When it comes to houseplants and orchids – light matters! My guess is that over 95% of indoor hobby growers don’t understand how important light really is, and while there’s no shame in that, the devil is in the details when it comes to light and our ability to grow plants better. The quality and intensity of light you offer your plants directly affects how well they will perform for you (how many blooms you get, how fast they grow, and how resilient they are to pathogens). The problem with understanding light though, is that it’s conceptual. We can’t hold it or touch it or easily measure it. It’s abstract, theoretical, and the explanation is never short—and personally, I find that the technicalities make it a bit dry/boring. However, if you want to grow impressive plants that reward you with flowers and growth, it’s a topic you need to get familiar with.
While I can’t make the technicalities of light any more exciting, what I have done is put together the different blocks of information into one place so you can learn the details, understand light a little better, and become a more-informed consumer. I can also say from experience, good lighting will fundamentally change how well you can grow indoors. It’s in your best interest to spend some time to understand the concepts outlined here, especially if you’re purchasing grow lights in the near future.
Update: I’ve had some people go, “Dustin, thanks for all the info about light…I still find it complicated, can you just recommend a brand?” If you fall into this group, that’s cool; I get it – it’s daunting learning everything when you just want a fricken light that works. I’ve carved out a section at the bottom of this post just for you.
Photo of Orchids & Houseplants Growing Under LED Grow Lights (at an East-Facing Window)
More Orchids Growing Under Full Spectrum ‘Flower’ LED Grow Lights
Learn About Plant Grow Lights, PAR, Photosynthesis, & Spectrum
To begin, watch these videos (there is a playlist of 5); they explain the foundation of photosynthesis, chlorophyll, light, spectrum and the most-recent advancements in our understanding of plant growth AND they will help you understand the other details I outlined further down in this article:
Grow Light Terms to Know & Things to Evaluate When Buying LEDs & Fixtures
PAR – Photosynthetically Active Radiation – a unit for measuring light values specifically for plant growth.
PPF – Photosynthetic Photon Flux (umol/s) – the measurement parameter of PAR that tells you how much light a fixture emits every second.
Example PPF Output for an indoor brand grow LED: 285 μmol/s
PPFD – Photosynthetic Photon Flux DENSITY – the amount of light falling on a given area (typically the measurement is per meter2/sec – but I use a /ft2 [per square foot] because I think in imperial measurements for physical space)
Example PPFD Output for that same LED: 285 μmol/s / 2m2 = 142.5 umol/m2/s (at 12″ from the light)
Efficiency – umol/j – a joule is a watt/second, so this tells you how many PAR per watt per second; it’s super valuable because it tells you, “how much power does the fixture convert to plant-usable light?” The larger the number, the more efficient (and better) the fixture is at converting power to light.
Example Efficiency for that LED: 285 μmol/s / 125w = 2.28 μmol/J
Examples of Other Fixture Types & Their Efficiencies T8 Fluorescents: .84 umol/j
T5 Fluorescents: 1.23umol/j
Metal Halide: 1.25–1.46 umol/j
Classic “Older” LEDs: 1.5–1.8 umol/j
Modern LEDs: 1.8–3.2 umol/j
Uniformity – The light footprint and range of intensity across that space; think “minimum/maximum & average PAR”
Spectrum – Spectral distribution of wavelengths from a fixture (“Full Spectrum” = White vs. Burple “red/blue”)
Size – How big is the hardware? Does it fit your grow space? Is the fixture a long tube that covers 4 feet; or its it a spotlight that only covers a 3 foot cubic area?
Proximity – How close can you get the lights to the plants? Higher output often means you need more distance between the light and the plant. In some cases you may want lower output lights so you can get them closer to the plants.
Emerson Effect – the increase in the rate of photosynthesis after chloroplasts are exposed to light of wavelength 680 nm (deep red spectrum) and more than 680 nm (far red spectrum)
Photomorphogenesis – The physical affect of light on plants (shape, appearance, color, taste, etc)
Grow Lights & LEDs – Understand What You’re Buying (Recap)
When purchasing grow lights, try to find brands which provide the PAR numbers. You will tend to find that brands which provide these numbers are going to be more serious about the products they’re selling for growing plants. You’ll recognize PAR measurements as a “umol” [that’s a micromole and it’s technically written, μmol/(m-s)] and the more you can familiarize yourself w/ PPF, PPFD, and efficiency ratings (see below), the better. Just be aware there are a LOT of lights that don’t provide their PAR ratings, and that doesn’t mean you can’t purchase those lights, but it does mean your ability to benchmark their effectiveness for growing plants, will be more difficult. I’ve covered that in more details further down below.
