What’s the “right light”?
Optimal light intensity for different genera and species of orchids has been an ongoing question and quest of our orchid culture. Over the years, I have continually experimented and refined the placement of orchids indoors under LED grow lights, and outdoors in several growing areas.
We have not been able to build a greenhouse in open space, with straightforward light modifications from shade cloth and/or summer white-washing. Instead, we have several smaller growing areas in a semi-circle around our house. One is directly attached to the house. Another is along a pathway. A group of Cymbidium live on covered benches beneath a deciduous tree. Plus there’s our original small cold-frame outfitted as a shade house.
Indoors, we have three growing areas: two rooms outfitted with LED grow lights (warm and cool), and an intermediate area in front of six glass doors facing from east to southeast.
While more complicated in many ways, this arrangement has produced two very beneficial results. First, we are able to grow a diversity of orchids in our “microclimates.” Second, I have become a much more observant, skilled, and, overall better orchid grower by studying the effects of light intensity in different situations. A challenge can bring out the best!
In this article, I would like to share reflections on finding the “right light” in our varied indoor and outdoor growing areas. Specifically, I offer insights on three aspects of creating optimal growing light conditions: 1) significance of location/latitude, 2) photoperiod duration, and 3) quality of light spectrum. These generally do not receive so much attention in orchid culture recommendations, and I have learned many nuances from trial and error. With our experience, my goal is to help you think about your own growing area (opportunities!) in new ways.
More light is better?
One of the recurrent themes of discussions (and problems) regarding growing orchids is concerns over inadequate light. In the beginning, I implicitly assumed more light was generally better — and necessary to ensure successful growth and flowering. Quite serious about proper light intensity, I procured a good quality light meter. I consistently used it to analyze our growing areas, as well as compare with other growers’ greenhouse light conditions.
I set my sights on the often-repeated cultural recommendations for different genera, e.g., 3000-6000 fc for Cymbidium, 500-1500 for Paphiopedilum, etc. I was always concerned that our orchids would not receive enough sunlight in our less-than-ideal outdoor locations, or too few hours of LED light indoors.
After several years of observations, my approach has swung around 180 degrees. Instead of being concerned with too little light, I have become much more watchful of too MUCH light for flowering and beautiful foliage. Consistently, I discovered that the common cultural recommendations were too high for our growing areas. For example, the Dendrobium jonesii were not happy at 3500 fc; 1500-2000fc was much better. Some Cymbidium species looked like they wanted to hide under the bench to escape 4000fc of light. The Phalaenopsis turned purple under fluorescent lights at 600fc. Nine hours/day of 400W LED light was yellowing even the highest light Dendrobium species.
With the exception of one Paphiopedilum rothchildianum hybrid, I cannot think of any orchid that wasn’t receiving MORE than enough light to grow and flower. Why were the standard recommendations generally too high for our orchids?
Location: latitude & light intensity
I mentioned these observations to a grower in southern Canada, and she provided a key insight into our history of light intensity results. Many cultural recommendations have originated in Florida. At her location in southern Canada, she blooms Cattleya at 2000fc with no difficulty: the oft-recommended 3000fc would “fry them” due to higher summer light exposure at her latitude (43 degrees N).
Even at our more southerly location in central California (38 degrees N), we are much higher latitude than Florida (25-30 degrees N). She provided an important piece of the puzzle regarding our consistent success with “lower” light levels: latitude is an often overlooked, but incredibly important consideration when determining a “right light” intensity.
Latitude contributes several factors to growing areas. First, the midday sun is higher in the sky at more southerly latitudes. This can create greater overhead light intensity, with increased risk of leaf burn outdoors. Depending upon your early-mid summer temperatures, you might need to add additional shade cloth to buffer the effect of more perpendicular sun angle, or move orchids during heat waves. I particularly notice the combined effect of sun angle and heat upon Vanda hanging high in the shade house.
Second, there is the changing position of the sun from its northerly position (summer solstice) to its southerly position (winter solstice). This shift is more extreme at higher latitudes. We need to add extra shade cloth to one growing area in September to reduce increased direct southerly light. The direction of sun entering your growing area during the peak growing season will also affect how you orient your greenhouse and/or benches.
Third, there is greater difference in seasonal day length with higher latitude; I will discuss the effect of photoperiod in the next section.
Indoor growing can also be affected by latitude. For our southeastern exposure, summer is the shadier time of year due to the northerly position of the sun. The “dark months” of May to August require supplemental LED lights to support growth. Further north, this shift would be even more pronounced, and especially impactful if you do not move your orchids outdoors for the summer. Even here, our regular summer nights (50-55F, 10-13C) are too cool for many orchids to have a summer vacation in the backyard. (For more information on the shelving setup in the photograph, please see my article on indoor/outdoor orchid benches.)
Duration + Light Intensity = Growth
The next aspect of creating optimal light intensity was revealed to me when I began to study horticultural LED lighting in more depth. There is a new way of calculating the light needed for a plant to grow, called the “daily light integral” (DLI). This calculation considers both the intensity AND the photoperiod for a given type of plant to receive an adequate number of photons for healthy growth. Light duration is very notably absent from most light level recommendations for orchid growing.
