Light Spectrum's Effects on Plants -- Part II: A Few Problems in the Details

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Light Spectrum's Effects on Plants -- Part II

By drdawg
February 23, 2015

Previously I wrote about light spectrum ranges and how those ranges affect plants. This light range is measured in nanometers, or what's called Kelvin (the K number is printed on fluorescent tubes/bulbs). What is also important is the light intensity. If the light intensity is not great enough, it really won't matter much what the Kelvin number is. Light intensity is measured in lumens.

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Knoxville, TN (Zone 7a)
Region: Tennessee
brandon7
Feb 28, 2015 4:09 PM CST
Overall I think the Dr has done a good job with this article, and I agree with a lot of it. However, there are some problems in the details.

As described in the article, bulb with ratings of 3500K or less are not well suited for growing plants. They CAN be used because they do put a spectrum of light which includes the wavelengths used in photosynthesis. The problem is that much of their energy is wasted. A large percentage of the total output is not used by plants. A grower could use a greater number of bulbs and produce light useful for photosynthesis (as well as waste energy and money).

Bulbs rated at 4100K actually work pretty well for producing plant growth (supplying energy for photosynthesis), and, in fact, their light spectrum contains about as much light in the higher frequency spectrum (useful for plant growth) as do bulbs with higher peak ratings. That's where the K-rating comes from, BTW; it represents the peak of the light spectrum produced by a bulb. The reason that shorter wavelength (higher frequency, higher K) bulbs are not much better for plants is that they put out less light per amount of inputted energy. They do put out a higher percentage of useful light, but the useful spectrum is not significantly (or any, depending on the specifics of the bulb) greater than bulbs in the 4100K to 5000K range. Bulbs with even shorter wavelength (higher K) are even less efficient and may put out about the same or even less (depending on the bulb) useable light.

Finally, mixing bulbs of different wavelengths does not expand your light wave range. As I mentioned above, all the lights put out a fairly broad spectrum of light. The differences are where the peak occurs and how much light is produced per given amount of energy. Because the shorter wavelength bulbs put out less total light per amount of energy, they generally won't add significantly (or any) to the upper spectrum of light as compared to using the same total number of longer wavelength bulbs. In other words, the useful light of two 5000K bulbs will look pretty close to one 5000K bulb plus one 6500K bulb.

If I had time, I'd make a graph, but just think of looking at only the right hand side (representing the useful light range) of a pile of dirt made up of two scoops. The first instance (representing the two 5000K bulbs) is a pile made from dumping two large scoops, one on top of the other. What you will be comparing is one side of the pile. Next, picture a pile made from dumping one big scoop and then dumping another smaller scoop on the right hand side slope of the big scoop (this will represent one 5000K plus one 6500K bulb). What you will see (depending on the bulb/scoop size) is pretty close to the same.

Hope this made sense. It's a little longer than I had planned....
[Last edited by brandon7 - Feb 28, 2015 8:51 PM (+)]
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Name: Ken Ramsey
Starkville, MS (Zone 8a)
[url=www.tropicalplantsandmore.com]
Orchids Greenhouse Vegetable Grower Ferns Region: United States of America Hummingbirder
Composter Bromeliad Master Gardener: Mississippi Cat Lover Tropicals Plumerias
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drdawg
Feb 28, 2015 4:35 PM CST
Thanks, Brandon, and welcome to ATP. I attempted not to be all that "scientific" about light, but just to make the confusion less when it comes to what are the best (peak-Kelvin) bulbs/tubes to use. There are books written about light and its affects on plants (and also we humans) but not many people will take the time to read them. My information is based more on what I use in my own greenhouses, growing lots of various tropical plants, but mostly orchids.

Since I wrote that article several months ago, I have changed my T5HO fixtures so that they each now have four, 6500K tubes and two, 3000K tubes. Are these fixtures any better for growing my plants than the six, 6500K tubes fixtures? Time will tell.

Brandon, do you have a background/education in physics or even perhaps commercial greenhouse growing? I welcome any correction or expansion on what I post. Needless to say, my experience and education is more limited than many. I'm just a hobbyist grower.
drdawg (Ken Ramsey) - Tropical Plants & More
[url=www.tropicalplantsandmore.com]www.tropicalplantsandmore.com[/url]
If God wanted me to touch my toes, he would have put them on my knees.
Knoxville, TN (Zone 7a)
Region: Tennessee
brandon7
Feb 28, 2015 10:07 PM CST
I do have a physics-related degree and quite a bit of experience in horticulture, but actually, my understanding in regards to what I added above is mostly from conversations with growers and plant lighting specialists. My initial assumptions were more inline with the way you had explained some things (like using two different spectrum peaks). When I heard others making the points I made above, I started looking into it a little more and came to the conclusion that they were correct.

I think your article is a great contribution and our conclusions about what bulbs to buy are pretty close. Any type of fluorescent bulb can be made to work, but some work better (are more efficient). The 3000K and 3500K bulbs are not as well suited (less efficient) for growing plants (supplying light for photosynthesis). Anything above that is probably OK. Personally, when I use fluorescent bulbs for grow lights, I use mostly 5000K bulbs and sometimes 4100K bulbs. There's nothing wrong with mixing them, but, there's also no advantage. If you add the spectrum distributions and look at the useable (by plants for photosynthesis) part of the resulting distribution, there's just no significant benefit to the mix. Things change if you also want to encourage flowering, BTW; in that case, mixing bulbs may be more efficient.
Name: Ken Ramsey
Starkville, MS (Zone 8a)
[url=www.tropicalplantsandmore.com]
Orchids Greenhouse Vegetable Grower Ferns Region: United States of America Hummingbirder
Composter Bromeliad Master Gardener: Mississippi Cat Lover Tropicals Plumerias
Image
drdawg
Mar 1, 2015 7:17 AM CST
I am not only growing but flowering my orchids beneath these particular T5HO fixtures.

Again, thanks for your valuable input. Keep the posts coming, Brandon 7. Go Dawgs!
drdawg (Ken Ramsey) - Tropical Plants & More
[url=www.tropicalplantsandmore.com]www.tropicalplantsandmore.com[/url]
If God wanted me to touch my toes, he would have put them on my knees.

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