Target PPFD for Orchids and Tropical Plants

Innovation in the artificial lighting industry, for the last few decades at least, has largely been driven by the cannabis industry. Houseplant growers have just in the last handful of years figured out that we can use these lights to expand our grow spaces and grow better plants under them. Now, with cheap and available lights, we can grow an amazing variety of plants that wouldn't have been possible with windowsill light alone. 

There is a ton of information about how to grow cannabis under lights, and the ideal light levels, spectra, and photoperiod for it. But there is very little information about houseplants, and our specialty, orchids. 

This article focuses on how we decided at what levels of light to grow our plants, what we've learned. I will talk specifically about what we are growing at what light levels. 

To begin, you must understand the unit I am using. We are measuring light in PAR/PPFD. To learn more about PAR and why it is important for measuring plant light, check out my other blog post here: PAR and PPFD for Growing Orchids and Houseplants.

Before we built the 3,000 sq. foot, HDO "Growhouse", we needed to do a little research about growing orchids under lights. We wanted to know the specific PAR values that were recommended for growing orchids and tropical plants. After some furious Googling and a whole lot of time spent scouring Google Scholar, we came up relatively empty to the question of "How much artificial light do orchids need?", this was fairly frustrating. There are so many products on the market. A Google search of the term "LED Growlight for sale" comes up with a whopping 144 million results. Why can't I find even one scientific article with the numbers I need? 

After coming up empty on the internet, we started to consult other types of plant growers. We talked to light companies and we pleaded with orchid growers on Facebook. No one really had the data we were looking for. We just needed to do the research ourselves. 

We purchased a Quantum PAR meter, the Apogee Full-Spectrum Quantum Sensor, a $500 instrument! I took this PAR meter out to my natural light greenhouse, where I had been growing a huge variety of orchids for many years. I was comfortable with my knowledge of how the light behaved in my greenhouse. I used this knowledge to take natural light values using my PAR meter, and convert that into target light levels for our artificial light growhouse.

For example, I had a standard Cattleya hybrid (an Rlc. Kat Green Power) growing on the same spot on my bench for the last three years. This plant flowers reliably twice a year, has very bright apple green leaves, and generally grows very well in that spot. Therefore, I knew the light in this spot was bright Cattleya light. I took a PAR measurement in this spot, and various other spots in my natural-light greenhouse. I repeated this several times throughout the day (from 9 am until dark) and I averaged the values together into one ideal target light value for each type of light I was looking at (i.e. shady, medium, bright, very bright) 

Here's what we came up with: 

50-70 umol/m2/s - Under bench (Mostly growing Phalaenopsis here, some mottled-leaf Paphs, Aerangis) 

100-150 umol/m2/s - Shadiest part of bench level, Northeast corner of Greenhouse (Here I grow Masdevallia, many Aerangis species, miniature Dendrobium species, some Bulbophyllum) 

200-300 umol/m2/s - Bright bench level (at 200 umol level, I grow miniature Cattleya, many Sophronitis species. At the 300umol level, I grow Dendrobium species, larger Cattleya, Rlc. hybrids) 

600-800 umol/m2/s - 6" below the roof level, VERY bright light (Here I grow Brassavola species and hybrids, Rupiculous Laelias, and bifoliate Cattleya) 

2,000-2,200 umol/m2/s - Full sun 

You might think you can just crank your lights up to these values and be good, but it isn't quite so easy. 

Fig. 1 Natural vs. Artificial Light Dose

Because of the nature of artificial light, it really only has two settings, on and off. Sure, you can certainly buy fancy and expensive dimmers that can replicate the path of the sun. But for the most part your lights will follow the same path as the black line in Figure 1. The lights turn on at 6am and off at 6pm. They are on at a constant brightness all day. The other curve is the average path the sun would take, on a day with a 12 hour day/night cycle.

Imagine your plant as an empty cup, to be filled with water (in this analogy water = photons of light). Your goal each day is to fill your plants' cup with just the right amount of water, drip by drip (photon by photon). You can fill this cup as the sun would, by starting out at a slower rate, and then peaking around mid-day, and then slowing down again before going dark. Or, you can fill your plants' cup with a constant, but overall lower rate. Take another look at Figure 1. The area under these two curves is identical. Meaning the plants are being given the same dose of light in both situations. Though the artificial light doesn't have the same intensity peak, it is still receiving the same amount of light it would have if it had been given a more natural daylight dose of light. 

If I grow my plants at the peak intensity reading that I got from measuring my greenhouse, I'm going to be overdosing my plants on light!  Based on the results of this graph, we found that on a 12 hour photoperiod, plants need about 30-50% less than the full natural light values cited in our original light target estimates. We started growing using the averaged light measurements taken throughout the day, but we found that even this was still too high, and our plants are much happier at surprisingly low levels.

Here is a list of what we grow under what light levels: 

Low light bench: 40umol/m2/s

Jewel orchids (Macodes, Ludisia discolor, a variety of Anoectochilus species such as roxburghii, formosanus, burmannicus, Goodyera species, Cheirostylis species) Begonia (rex Begonia hybrids), various ferns, Masdevallia species, Pleurothallis species, Phalaenopsis species and hybrids, limited low-light Paphiopedilum (Maudiae hybrids, Paph. thaianum, Paph. armeniacum, Paph. micranthum), Habenaria. 

Medium light bench: 150 umol/m2/s

Miniature Cattleya (Sophrolaeliocattleya hybrids, Hadrolaelia species, various other small Cattleya), higher light Paphiopedilum (strap-leaf Paph species and hybrids), Phragmipedium species and hybrids, miniature Dendrobium species, Oncidium intergeneric hybrids (such as Oncidium Sharry Baby), Zygopetalum hybrids, Gongora species, Cynoches hybrids. Angraecum species and hybrids (such as Angraecum scottianum, Angraecum pseudofilicornu, Angraecum Gem Star, Angraecum magdalenae, Angraecum didieri, and others), Carnivorous Pinguicula, Utricularia and Drosera, and Pothos.  

High light bench: 250 umol/m2/s 

Standard Cattleya species and hybrids (Labiate Cattleya, Rlc. hybrids), Dendrobium species (Including Dendrobium farmeri, Dendrobium aggregatum, Dendrobium trigonopus, Dendrobium chrysotoxum, and many other species), Dendrobium hybrids (Dendrobium Densiflorum hybrids, Phalaenopsis Dendrobium hybrids, Nobile Dendrobium hybrids), Rupiculous Laelia species and hybrids, Miniature Vandaceous plants (Holcoglossum, Ascocentrum, Renanthera, Vanda), Carnivorous Pinguicula and Utricularia, Broughtonia species and hybrids, Epidendrum species and hybrids.