Many first time grow light users or those who want to start to dabble into growing their plants under artificial lighting at first are very confused about all the different color temperatures or color spectrums of T5 grow light bulbs, so here is a little guide to help you understand them and to use your lights better and to their full potential.
First off I should explain a bit more about the terms that will be used in this article and that need to be known for you to fully understand grow lights. I previously mentioned a color temperature, and what that means is that the color temperature is the measurement that shows in what hue your grow light will be, and it is measured in Degrees Kelvin or Kelvins for short. Color spectrum or the visible spectrum is the light that we can see with our eyes, we can differ various colors or hues of the light, but when it comes to be precise then better to stick with the color temperature measurement to ensure that your plants get the light they need in the specific stage of their growth.
6,500 Kelvin bulbs
The most common bulbs sold are ones with the color temperature of 6,500 Kelvins, because usually these types of bulbs come with T5 fixtures and have the biggest selection among different manufacturers. Sometimes there are 6,400 Kelvin or even 6,505 Kelvin bulbs that are essentially the same thing, because all these bulbs will emit light that is similar to the light that we see during daylight hours, it is called a daylight white light and it has blue tinge to it. In fact any bulbs that have the color temperature of 5,000 Kelvins and up will emit daylight white light, it only will slightly differ how cool or blue the light will be.
This type of light is best and most useful for plants that are growing or in their vegetative stages of their growth, because the light is similar to light that they would get if they would grow outside during daylight hours. Also this light is good for seedling and cloning of the plants because of the same daylight reason.
3,000 Kelvin bulbs
The other most common color temperature in T5 bulbs is 3,000 Kelvins. Some manufacturers make bulbs with 2,700 Kelvin or 2,900 Kelvin color temperature, and again they are produced for the same purpose as 3,000 Kelvin bulbs, and the difference in the color temperature really isn’t noticeable, because any bulbs from 2,700 to 3,000 Kelvins are in warm white or bright white color and have more red and orange hues to the light.
And thanks to the fact that these lights have more warmer tones to the emitted light, that simulates the color the daylight would be in sunset hours, which promotes higher rates of photosynthesis which in return affects plant flowering and budding and essentially makes the yield of your plants bigger.
4,100 Kelvin bulbs
There are also bulbs that have the color temperature of 4,100 Kelvins, that although aren’t that common, is still useful to the plants and worth looking into. The color temperature from 4,000 to about 5,000 Kelvins creates a neutral weight light, that have balanced level of blue and red/orange tones to it. To the human eye they can seem as they have light blue tinge to them though.
Because the light is neutral, not leaning in favor of red or blue spectrum, it can be described as standard and therefore can promote plant leaf as well as stem growth, but can be also used when the plants are flowering. The only thing is this light won’t have as big of an effect on the plants – their growth or yield – as would two previous color temperatures would, but it still will help the plants especially if they are grown in winter. These bulbs are good for those who don’t want to switch the bulbs or fixtures and use only one type of light and bulbs all around.
10,000 Kelvin bulbs
And you can also find T5 bulbs with the color temperature of 10,000 Kelvins. Most commonly they are used as aquarium lights. The 10,000 Kelvin color temperature means that they are very blue based lights so they can be used as grow lights for extreme plant growth, because of the cool color of them.
But they usually are used for marine aquariums that have reefs and fresh water type of marine life in them, because the color spectrum also is quite similar to the light that could be seen in deep waters or ocean. So for all those who own fresh water aquariums or simply want to experiment with plant growth these bulbs are for you.
59 Comments
Anil Kumar Chopra
Dear Sir
Interesting and veryuseful article. Ihave bouhgt tube lights T8: 4000Kelvin and the other
10000 Kelvin. Can i combine these to?
Thanks
Ben Thorton
Hi Anil,
Thank you! And yes, you can combine them as long as both T8 lights have the same wattage. You can combine different color temperature lights in one fixture, but it wouldn’t be advisable to do that with different wattage lights.
Regards,
Ben
Jerry
What are lights that are rated 3500? Are they just a mix of 3000 and 4000? And what would have the bigger effect on yields, a plant grown in a 3000 that produces a smaller plant, or a plant grown in something like a 4000 that becomes taller because of it?
