Spectrum and Photoperiod: What is best for your houseplants and orchids?

The last questions I hope to help you answer have to do with spectrum and photoperiod. We will start with spectrum. Just like fixture types, there is a seemingly endless array of possible options, and as a layperson it’s exceptionally difficult to decide what is best for your plants.

Unfortunately, the study of growing orchids and houseplants under lights is still relatively new and not a lot is known about how subtle spectral changes affect orchid growth and in particular, flowering. The spectra of light emitted by LED diodes can be very carefully controlled, even down to 5nm, so how do you as a casual consumer decided what type of spectrum to buy?

The point I hope to make in this article is that a subtle spectral difference between two types of LED fixtures is probably not that important. Err on the side of more wavelengths (i.e. always buy “Full Spectrum” fixtures that are intended for use as horticultural grow lights) and stay away from gimmicky pink or blue fixtures. If the manufacturer doesn’t provide any spectral data on their product, it is unlikely to be as suitable as a fixture that does have this data.

Figure 1: Chlorophyll Absorption Coefficients (data from PhotoChemCAD database (Taniguchi & Masahiko, 2018))

Fairly often I come across a graph like Figure 1, touted as the absorption of light by chlorophyll at different wavelengths. The data in these graphs are collected from the chlorophyll molecule having been isolated and suspended in a solution. The Y axis represents an “absorption coefficient”, which is a parameter of the so-called Beer-Lambert law of light absorption. Absorption coefficient graphs like Figure 1 have been widely misused for LED marketing purposes, and they have led to the popularity of "blurple" lights. Blurple is a term used to describe LED lights with just red and blue diodes. These lights appear as a dark fuchsia color to our eyes, and render green foliage grown under them into an alien black color. To a lover of emerald foliage, these lamps can be particularly offensive.

Graphs like Figure 1 aren’t very helpful for understanding how light interacts with plants; the data is just too limited.  Absorption coefficients are hard to interpret on their own: to figure out how much light is actually absorbed; you also need to know the chlorophyll concentration and the length that light travels through the solution. While it may be true that chlorophyll on its own absorbs mostly red and blue light, inside a plant there are other light-absorbing molecules and other physical processes at work.

Figure 2: McCree, 1971 Leaf Absorptance

In contrast, when the light absorbed by a whole leaf is plotted, and not just the isolated chlorophyll like (Taniguchi & Masahiko 2018), we get a much different line (McCree, 1971). All wavelengths of light are absorbed, showing the importance of a full spectrum fixture for best plant growth.

Figure 3. Sun vs. Artificial Light Spectra

Figure 3 is a comparison between a few different light sources, assuming equal PPF (for more info on what PPF is, please read the first article of this series, An Introduction to PAR and PPFD). The red line in the plot is a blurple light; look how much it is missing compared to the full spectrum LEDs (blue and green lines). The yellow line is a full spectrum fluorescent bulb.

Another idea which is heavily marketed by lighting companies is that blue light influences plant growth, and red light is responsible for flowering. The effect of blue light vs. red light has been studied on food crops and cannabis, and evidence from these studies does show a significant effect of light color on plant growth (Brown and Schuerger, 1995). Plants grown under pure red light experienced etiolation and a smaller biomass, which was alleviated when blue light was added. Plants evolved to grow under full spectrum sunlight, and experiments have shown that plants grow much better under artificial sunlight spectra than limited spectra such as fluorescent or blurple (Hogewoning, 2010).

Full spectrum bulbs are widely available on the market today. As a home grower, you should pick a light that provides as many wavelengths as possible and makes your plants attractive to your eye, as some are tinted more pink, blue, or yellow. The subtle spectral difference between one bulb or another likely doesn’t matter for hobbyist applications, and more research needs to be done to provide evidence for the “best” spectrum of light to promote optimal orchid growth and flowering. Choosing a bulb that is as close to natural sunlight as possible will bring the greatest benefits to your plants.

Photoperiod

 The final question I hope to answer for you is what the ideal photoperiod for orchids is and should that photoperiod change seasonally. Photoperiod is the term used to describe the total amount of time a plant receives light during a 24-hour period.

The lights at the High Desert Orchids growhouse have been on a 12-hour photoperiod since we turned the lights on in July 2020. My rational for this 12-hour photoperiod is that we grow mostly epiphytic orchids, and the ecological distribution of these plants usually occurs around the equator. The equator doesn’t get the +/-6-hour seasonal shift in daylength like we do in the northern hemisphere. Day length in these orchid regions varies only +/-2 hours throughout the year. So, to make my life simple, I started with a 12-hour photoperiod.

The other question I get asked often is if plants need a seasonal change in photoperiod to induce flowering. This means changing the amount of time your lights are on in your grow-space to emulate natural daylengths.  It seems to be rather widely accepted that some seasonal change is required, but I have not found any scientific evidence that agrees with this notion. As stated above, orchids don’t get a shift in nature, so why would a seasonal change be required to make plants flower in our homes?

I mentioned my natural light greenhouse in the second article of this series, Target PPFD for Orchids and Tropical Plants. Both the growhouse and my natural light greenhouse maintain intermediate temperatures (55-85F), and the humidity is about the same (75-85%RH). The primary difference between these two places is the amount of light the plants get. At the growhouse, plants always receive a 12-hour photoperiod, while the natural light greenhouse gets a natural seasonal daylength change. I grow many mericlones, species, and hybrids in both facilities to study the effect of photoperiod on flowering. So far, I have not come across a single instance of a plant flowering in the natural light greenhouse but not flowering in the growhouse. In fact, in most cases, plants grown in the two facilities open their flowers within a week of each other. I have even had single-day flowering Diplocaulobium species and Dendrobium pachyphyllum types flower on the same day in both facilities. This tells me that light is not influencing their flowers at all, and most likely they are being induced to flower by temperature fluctuations instead.

I hope you have enjoyed reading this series of articles on lighting. Please feel free to email me with further questions or visit my website highdesertorchids.com for more information on lighting and orchid growing.

Brown, Christopher s, and Andrew C Schuerger. “Growth and Photomorphogenesis of Pepper Plants under Red Light-emitting Diodes with Supplemental Blue or Far-red Lighting” Journal of the American Society for Horticultural Science, vol. 120, no. 5, Sept. 1995, pp. 808–812., doi:10.21273/jashs.120.5.882. 

Hogewoning SW, Douwstra P, Trouwborst G, van Ieperen W and Harbinson J. 2010. An artificial   solar spectrum substantially alters plant development compared with usual climate room irradiance spectra. Journal of Experimental Botany, 61: 1267-1276

Taniguchi, Masahiko, and Jonathan S. Lindsey. “Database of Absorption and Fluorescence Spectra of >300 Common Compounds for Use IN PhotochemCAD.” Wiley Online Library, John Wiley & Sons, Ltd, 13 Feb. 2018, onlinelibrary.wiley.com/doi/abs/10.1111/php.12860.