My Grow Light Experiment (T8 vs. T5)

Ramona, CA(Zone 9b)

The topic of regular fluorescent “shop lights” versus specialty “grow lights” has come up several times. Since someone recently gave me a grow light (T5), I decided to do a little comparison. So, here is a very UN-scientific experiment between my old faithful shop lights (which use T8 bulbs) and my new T5 fixture.

My T8 lights include: 2 four-foot fixtures side-by-side with 2 T8 lights in each fixture and a reflecting hood. I replaced the bulbs this year since I heard that old bulbs get dim and less effective. I didn’t want this to be a factor in my comparison. I bought “daylight” (high spectrum) bulbs. The T8 fixtures are on pulleys so I can raise and lower them as needed to keep them close to the plants. The plants under these lights were kept about 1-2” away from the bulbs.

My T5 lights: A single 2-foot fixture with 4 of the daylight (like around 5,600 spectrum) bulbs. The plants under this fixture were kept about 5-6 inches from the bulbs. There is no need to put T5 lights on pulleys because the plants get plenty of light without needing to be close.

I used all tomato plants in the comparison. They were all planted on the same day, potted up on the same day and in the same type of soil. I used the same variety plant – one under the T5’s and one under the T8's.

Here are the results:

Growth: There wasn't any difference in growth that I could see. Sometimes the T5 plants were bigger and other times the T8. Any differences seemed to be other factors.

Heat: I had expected the T5 light to be HOT, but it really wasn’t. It was a tad warmer, but nothing significant. Later on after the experiment, some larger plants grew up into the T5’s. Some, not all of the plants that came in direct contact with the T5 bulbs showed light areas on the leaves from too much heat.

Color: I was very surprised to see a difference in plant color. The plants under the T5 lights were typically darker green. In photo #1, the T5 plant is on the left. In photo #2, the T5 plants are on the left. Not sure if darker green has any significance, but this was the most consistent difference in the test.

My conclusions after all this . . .

T5 Advantages: Hardening off the plants was easier. After being under the T5’s I was able to move the plants to full sun, pretty much right away. I also liked not having to move the T5 lights up and down which made watering and viewing the back plants easier.

T8 Advantages: Cheaper. Same growth as T5's assuming you keep the lights within an inch of the plants to give them as much light as possible (otherwise leggy). Cheaper. T8's don't give off heat so no issue if plants touch the lights. Did I say cheaper?

UNscientific, but hopefully helpful. Now, if someone would only give me an LED light … (*smile*)

This message was edited Apr 29, 2013 12:21 AM

Thumbnail by TLeaves Thumbnail by TLeaves
New Orleans, LA(Zone 9a)

I am so glad you posted this. I currently use T8s as my growing lights, and was thinking about replacing them with T5s next year. And I wasn't looking forward to the expense! Like you, the plants under my T8s (tomatoes & peppers) were a light green color, but all seemed healthy & grew well once transplanted outdoors. All the plants were kept within an inch of the lights.

Maybe I'll try my T8s another year with some new bulbs.

Charlotte, NC(Zone 7b)

I don't have many T5s but have noticed the seedlings do better. I still keep the tubes as close to the seedlings as possible.

I have not used my seed starting set-up this spring as I'm not growing a vegetable garden this summer.

Thanks for sharing your experiences. They are a great help.

Contra Costa County, CA(Zone 9b)

The reflector plays a REALLY big part of the better performance of the T-5 bulbs over the T-8s.

Think about a ray of light from the bulb. It is aimed away from the plants, up toward the reflector. The basic flat reflector just reflects that ray of light right back to the bulb. It is lost. A really good reflector will have a sort of indentation directly above the bulb that reflects those rays sideways around the bulb, then down onto the plants.

If anyone still needs to buy a fixture got to Home Depot and get a shop fixture with DIAMOND PLATE reflector.
In tests this reflector will make T-8 bulbs perform about as well as T-5 bulbs.

Yes, the color spectrum of the bulb is very important. Plants use certain wavelengths in the red end of the spectrum, and certain wavelengths in the blue end of the spectrum. Getting specialty plant bulbs will help a lot! Look for a PAR rating. PAR is a measure of how well the bulb promoted photosynthesis. You may have to research on line to find the PAR ratings of bulbs. Look at some Aquarium web sites, like ...
....then go to the lighting forum at that site.
Our eyes see the yellow and green part of the spectrum best. Bulbs designed for humans will have a lot of the yellow and green wavelengths, so look bright, but to a plant that bulb is not very good. When we look at a bulb designed for plants it will look sort of purple-pink, and not very bright.

Charlotte, NC(Zone 7b)

Diana - I used to grow African violets, and with them, full-spectrum lighting is essential. With vegetable seeds, that are raised to their 2nd or 4th true-leaf stage before being transferred to the outdoors, I have found cool-white tubes to be adequate.

I like your suggestion of diamond plate reflectors and will have to look into purchasing these next year.

Liberty Hill, TX(Zone 8a)

Cool spectrum for seedlings warm spectrum for flowering.

Great idea about the reflector.

Ramona, CA(Zone 9b)

Glad this was helpful. I totally agree about the reflection of light being important. My old T8's have a very nice silver reflector and I wish I could find more like them. Shop lights with a good reflective surface above the lights seem to be harder to find these days and the diamond lights Diana mentions seem to be some of the few left. This year, I added Mylar sheeting behind my shelves for added reflection. You can definitely see the difference in light in the room, but I didn't try to do an experiment to see if it made a difference however. :)

New York City, United States

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New York City, NY

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