Light intensity falls off very quickly and sometimes the difference between too much light and the optimal amount is the matter of moving the plant 6-12″ further from the light. Excessive light, especially in the blue spectrum, can lead to chlorosis (which typically looks like yellowing mottling of newer leaves as a result of chlorophyll damage from toxicity). In some cases you can mitigate chlorotic issues like this, by adjusting your fertilizing parameters like frequency, volume and pH—but it’s not always a cure. You can use this chlorotic symptom as a simple visual benchmark to know when light is too intense and then make micro adjustments until you find the right spot.
Beware: chlorosis from too much light, is not the same as leaf burn from heat, which happens quickly, kills the tissue, and is irreversible. Moving a dark-green plant that has been in your low-light bathroom, to a south-facing window (in full sun) is a really easy way to cause this physical burning of the leaf from high temperatures. The leaf is dark and the sun is hot, so your leaf fries. Gradually acclimating some types of plants to higher light can prevent burning like this—but not all plants are able to withstand high levels of light, so when it comes to proximity and intensity, you have to tinker with each plant to find it’s ideal spot.
For plants under grow lights, I’ll typically place the lights about 12-18” from the tops of the plants but if you have a very high-output light like MarsHydro…then placing the plants that close is a really bad idea. Understand that you’ll need to either increase/decrease the distance between the plants and lights if the PAR is too high or too low, or increase/decrease the light output (by purchasing lights that match the needs of your plant within the available space) to compensate for whatever distance you’re working within.
Growing Near Windows? Beware of shifting variables
Window direction, time of day (angle of sun to windows), seasons, and window treatments (tints/glass thickness) will all affect the intensity of light throughout the day. Beware of these types of variables and understand that they all can influence how much light your plants are getting.
Tips for Buying LED Grow Lights Without a PAR Rating
What if a light manufacturer doesn’t provide the PPF, PPFD, or efficiency numbers? (cuz a lot don’t!)
I default to reviewing a combination of 4 things…but you need to understand these are all imperfect methods of evaluating a light and without PPF or PPFD, you’re not going to get a lot of certainty about the quality of light your fixture can provide…but you can make an educated guess.
Wattage – this is the amount of power a light pulls from your wall and generally the idea is more watts means more light. I try to target about 12w/ft2…24w for moderate light plants, and 36w for high-light orchids. The use of watts as a measuring tool quickly loses reliability and you won’t want to use this variable alone. Why?:
Challenge #1: Different Fixture Types/Technologies – If you’re comparing LEDs to fluorescents, the LEDs are more efficient and will convert more of the power into light than the florescent fixture will. So, if you have a 40w LED and a 50w florescent…the LED is going to putout more light.
Challenge #2: Different Efficiencies – If you’re comparing two LEDs (a more-direct evaluation) it still isn’t exactly perfect. LEDs have different efficiencies and some will convert more power to photons than others. But, you can probably safely assume that if you have a 12w LED light and 22w light, the 22w will provide more light.
Challenge #3: Different Spectrum – Even if the light output from a given fixture is higher, you still don’t know that it’s the best spectrum for plant growth. So a 50w light that puts out 70% of its spectrum as green light won’t be as effective as a full-spectrum grow light.
Lumens/Lux/Footcandles – an actual measurement of light output. These are methods of evaluating the “brightness” of light. The problem with each of these three options is that they are units of measurement for visible brightness and they don’t account for spectrum or Photosynthetically Active Radiation (PAR) – they don’t tell you if the light is good for your plant.
Color Temperature & Spectrum – These numbers and charts are intended to give you an idea of the composition of light that your fixture will emit.
Color Temperature is a single number reading that tells you “the sum of all parts”. A ‘flower spectrum’ light will be a warm, orange-yellow color and around 3,000K (K, for Kelvin), and a 6,500K light will be a more natural white light (typically used for vegetative growing), 10,000k is very blue and is typically used for reef aquariums. Unfortunately, the color of a light, really doesn’t tell you the spectrum or its value to the plant… and it’s an average of the colors within the light meaning that the ratios may not favour plant growth.
Examples of Different Kelvin Color Temperatures
Spectrum charts give you an idea of the distribution of wavelengths within a light spectrum. They provide the detail that color/temperature doesn’t provide; however, a spectrum chart won’t give you insight into how much total light is being output. So, you could have a really great spectrum…but really low output. This is where looking at lumens and wattage can help you infer about the quality of the light in combination with the spectrum. If you want the details of growlight spectrum, check out this post from Robert Pavlis owner of the Garden Myths & Garden Fundamentals blogs (from Ontario).
Comparison of Different Light Types & Associated Spectrum
Light absorption in a plant leaf: the spectrum a plant actually requires
Customer Purchase Reviews/Ratings – This one is a fall back; however, I have a hard time trusting reviews. Read the reviews and skim for people who included photos and well-thought-out responses. Generally though, if you look to lights that have more reviews overall and with a better star-rating, you’re setting yourself up for success. Just be aware it’s easy for people to fudge those numbers. A couple stories about that:
For the better part of the first year after I started my blog, I’d get contacted by Amazon sellers offering a free light, if I paid for it on amazon provided I left a good review (for which they would later reimburse me the cost of the light). I never did this but I have seen others do this.