Ben Thorton
Hi Jerry,
3,500 Kelvin light bulbs aren’t a mix of the two, rather they are in between the to in terms of the color temperature spectrum. Meaning that they are warm white bulbs, whiter in color than 3,000 Kelvin bulbs, but more yellow in color than 4,000 Kelvin bulbs. As for which will have a bigger effect on your yield, it depends on what plants you are growing, because there are plants that respond better to yellow bulbs and ones that like while light better. So you need to do your research.
A general rule of thumb, though, is that growers use 6,400 to 6,500 Kelvin bulbs when their plants are growing (in the vegetative stages of plant growth), and 3,000 to 3,500 Kelvin bulbs once their plants start blooming (in the flowering stages of plant growth). And this formula usually provides the best results.
Regards,
Ben
Kim
Such a useful article. I’ve been scouring the internet for days to find this info and kept getting totally confused, so I truly appreciate how simply you laid out all the facts.
Ben Thorton
Thanks, Kim. Glad you found the article useful!
Regards,
Ben
Carl
I find color temperature misleading and question the science behind changing during flowering. That natural sunlight varies with season does not necessarily mean by having more red you have a higher yield. All of these light sources generate light of many wavelengths. If the intensity by wavelength of red and blue is greater artificial bulbs rated at 2500 or 6000 then Kelvin is immaterial. Does the data really show plants produce more if you start with more blue and end with more red because that is what the Sun does? Or again if you matched the amount of light at each color with the sun, and kept it at that level the whole time, I think it would be most productive. It seems to me the terms in defining Kelvin is almost backwards in meaning that that of the physics of light.
Ben Thorton
Thanks for your comment Carl.
Plants best absorb light in 2 main wavelengths – 450 nanometers (blue light) and 650 nm (red light) and each of these wavelengths trigger a different response from your plants – red light induces flowering, while blue light helps with vegetative growth. That is the reason why you should switch to different bulbs once your plants start to flower.
Bun
I want to use LED lights preferably in a four foot length like a shop light- What would be the best combo of kelvin bulbs to use for vegetative growth?? And whats the max amount of lumens?? I heard you can actually have too much light and burn your plants. Trying to figure out the right amount of light for my tomato plants 🙂
Ben Thorton
For vegetative growth, I always recommend 5,000 to 6,500 Kelvin bulbs. Specifically for tomatoes, 6,000 Kelvins are optimal.
As for burning your plants, usually, that happens from too much heat rather than too much light. So make sure that your lights aren’t placed too close to the plants and therefore don’t receive too much heat. However, if you are using LEDs then that shouldn’t be a problem since usually LED lights run pretty cool.
Chase
Hello, I am growing herbs indoors with a 5000k bulb. I only have one socket in the light fixture that I am using. Would it be better if i used a slightly warmer light? Also how far above my plants should the light be and how many lumens for one plant?
Ben Thorton
The 5,000 Kelvin color temperature is good for most plants, Chace. I wouldn’t go any warmer, though, if you want lights that are not specific for flowering plants, but you can go cooler, for example, use 6,500 Kelvin bulbs. As for how close or far from your plants should you please your grow lights, I usually recommend about 6 to 8-inch distance from the plant canopies. Lastly, I can’t say how many lumens for each plant should you have, as plants differ in size, but you can aim to have about 5,000 lumens per square feet.
Steve L
Hi, Ben
I am curious as to the specs of lights i recently purchased at Costco. They look like traditional fluorescent two-tube fixtures you would see in a shop setting, except if course they are LED’s. The interesting thing isnthat I do not see individual LED’s inside the tubes, just one bright white light that really does mimic the light from an old fluorescent tube.
I’d love to know more about threse lights’ suitability for indoor gardening, thwir color spectrum, etc. They put off an impressive very bright white light.
Ben Thorton
Most likely you purchased linear LED bulbs, that have the shape of a T5 bulb, but that actually are LEDs.
Generally speaking, LED bulbs should be just as great for indoors growing as T5 bulbs. They emit bright light and usually come in the color spectrum that is suitable for growing plants under it.
