fertilization technique.
So what is the deal with fertilizing the foliage? It is virtually impossible to fertilize the foliage without some fertilizer reaching the medium. Also, fertilizer left on the foliage will rinse into the media when the plant is misted or otherwise rinsed or watered. Almost certainly, none of the fertilizer is absorbed directly by the leaves and instead drains into the medium and is absorbed by the roots.
Does fertilizing the foliage work? Maybe. But it is unlikely that it is due to the plant absorbing fertilizer through the leaves. The cuticle prevents that. Much more likely, it is the random amount of fertilizer that ends up in the medium that is the actual source of fertilization. I prefer to fertilize the roots at a specific concentration and rate rather than to take chances with random overspray and whatever fertilizer washes into the medium off the foliage.
OK, now to the conventional wisdom that plants must be absolutely dry by evening. I have done a fair bit of travel into tropical climates, mostly to SCUBA dive and to encounter different terrestrial environments. In my experience, rain falls in the tropics at any time of the day or night. Is there any reason to think that orchids have not evolved to deal with this? Because they suffer no ill effects in nature from being wet at night, what might be going on?
Epiphytic orchids have evolved in a parallel way to plants that have to deal with a very dry environment: cacti and desert succulents (Zhang et. al. 2016) This may seem a contradiction, but consider that epiphytic orchids get drenched and then dry very quickly due to air movement in their natural environment. Their roots grow exposed to the air, and that is a very xeric condition. They have evolved to conserve water that they store in pseudobulbs and thick fleshy leaves. They open their stomata at night when humidity is high and absorb CO2. Because there is no light at that time, they store the CO2 in organic acids. During the day, they break down the organic acids to produce CO2 and use light to produce sugars (food). At this time, the stomata are closed. Note that this is a simplification of crassulacean acid metabolism (CAM), and all epiphytic orchids may not be obligate CAM plants (Tay et. al. 2019; Rodrigues et. al. 2013).
What I glean from this is that, in nature, orchids can be wet at any time, and they have adapted to that. However, they experience a fair amount of air movement and grow fairly well separated. So I think the problem is not when the plant is wet or dry but, much more importantly, how much air movement it experiences while wet. In my experience, stagnant air and overcrowding are the problem, not that plants remain wet after dark. Plants jammed together severely limit the movement of air around the plants. This is especially true when strong air movement is not provided. I believe stagnant air around the plant causes the problems attributed to having the foliage wet after dark.
Now, the argument has been made that plants such as Phalaenopsis grow with their crown facing down in nature. My observations are that not all plants do. Moreover, wind during tropical rains surely blows copious moisture into the crown whatever its orientation. Additionally, the phalaenopsis in my collection have their crowns facing upright. They collect moisture in the crown whenever I mist them.
I routinely mist my plants thoroughly after dark to keep the humidity high while their stomata are open: what they would experience in nature. I also provide continuous, strong air movement — again, the natural condition. Cattleyas, cattlianthes, rhyncholaeliocattleyas, phalaenopsis, dendrobiums and vandachostylis all get heavily misted after dark and seem to love it. But they are not crowded and dry quickly with the air movement the fans provide. My observation is that these plants do not experience any ill effects due to this treatment. Quite the contrary, the health of the plants and the number and health of aerial roots tell me that I am likely doing something right.
One more is the idea that we should grow in clear plastic pots so that the roots can photosynthesize. Sorry. The amount of photosynthesis done by the roots is minuscule compared with that done by the leaves and pseudobulbs. The roots contribute almost nothing with respect to food production, and plastic pots can be problematic. Air exchange is prevented. They can keep the medium from drying out quickly enough, and that can cause problems. Also, sunlight passing through the clear plastic can heat the roots beyond what may be optimal. Clear pots may also allow the growth of algae or moss on the roots. These can interfere with fertilizer uptake by competing with the roots. True, you can keep an eye on the root growth in clear plastic, but I find growth in my collection, under my conditions, is far superior in squat clay pots.
Last but not least is the conventional wisdom that leaves should be light green in color. If not, then they are not getting enough light, and light intensity should be increased. Well … sometimes. Some plants have pigments called flavonoids (carotenoids, anthocyanins and betacyanins) in addition to chlorophyll. These are involved in photosynthesis and photoprotection. When light is too strong, chlorophyll will begin to bleach out, and the flavonoids will increase in concentration to protect the leaves from excessive light intensity.
What is the result? Too much light in some plants will cause the leaves to become darker. Following the rule of thumb, light will be increased, resulting in the leaves becoming even darker. Then light is again increased, and the leaves become still darker. Eventually, the light is increased to a level at which the plant is killed.