Some well-known YouTubers have done LED product reviews on Amazon (over 10 reviews from a single person) and not one of the reviews for the different brands was less than a 5-star rating. As a consumer, it makes it hard to evaluate the effectiveness of a light if every light is 5-stars, right?
What About Light Meter Apps – Do they work for evaluating LEDs & Grow Lights?
You can use them, but they might give you false data. Some of the things you need to consider if you plan on using a mobile app to measure light output include:
Spectrum: The evaluation of light using your phone camera’s lens will likely be based on visible spectrum (because they’re typically used by photographers for light balance) – the resulting measurement for lux/lumens or footcandles from an app are going to be an estimation at best. They definitely won’t account or correct for PAR and/or lights that are heavily in the red spectrum are going to give “low light” readings (so there’s a really good chance you’ll get false readings).
Each mobile device (eg. iPhone 6, iPhone 8, Samsung Galaxy, Google Pixel 2, etc) has a different set of camera hardware. The likelihood that all devices have been effectively tested by the developer is pretty slim. The team I work for once built a light meter app; we ended up having to stop development because of the number of inconsistencies we would get from readings across the many devices we tested.
The angle of your lens to the light source will matter too – is it pointed at a 90-degree angle? 45? 15? the amount of light entering your camera lens can vary based simply on its angle to the light source.
Test Accuracy: An easy way to find out if your light meter even “kind of works”, test it’s accuracy under conditions where you KNOW you’ll get an expected outcome. In full sunlight at noon when the sun is highest in the sky, you should get a reading of 10,000 footcandles. If your light meter is in lumens, it should read ~110,000 lux in full sun at noon. If it reads above or below that number by more than…10% error, then light meter is off and you cannot rely on it to measure the idiosyncratic details of your indoor grow lights.
I hope this post has provided you with enough information that you feel like a more confident consumer. I know the process of buying lights is a bit daunting and at some point, you just pull the trigger and try/buy a light. In general, the more expensive premium brands tend to offer a better product and you can expect to spend $150-500 for really good LED grow lights. However, if you’re on a budget, you can still get better growth with some of the more affordable options on Amazon.
LED Grow Light Recommendations
Alright, so you’ve either read all of the details and you’re still looking for more info OR you just want to know what LEDs I use and you’ve skipped all the details of this post. Whatever the reason, here are the lights I currently use and regularly recommend to others; please note though…I haven’t tried ALL of the LEDs available on the market — I haven’t even tried that many overall! Without any further blabbing, these are the lights I use and like:
Botanical LEDs (formerly The Orchid Hobbyist) who now sell a high-output grow light (that I love) – The owner, Jeff has been spending a lot of time and effort refining the quality of his LEDs. The lights are likely more affordable and the spectrum quality is very good for plant growth. I use these lights on my seedling orchids and they get no additional natural sunlight. I also now use this in above my bedroom plants and I’m very happy with the output and growth under them. For the lower-output lights, you’ll need about 2 lights per shelf for low light orchids or plants like phals and paphs; or 1 of the high-output lights for high-light plants like oncidiums, cattleyas and so on. Really fantastic lights and good customer service too.
Sunblaster LEDs – These are probably the most common “brand” of LEDs that I’ve seen available. They’re good, moderately-high output and work well to grow most plants. The light spectrum leans heavily in the blue spectrum which can lead to chlorophyll damage if some low-light plants are too close to the lights. Chlorotic spotting or light damage looks like yellow mottling on the leaf in early stages and late stages the leaf can go fully yellow. They’re good and if you can’t get the first two options, this is good third choice—just be aware that you can overdo it with these lights and there is such thing as too much light.
Arcadia Jungle Dawn Full Spectrum LEDs, they’re a brand for reptile terrariums but their spectrum really pushes red color in plants that express anthocyanins. I was using three, 40w spotlights on my 125 gallon terrarium, and had great growth. This brand might be more common in the UK, but if you can find it, they’re a good choice; the colors of my plants (specifically nepenthes) under these lights was amazing. They’ve recently released a strip light version and if they were easier to get a hold of over here, I’d get one or two of those to help round out the spectrum of the light I currently run.
Fin – The End
Thank You: The information in this post has been extracted from numerous sources (including the videos from the playlist) and the talk Nick Klase gave on LEDs. Thank you to all of those who dedicate their time and effort to the hobby and providing valuable information and products to others. If you want to “pay it forward”, support to those channels on YouTube, and subscribe to them for more of their great plant content.
