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Grow Light vs Cloudy Day Lumenosity

Posted by oilrigg 9 (My Page) on
Mon, Feb 18, 13 at 17:04

I'm growing tomato and pepper seedlings under 4 100W 1600 Lumen CFLs. I'm thinking that the lumen output isn't enough and was wondering if it would be better to put my seedlings outside?

When it's sunny outside I put my plants out, but when it's a cloudy day I keep them under the lights. Is the lumen output on a cloudy day lower than the lumen output of my growing light setup?

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RE: Grow Light vs Cloudy Day Lumenosity

Hi oilrigg,

Well, as you search the web for information about the illumination on a cloudy day, you find all kinds of answers. On cloudy days, I'm sure the brightness does not remain constant. A rough estimate at mid day on a typical cloudy day would probably be somewhere between about 5,000 and 15,000 lux.

Tomato plants start to synthesize at around 2000 lux and for normal growth, flowering and fruit setting they need a minimum of 5,000 lux (preferably 10,000-20,000). So, I think the tomato plants would do fine outside on a cloudy day.

I have a light meter and I just measured the light about 4 or 5 inches under one of the 4 foot T8 bulbs. The bulbs are 6500K, rated at 2750 lumens. The lux was about 15,000. That's probably a little better than the average cloudy day. With your 1600 lumen CFL's however, I'd probably put the plants outside.

Hope this is helpful,


This post was edited by art33 on Thu, Feb 21, 13 at 12:53

RE: Grow Light vs Cloudy Day Lumenosity

Thanks for the info Art!

I'm planning on growing my tomato and pepper plants indoors for about a month or two then bring them outside when the weather gets warm. I don't want them to start flowering yet so would a 5,000 lux light setup work just fine for now?

I guess if I knew the exact lux output from my light setup I could figure out if it would be best to put them outside or keep them in. A light meter sounds very handy.

RE: Grow Light vs Cloudy Day Lumenosity


First of all let me say that I don't grow vegetables under lights, just flowers. So, I'm probably not the best one to give you suggestions :-) If tomato plants are anything like the flowering plants I grow, keeping the lighting low may not be a good idea. My guess is that the tomato plants would simply react to the low light by stretching out, reaching for more light. That of course would cause the plants to grow tall and leggy. And, if you increase the lighting, I don't know how long you could keep the plants from trying to flower :-) I do think that growing them in a cool area would be helpful.

I don't know how far your lights are from the plants but your 1600 lumen CFL's are probably already giving them more than 5000 lux. If you don't want your plants to flower and they're looking healthy, you might be just fine to stick with what you have and keep the plants inside.

One good thing (about tomatoes) is that if the plants do get a little leggy, you can always plant them deeper, when you move them outside. Anyway, sorry I can't be of more help but maybe someone with tomato growing experience will come along with some better ideas.


RE: Grow Light vs Cloudy Day Lumenosity


I think if it starts flowering I would probably prune it back and remove any flowers. But I don't think it would flower for another 2 months and by that time I should be able to leave them outside.

The distance varies. The closest is about 5" while the furthest is maybe 12-15" away.

I'm not sure what the lumen to lux conversion is but if I have 4 1600 lumen CFLs, would my total output be 6400 lumens?

I did plant my tomato and pepper seedlings deeper after they sprouted. I have some tomatoes growing faster than others and some which seem to not grow at all, which is why I was thinking that my lights were inadequate.

Thanks for the info Art.

RE: Grow Light vs Cloudy Day Lumenosity

"I'm not sure what the lumen to lux conversion is but if I have 4 1600 lumen CFLs, would my total output be 6400 lumens? "

Well, yes you would have a total of 6400 lumens but not in any one area over your plants. Unless, of course, the four bulbs were all in a small bundle where they would act, more or less, as one large bulb. Normally, when you're using several bulbs, they're spaced apart to cover a larger area over several plants. Just as four 100 watt light bulbs separated some distance apart would have a different effect on a light meter than one 400 watt bulb.

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