I killed a Vandaenopsis Irene Dobkin just this way. It is important to evaluate if the plant in question lacks flavonoids in quantity and should have a light green color or if it has an abundance of flavonoids and pushing for a light green leaf color is detrimental. Plants with a lot of flavonoids show a reddish tinge along leaf edges and sometimes a reddish blush through the leaf or on the bottom side. It can also have red spots over the leaf. The reddish tinge shows that the plant is getting plenty of light. It would be unwise to increase light in an attempt to get the leaves to become light green.
In summary, conventional wisdom might be a place to start. But it is probably wise to do some thinking of our own along the lines of what the plants experience in nature and how that fits with and can be reproduced, or at least approximated, in our own conditions. We need to do some reading about the anatomy, ecology, evolution and physiology of the type orchids we grow and see what that tells us.
Conventional wisdom mostly works. Of course it does or else it would soon be discarded. But, heretical as it might sound, conventional wisdom does not always work for the reason you think, nor does it always work the best. Sometimes following it can even surprise you with unintended consequences. It is essential to evaluate our unique conditions and make necessary adjustments based on what we can learn about the botany (anatomy, ecology, physiology, etc.) of the type orchids we are growing. No two growing environments are identical. What works in some might not be optimal or might even be detrimental in others.
— Larry Litwin received a BA in natural sciences and math from Edinboro University in 1969 with a major in biology. He received his MA in biology from Bucknell University in 1972 with a concentration in plant physiology. He retired from the New York State Department of Health in 2007 after 30-plus years, where he worked as a software engineer and applications analyst. He has been growing orchids since 1970, and though his employment was outside the field of botany, he never stopped investigating and experimenting. His primary interests are the physiology of epiphytic orchids as it relates to their culture and the tropisms exhibited by orchids, particularly in the inflorescence.
References
Bradt, S. 2007. First Orchid Fossil puts Showy Blooms at Some 80 Million Years Old. https://news.harvard. edu/gazette/story/2007/08/first-orchid-fossil-puts-showy-blooms-at-some-80-million-years-old/. Accessed July 24, 2023
De, L.C. and S.S. Biswas. 2022. Adaptational Mechanisms of Epiphytic Orchids: A Review. International Journal of Bio-resource and Stress Management 13(11):1312–1322.
Luttge, U. 1989. Vascular Plants as Epiphytes: Evolution and Ecophysiology. Edition 1, XII, Springer-Verlag, Berlin, Germany.
Poinar, G. and F.N. Rassussen. 2017. Orchids from the Past, With a New Species in Baltic Amber. Botanical Journal of the Linnean Society 183(3):327. Rodrigues, M.A., A. Matiz, A.B. Cruz, A.T. Matsumura, C.A. Takahashi, L. Hamachi, L.M. Félix, P. N. Pereira, S.R. Latansio-Aidar, M.P.M. Aidar, D. Demarco, L. Freschi, H. Mercier and G.B. Kerbauy. 2013. Spatial Patterns of Photosynthesis in Thin- and Thick-Leaved Epiphytic Orchids: Unraveling C3–CAM Plasticity in an Organ-Compartmented Way. Annals of Botany 112(1):17–29.
Siegel, C. 2015. The Secret Life of Orchid Roots. Orchid Digest 2015:28-41.
Tay, S., J. He and T.W. Yam. 2019. CAM Plasticity in Epiphytic Tropical Orchid Species Responding to Environmental Stress. Springer Open. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6513927/.
Zhang, L., F. Chen, G.-Q. Zhang, Y.-Q. Zhang, S. Niu, J.-S. Xiong, Z. Lin, Z.-M. (Max) Cheng and Z.-J. Liu 2016. Origin and Mechanism of Crassulacean Acid Metabolism in Orchids as Implied by Comparative Transcriptomics and Genomics of the Carbon Fixation Pathway. Wiley Online Library: The Plant Journal. https://onlinelibrary. wiley.com/doi/full/10.1111/tpj.13159. Accessed July 24, 2023. Zhang, S., Y. Yang, J. Li, J. Qin, W. Zhang, W. Huang and H. Hu. 2018. Physiological Diversity of Orchids. 2018. Plant Diversity 40(4):196–208.
Zotz G. and U. Winkler. 2013. Aerial Roots of Epiphytic Orchids: The Velamen Radicum and its Role in Water and Nutrient Uptake. Oecologia 2013;171:733–741