Bogdan
This is what I was looking for. Thanks. Lately I am eyeing a 4′ 26w t5 led tube that gives 3900 lm (150 lm/w Philips efficiency) for the 4000K and 6500K types. I am thinking of replacing the tubes in the 4x 54w T5HO fixture that I use for my orchids during the entire year, with which I manage to bloom high light types, Teoretically, 15600 lm (LED) at an angle of 160 degrees vs less than 18000 spread all over – even with the reflectors the eficiency is not 100% – would be a good idea? The 4000K has a more balanced spectrum, whereas 6500K is rich in blue. What is your stance on this?
Once again, thank you.
Ben Thorton
I think that set up could definitely work. And as for light color temperatures, in my opinion, the 4,000 Kelvin bulbs are a good idea as well. Especially, since many orchid growers use a mix of two 6,500 Kelvin and 3,000 Kelvin bulbs in a 4-tube fixture and 4,000K bulbs would provide roughly the same type of light.
Bill
Using a combination of 5000k and 6000k LED bulbs. Seem to be working great for veg growth. Would 7000k bulbs be better and produce faster and more veg growth? I plan on cloning also so are the higher k lights better for cloning. I thought 3000k was a better light for stating seedlings and clones because of the penetration into the soil being better with 3000k than 5000 or 6000k lights.
Ben Thorton
Light bulbs that have the color temperature of around 7,000 Kelvins is very blue, which means that although it is good for fast-growing plants, I would stick to 4,200 to 6,500 K bulbs if your plants are not fast-growing.
As for cloning plants and seedlings, in my experience, 6,500 K bulbs are better.
Pulak Bhatnagar
Hi, can you please advise me what light i should use if I have indoor plants like areca palm and fiddle leaf fig? The palm is roughly 6 ft tall. I currently have one 40w 6500k tubelight in the room. Is that enough? If not, what light and how many of them do I need? Thanks in advance.
Ben Thorton
I think that you should definitely invest in a T5 fixture with more than one bulbs. I would recommend you go for a 4-tube 4-feet or 4-tube 8-feet setup.
Mo
Hello Ben ,
I am performing some deep R&D to develope my own growlight for the startup of a hydroponic farm .
Tested the wavelength of 16 different LED samples ( red and blue ) & the blue was ranging from 440-460 nm while the red was ranging from 631-640 nm ( could not find deep red ).
So I contacted a professor from McGill Uni & he advised me to achieve 16-21 Mol/m2/day as an optimum range for the leafy green products & their maximum growth .
When I started converting some equations I got from a photometric lab , I was far away from these figures ( 1 mol/m2/day ) & thus the tested sample are expected to produce nothing but correct wavelength and not enough figures for the PPFD .
So today am heart broken & I don’t want to buy ready made grow lights because I do not trust the Chinese products & I cannot offer the elite ones such as Philips ( one grow light unit is more than 800$ ).
Should I shift to T5 ? What’s the best T5 combination for lettuce & for the second farm ( strawberry )?
Thanks allot for your support & care Ben
Ben Thorton
I think you can definitely switch to T5s, Mo since they are much cheaper than LED lamps. As far as lettuce and strawberry growing goes, any T5 fixture that covers your whole plant area with light and that has full spectrum bulbs with the color temperature around 4,100 to 6,500 Kelvins should work great.
Dion Magestro
Great article Ben
i am specifically wanted to grow tree seedlings from seed what is you recommendation with T5 bulbs as far as kelvins? Or the best alternative if not t5’s ?
Ben Thorton
For seedlings I would go for T5 bulbs that are around 6,500 Kelvins in color temperature.
Alex
I ordered SUN-901588 GE F54T5/865 – 4 ft. – 54 Watt – T5 – 6500K Blue – 5000 Lumens – High Output – Fluorescent Grow Lights. Are they supposed to glow blue? Cause they’re not.
Ben Thorton
The emitted light shouldn’t be blue, 6,500 Kelvin color temperature bulbs emit white light that has a blue tinge to it (cool or blue-white light rather than warm/yellow-white light), not completely blue light.
Somveer
This article was brilliant! Thank you
Alicia
A long time ago I bought this product on Amazon: https://www.amazon.com/gp/product/B004YZVQUA/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
It’s now discontinued, but I’ve had great success (for years!) growing cacti and succulents in my (windowless) bathroom with them.
