CFLs as Grow Lights

saoodhashimOctober 22, 2013

I bought six 20W daylight CFL and am keeping it each one over one 4" pot - having 3 seedlings. The CFLs are like 1-2" above the seedlings. Using a light meter I measured the approx. foot candles being hit around the top of the seedling and it seemed to be around 2500-3000 fc. and I could sense that they generate some heat around near to the bulbs. The temperatures reading near the bulbs are 5-7F higher than the normal room temperatures. I was reading on one of the FAQs on GW that seedling require 750 fc and also it is advised not to expose them to direct sunlight (or even windows getting direct sunlight). Now I was thinking if I should maintain this distance or increase it so that the foot-candles reading is more closer to 750 fc? Is the 3000 fc reading too intense for the seedlings?

Would highly appreciate your inputs.

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Hi saood,

Actually, I think your light readings are pretty accurate. I didn't have a 20 watt CFL but I did have a 14 watt daylight CFL. Measuring 1-2" under that light (using my Dr.Meter) I got about 2400 foot-candles. Your CFL lights are a little stronger so I'd say you're readings look good. It's really impossible to make accurate comparisons with other folks because there are too many variables. One big variable is the kind of reflector a person is using.

Considering the bulbs you're using, I do think that 1-2" is probably too close. What foot-candle to actually shoot for would depend on what kind of seedlings you're growing. Some plants require more light than others, but I think anywhere between about 500 and 2000 foot-candles is considered pretty normal. Most folks keep their lights on for about 16 hours a day, but that too is a variable, depending on the plants you're growing.

Personally, I'd suggest increasing your lighting distance. That will also help keep the heat away from your plants. The foot-candle reading should drop fairly quickly as you increase the distance.

Over time, you'll better know where to keep those lights for best plant growth. The plants will let you know if they're not happy :-)


This post was edited by art33 on Wed, Oct 23, 13 at 0:27

    Bookmark   October 22, 2013 at 10:51PM
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Thanks Art

The regular tomato and the cucumber seedlings have I think given a good result to this light exposure. But the pepper seedlings and some cherry tomatoes don't just seem to move a lot (peppers are the worst - many just died just after germinating). Could this be that the heat and / or the light is too intense for them? The thermometer reading around the plants (close to CFLs) is around 85F. Whereas the normal room temp inside my home is 75-77F.

I believe you know that the fc readings are from that android application that I downloaded yesterday? So perhaps that also seems to be fairly accurate?

    Bookmark   October 23, 2013 at 2:17AM
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Hi again saood,

First of all, yes I knew your fc readings were from the android application. And yes, from what I can tell, the readings seem to be fairly accurate. Even if not perfect (the Dr.Meter is probably not perfectly accurate either), certainly close enough for your purposes.

Regarding your plants, the vegetables you're growing are, of course, warm weather plants that like lots of sun. According to some notes I made last year, "Tomato plants start to synthesize at around 200 foot candles, and for normal growth they need at least 1000 to 1500 foot candles of light per square foot of growing surface". Temperature should be 75 to 80 degrees F.

I don't know why you had problems with the peppers and cherry tomatoes. I kind of doubt that the heat was the problem, but I do think your lights may be a little too close and it wouldn't hurt to increase the distance some.

As for some peppers dying just after germination, that could be dampening off disease
( ), try to avoid excess watering.

Wish I had your weather, it's getting cold here! We had our first frost last night.


This post was edited by art33 on Wed, Oct 23, 13 at 14:06

    Bookmark   October 23, 2013 at 1:53PM
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Your notes that you made for tomatoes regarding the 200 foot candles and 1000-1500 for normal growth, is there any website detailing that for tomatoes and other vegetables. I have been looking for such info for quite some time on the net but have not been able find it. I would appreciate if you could share that.

As for your wish to have my weather, I wish the same for your weather when its summer over here :) Perhaps, in this world, all our wishes do not come to pass. We have to make the best use of the circumstances in which God decided for us. I hear that paradise is the place where every wish is fulfilled immediately.......... but then you have to earn it - no free lunch :)

    Bookmark   October 24, 2013 at 3:11AM
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Hi saood,

Regarding my notes about the fc requirements for tomatoes, I got that information last year from ...
However, they've made a lot of changes on their site and if that information is still there someplace, I can't seem to find it.

I don't grow vegetables under lights, just flowering plants (mostly annuals). I sometimes enjoy experimenting with different tools or instruments, such as light meters. However, a good understanding of lighting can really get quite involved (FC, Lumens, Lux, PAR, DLI) and isn't something I've been willing to spend a lot of time on ... the things I grow just aren't that complicated :-)

However, I have (several times) searched on the web for information regarding the foot-candles required for various plants and came up pretty much empty handed. Mostly just general information such as low, medium, or high light plant requirements.

Wish I could be of more help in that area.


    Bookmark   October 24, 2013 at 5:41PM
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