For the ubiquitous footcandle or lux recommendations, is it assumed that an ideal greenhouse receives eight hours of light a day? If you have partial day shade from a tree, or strong seasonal variability like coastal fog, then you quickly find the limitation of applying a static number to your light intensity expectations. It is quite easy to provide too much or too little light if you do not account for the combination of duration and intensity.
For example, we easily bloom high light Cymbidium with three hours of direct morning sun, heavy afternoon (deciduous tree) shade, and filtered late day light. Their leaves are bright green, usually notably lighter green than Cymbidium that I see displayed from local coastal greenhouses. The Cymbidium tolerate the high light in the morning when it is cool, and heavy afternoon shade protects them from our increasingly hot summer weather. The orchids are able to be more photosynthetically active in the cooler, moist morning before having to use physiological safeguards to protect themselves from afternoon heat and excessive moisture loss.
A similar approach applies to our shade house for Laelia and Dendrobium, but they receive 50% light for about five hours. More sun would not be tolerated, especially by the thinner-leaved species, so lower light for longer duration suits these orchids well.
Indoors, under LED grow lights, we take another approach. We grow a diversity of genera (e.g., Paphiopedilum, Dendrobium, Leptotes, Epidendrum, Sarcochilus, Vanda) at 7.5-9 hours/day of light. Rather than varying the photoperiod, we modulate the light intensity by 1) using a different number of grow light bulbs for different shelves, and 2) varying the plant distance from the light fixture.
For example, a shelf with Paphiopedilum uses two T5 HO LED grow light bulbs while the same size shelf for higher light Dendrobium utilizes four bulbs. In the larger growing space (4’x’4’x4′, 1.2m x 1.2m x 1.2m), orchids are positioned from 10″ (25cm) to 40″ (1m) from the LED light fixture. With this thoughtful placement, we accommodate a diversity of orchids from Mexican Laelia (high light) to Vanda falcata (lower light) in the same area.
Light spectrum
There is a cornerstone of light intensity that we take for granted: light spectrum. Obviously, the solar spectrum is a global constant, and not a variable for outdoor growing. But in the newly expanding world of indoor horticultural, it is a very serious consideration.
LED grow lights were a breakthrough from fluorescent lighting for their ability to offer a continuous light spectrum rather than simply isolated wavelengths along the visible spectrum. Higher quality LED grow lights are usually high CRI (color rendering index), which more closely mimics natural sunlight.
The most obvious aspect of high CRI grow lights is visual appearance: brighter, more vivid colors. Beyond the visual light spectrum, however, there is much more at the cutting edge of new technology. Plant growth is affected by non-visible light, such as UV and infrared, as well as far red wavelengths. Newer metrics are being applied, such as plant biologically active radiation (PBAR), to analyze and quantify the full range of wavelengths (240-800nm) that plants “see” and use for growth.
In our relatively small LED grow light setups, I have observed some interesting differences in orchid response to light spectra, possibly due to subtle photosynthetic optimization that has occurred in evolution for different habitats. Particularly, I have seen outstanding root growth, and strong flowering, for Paphiopedilum under a high CRI light spectrum rich in deep red wavelengths (“sun white” spectrum by Active Grow Horticultural Lighting). I speculate that this might be an adaption from shady, forest floor habitats where longer wavelength (redder) light has greater canopy penetration.
Similarly, we have had excellent growth and flowering with a lower CRI, but higher intensity, light spectrum (“bright white” spectrum by Active Grow) for higher light Dendrobium and Laelia species. These orchids, which naturally reside in higher canopy or open areas, would receive more short wavelengths compared to understory orchids.
The bottom (light) line: when growing under LED lights, more intensity or duration is not necessarily needed for optimal growth. The quality of light spectrum, and hence light useful for photosynthesis (PAR, or photosynthetically active radiation, 400-700nm) is fundamental to growing results.
If you are designing indoor orchid growing areas now or in the coming years, I recommend paying close attention to new developments in LED grow light spectra. For less energy usage (i.e., lower intensity/wattage, fewer hours operation) with an optimized light spectrum, you can realize better growth for your orchids.
Bringing it all together: orchid placement
The culmination of all of the factors described above — latitudinal/seasonal light variation, photoperiod, and light spectrum — is determining the optimal placement of an orchid in relationship to its light source (solar or synthetic). With a shift in any of these variables, the light intensity received by an orchid over its growing season will change.
Starting with a big picture understanding of light intensity in your growing area, you can next observe the creation of microclimates. For instance, shading from other orchids, height in the greenhouse or indoor light room, and even the reflective properties of materials outside the growing area (pavement, house siding, walls, etc.), can have a notable impact.
All of these influences contribute to a much more nuanced and complex picture of why some orchids thrive and some struggle in a given location. Are your orchids really receiving the “right light” intensity when you consider the many complex factors of their growing conditions during the year?
It is my hope that introducing these (new) considerations will help you go beyond general cultural recommendations to work with the specifics of your growing area to create optimal light environments. Like we did, you might even discover new places to grow orchids that you did not previously envision as adequate or ideal. We all need more growing space, after all …
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