One of the bulbs recently blew out, and I have no other information it (color spectrum, color temp); all I know is that it’s a T5 8W bulb.
The thing I loved most about these lights is that they gave off a purple (almost pinkish) color; not at all like the “daylight” T5 bulbs.
After many searches and purchases, I recently found a replacement bulb which has the purple color that I love!! However, I’m concerned because it’s an aquarium bulb and 18,000K (called Aqua-glo by Hagen Glo).
So my question is – will this 18,000K bulb fry my plants? Or will it just cause extreme growth (which I’m okay with!)
Ben Thorton
Very interesting question, Alicia! Although I haven’t used anything past 10,000 Kelvin bulbs myself, I would imagine 18,000 K ones would result in fast-growing plants. Although, usually the recommended color temperature for plant growth is up to 10,00 to 12,000 Kelvins, so I am not sure if 18,000 Kelvis will be the best option for your plant, especially since you are not using this light as aquarium lighting or to grow water-based plants.
Alicia
Thanks for the reply Ben! I installed the 18,000 K bulb about 10 days ago, and so far my plants are doing well with it, though it’s very early to say. I turn the lights on for about 14 hours a day. I’ll keep you posted on how they do!
Ben Thorton
Definitely, Alicia! I am interested to see how the 18,000 Kelvin bulbs work as plant grow lights.
Randolph
Hi Alicia. Any updates you can share with us?
Emma
I know this is super old but in case anyone ever looks up 18000K lights for plants, here’s my experience. I got some really cheap 18000K LED push lights two weeks ago and have been putting a small rubber plant underneath one of them for a couple hours every now and then. Not a very serious approach so far but I’m actually seeing results! I got that plant almost 3 months ago, it’s very small and had a budding leaf growing out of its stem. I kept it right in front of my window this whole time, repotted it, fertilized, and still nothing happened. Nothing ever moved. Ever since I started using those push lights the leaf has doubled in length and is now turning green. I think it might actually start unfurling soon. It’s crazy because it didn’t move for so long, you can actually see a dent where it was stuck in the stem for 2+ months lol. Super interesting!
Andy
I’m getting into growing microgreens and im having a hard time choosing a cost effective light source. Sorry if this is too much trouble but could you tell me if either of these would do the trick? Great article by the way!!
https://www.amazon.com/Barrina-Integrated-Fixture-Utility-electric/dp/B01HBT3BVM/ref=sr_1_4?ie=UTF8&qid=1509499190&sr=8-4&keywords=linkable+led+shop+light
https://www.dpciwholesale.com/4-led-shop-light-white-10030.html
Ben Thorton
This is definitely not too much trouble, Andy! And either of these fixtures would work great!
David Middlemiss
Hi Ben, this is somewhat following on from Alicia’s question,
I’m looking to stimulate vigourous winter growth of cacti and succulents, I’m wondering if I can stimulate dramatic growth during this period of North Eastern English winter with the 10000k lights, (I need the growth to be height and with also as in natural growth but just enhanced) or if the 6400/6500 would be suitable in a higher wattage to stimulate rapid growth
Regards
David
Ben Thorton
Since the optimal color temperature for succulents and cacti starts at about 5,000 Kelvins and they need at least 2,000 lumens of light for each square foot, I think 6,400/6,500 Kelvin bulbs in a higher wattage would work great. Do a light cycle of at least 14 hours on and you should be good to go!
Chris Williams
So could I still grow a plant with a decent yield with a 5000 kelvin bulb from start to finish??
Ben Thorton
Definitely, since bulb with 5,000 Kelvin color temperature will give out natural light that is fit for both flowering plants and plants that are still in the vegetative stages of their growth.
KENAN SIRIN
Hi,
I found flourecents with 6500K but each brand has different Lumen figures. Do you think that, Philips 6500K, 3250 lm would be good for MicroGreens production? Thanks.
Ben Thorton
I would say as long as the bulbs are the right length and the right model (meaning T5, not T8 bulbs), Philips bulbs should work with MicroGreens lights.
Leda
Hey Ben
Thanks for this amazing article!
