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The right light for seedlings

Posted by AbbeysDad z5 CNY (My Page) on
Fri, Feb 18, 05 at 10:38

I'm putting together a new seed starting unit in the basement. I have those plastic shelving units (2) that are 3' long by 2' deep, giving me 4, 6 foot shelves - enough space for 24 flats!
Planning to use 2 shop lights (4 tubes) per shelf. I don't have a light meter, but a single shop light (2 tubes) didn't seem like quite enough light.
What should I get for floresent tubes? There are expensive growlux and agro something...more reasonable seems to be GE Daylight - 40w at about 3800 lumins I think.
Your thoughts appreciated.

Follow-Up Postings:

RE: The right light for seedlings

GE have a 55W daylight lamp putting out 3800 lumens, the 40W one would typically put out about 2800 lumens. Four 40W tubes per shelf would be about a minimum useful level for most seedlings, if you include a reflector of some sort above the tubes. The same with 55W tubes would be plenty.

The GE daylight would work, but see if you can find a "deluxe" tube. Even a cool white or warm white deluxe (you can mix some of each) would be better. Daylight just means that it has a fairly high colour temperature and a decent spread of light across the spectrum. What is means in the cheapest lamps is that you get a lot of green light that tails off at the red end without the best part of the far red. The deluxe lamps have more of the useful far red light. Don't spend big bucks though because either will work.

The gro-lux and GE plant/aquarium bulbs don't bother with the green light and concentrate just on the red and blue parts of the spectrum that are most efficient for plants, so they have a weird pink or purple colour. They will make your plants look weird but they are a bit better than a regular bulb at the same wattage. I wouldn't spend big money to get one of these tubes either, I'd rather add a more powerful regular bulb if the plants need more light. Use these special lamps if you can get them for a sensible price, look around for a big package on the net, and maybe add a regular bulb in each shelf so you can see your plants without it looking like a french boudoir.

RE: The right light for seedlings

shrubs - thanks for the info.

I didn't see any 55 watt or deluxe tubes where I was looking. Did see some expensive plant/aquarium tubes.

I'll look some more. And I'm probably wrong, but I swear the GE Daylight 40 watt tubes I was looking at said they were 3800 lumins?

I just want to make sure I have enough light. Don't want to go to the trouble/expense and have spindly, wimpy seedlings.

RE: The right light for seedlings

  • Posted by rokal LongIsland/z6b (My Page) on
    Fri, Feb 18, 05 at 14:35


You could also look into overdriving the cheap $7.25 home depot 4 ft. shop light fixtures. I recently did this and the fixtures are MUCH brighter. Supposedly, the light output increases by 70%. I hang 2 fixtures (2 tubes each) over each shelf and use 32 watt t8 bulbs. The downside is that the bulbs don't last quite as long. Not a big deal since the regular Cool White Plus bulbs are relatively inexpensive.

Here is a link to the over driving thread:

I have found the BEST cheap flourescent ballast/fixture - cont'd.

RE: overdrive

thanks rokal, but I got the $10 shop lites at Lowes and I'm thinking I'll maybe spend a little more for tubes rather than putz with rewiring to overdrive. I think if I get the right tubes, with 4 per shelf, I should be okay with them on for 20 hours/day.

RE: The right light for seedlings

3800 lumens from 40W would be 95 lumens/watt. That's right up at about the limit for fluorescents so its possible. There are so many tubes these days that you need a complete catalog number to have any idea which one you mean. Anyway, don't worry too much about the exact lumens since they are for human eyes and not plants. The funny purple gro-lux types look very inefficient in lumens because all the light comes at frequencies where the human eye is not very sensitive, but they put out just as much or more light in plant-friendly wavelengths. A good rule of thumb is to provide 15W-20W for each square foot of growing surface. This is enough to grow on most seedlings to a size where they can go outside.


Overdriving really isn't worth it if you can get the light you want from regular tubes, because you are getting a bit more light for a lot more electricity, and burning out your tubes faster. Overdriving is useful for when you simply can't squeeze in enough tubes to get the light you want, and you don't want to upgrade to an HID lamp or a big fat CFL.

RE: The right light for seedlings

I believe someone on this forum suggested using one regular cool 40 watt and one hot or bathroom 40 watt bulb. I'm trying it this year, and it seems okay so far.

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