To me this is a totally new language lol. But I’m learning. I have already LIFX led lights
And there are many kelvin to change in one bulb. Is that ok as growing light? If so what’s the best for gardenia and what’s best for bonsai? And how many hours should I leave them on? And last question, is there a standard kelvin number that suits all kind of plants?
Thanks again, and sorry for having many questions
Ben Thorton
Glad you found the article helpful.
As long as you change your LIFX light to a light spectrum that is beneficial to plants, you should be good to go in terms of growing plants under the light.
Since Gardenia loves bright light, you should aim for about 6,500 Kelvins and keep the light on at least 6 to 8 hours a day.
As for Bonsai they too love bright light and a lot of it, so I would advise you aim for the same 6,500 Kelvins, but maybe extend the time that you leave the light on to 8 to 10 hours.
And yes, there is a standard Kelvin number that suits most plants, and it is the previously mentioned 6,500 Kelvins. Most plants under 5,000 to 6,500 Kelvin light will thrive!
WildCat
Great information, I don’t think most people know/understand about light color. The common thought of why does that room or picture look so yellow (2700K) Or that one is looking blue (6500).
Anyway can you please clarify a couple things for me. I am very familiar with color temp and have been buying 5000 – 6500 (daylight) bulbs for a long time, just for everyday use. I like seeing things without the yellow hue that the 2700 (warm white) bulbs give off. So my main question …. is there a difference between a 6500 bulb that I would buy at Wally World and use, lets say in my bathroom vs. a 6500 bulb that is labeled as a “Grow Light.” Is 6500, 6500, is “grow light” just a label so that they can sell them for more money or is there actually something else going on other than color temp. Can I just use a standard 5000 – 6500 bulb, such as this one in a light fixture something like this. Or would that not work the same as a “grow light?”
Another thought I have is that if I buy an LED grow light it has many LEDs in different spectrums. For instance this one says that it has 120LED – 78pcs red light/ 24pcs blue light/ 6pcs white light/ 6pcs UV/ 6pcs IR.
Is it better to use a 6500 for grow (18 hrs/day) and then switch to a 3500 for bloom (12 hrs/day) or are the multi-color LED setups better just changing the number of hours.
By the way,it is my understanding that T5 is merely a size to describe a tube light that is 5/8″ in diameter and not specifically made for grow setups even though that term gets used frequently to talk about a florescent tube grow light. Home Depot sells various T5 tubes, just NOT labeled grow. If I buy a 6500k T5 at Depot that isn’t labeled a grow light is it the same as the T5 grow light tubes that are abundantly available on Amazon.
Do different types of plants like different light color combinations.
Thanks
Ben Thorton
As far as color temperature goes, I doubt that grow lights and non-grow lights differ much, because if a bulb emits a specific shade of light, then it does it no matter if the bulb is a grow light or not. The thing that differs is how much heat the bulbs emit and how powerful they are. Usually, when we buy light bulbs for our home, we don’t really worry about their heat emission. However, when it comes to grow lights it is very important since bulbs that produce too much heat can easily damage your plants beyond repair. So grow lights are manufactured to be powerful, yet to emit as little heat as possible, so you can place your lights close enough to your plans that they get all the benefits from the lighting, yet so that the bulb doesn’t scorch the plants.
When it comes to LED lights, you can use either of the methods you described. But, since LEDs consist of multi-colored diodes, you can just switch up the number of hours you keep the light on, and not buy two different color-temperature LED bulbs, as it is with T5 lights.
And yes, T5 stands for tubular light bulbs that are five eights of an inch in diameter. There are T5 bulbs that are grow lights, and then there are ones that are manufactured just for regular use. And as I explained before, if you buy a non-grow light T5 bulb, you might end up damaging your plants, because non-grow lights usually emit more heat than grow lights do.
Lastly, yes different plants like different combinations of both light color as well as how much light they get. The fact that 6,500 Kelvin bulbs are mostly for vegetating plants and 3,000 Kelvin bulbs are for flowering ones is a universal guideline that WILL work on virtually all plant. However, different plants might respond better to slightly varied color temperature combinations.
Charlie Bradbury
Hi, thanks for much for this succinct and clear explanation, I would like to ask for further info though.
I have had a number of aquariums in the past and still have all the kit in storage. I’m looking to give my vegetable seedlings a boost this year to get them in the ground ASAP and wondered if I could achieve this using the aquarium bulbs and fixtures I already possess, which are:
T5 – PowerGlo – 18000k
T5 – AquaGlo – 18000k
T5 – SunGlo – 4200k.
Based on what you’ve said, the 4200K will give a balanced boost in both growing and flowering so seem the most obvious to use, but concerning the 18000ks, would they be super helpful or a waste of electricity, I can’t quite figure out which?
Thank you!
Ben Thorton
You are right about the 4,200 Kelvin bulbs, they will give your plants a nice boost. However, I wouldn’t recommend yo use the 18,000 Kelvin bulbs on your plants, at least not just these bulbs alone.
Brent
I recently put Anacharis plants in my 10 gallon fish tank – what type of bulbs do I need (the tank lid has 2 in there already but the plants are dying)? Also, where do I get the right bulbs?
Thanks in advance!
Ben Thorton
Since Anacharis plants don’t do well in low and too bright light conditions yet thrive in moderate to high lighting situations, I would suggest you, indeed, install one or two additional lamps, just to make sure that your Anacharis have enough light.
As for what bulbs, I would go for full spectrum T5s that have the color temperature in the range of 5,000 to 7,000 Kelvins. There are tons that you can find on Amazon, and your local hardware store should have some as well.
Megan
Hi, love the article!
I enjoy growing all my own vegetables (peppers, tomatoes etc.) and flowers (geraniums, petunias etc.) from seed and starting them indoors in early spring. I do not get enough window light for my plants to thrive. My question is, what is the best all around light for my seedlings? I understand the lower kelvins are better for flowers but I do not always see buds before they head out side. Is the 3,000 kelvins just for when they are in the stage of flowering?
Ben Thorton
Glad you liked the article, Megan. As for the best all-around lights for seedlings, I would go for bulbs that are around 6,500 Kelvins, since seedlings require the blue wavelengths to form successfully. And, yes, 3,000 Kelvin bulbs are the most useful at the flowering stages of plant growth, no matter if we are talking about flowers, fruits or veggies here.
Ash
Brilliant article… Many thanks Ben 🙂 for sharing the knowledge!!!
May YOU flourish ????????
Joe Foxx
What would be the best light temperature for germination within a growth chamber?
Ben Thorton
I would aim for around 6,500 Kelvins.
sravan kumar
Hi… Ben, i,m really glad for your valuable information. I’m very new to this, planing for lettuce and some other leafy vegetables. Can you suggest me which range kelvin bulb is suitable for leafy vegitables.
And one more thing,i got a thought after i read your article, what if we mimic the light exactly like sun light. I mean in early hour of day the sun light is reddish orange and after that it will gradually white , again in evening time reddish orange . Will there any extra benefit if we do like i said above ?
Bill
I have been growing plants under T5 6400K bulbs for years now and they always stretch toward the light (6” away from top of plants) and the main stems never bulk up the way I would like. I also grow under 2 Metal Halide bulbs 7200K and the plants stretch even more under them. We have been adjusting fertilizer which has helped a little with the stretching but not enough. So my thought is if the greenhouse growers in the area are just using the sun with no added light source during mid March till mid May is the color of the sun a different K value then? In other words do you think that using a different K value would correct my stretching issue. Thanks ahead for your help.
John Pradeep
Hydroponics gives both quality and quantity which is better than other forms of cultivation. The time span for the complete growth of the plants taken in hydroponics is also less. The crops grown in seven months in ordinary agriculture can be grown in only two months in hydroponics farming with double the amount of yield.
Bruce Critchlow
What is your thoughts on full spectrum T5 bulbs instead of cool and warm bulbs.
Specifically seedlings and early cloning.
Thanks
Ben Thorton
Full spectrum bulbs are a tricky thing since some manufacturers call bulbs with 6,500K color temperature full spectrum, while others give this title to bulbs with other color temperatures. The thing to remember here is that full spectrum bulbs usually (hopefully) have all wavelengths of visible light meaning, so they will be great for both seedlings and early cloning as long as the color temperature of them is close to that of natural daylight.