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Total light output needed for each plant shelf :T8 set-up

Posted by bank_beaver 4a (My Page) on
Thu, Mar 8, 07 at 16:25

I'm looking for advice. I'm designing and building my own plant rack. I'm growing woody cuttings/seed, vegetable starters vegetatively (also mature leafy veggies:lettuce,kale,micro-greens,herbs) and wheatgrass/sprouts. I want to use T8 48" 32W (2850 lumens each)bulbs in an industrial reflective fixture. My question is this:

I read that 20 watts per square foot is a good average for indoor growing with florescents. I know that a lumen figure needed for an 8 sq. ft. shelf would be a more accurate take on the amount of light needed per shelf. If each shelf is 4'x2'(8 sq. ft)that means i need 160 watts per shelf. One question is what is the total lumens needed per shelf? If i use the watts figure per shelf, then if i went with a 2 fixture (2 bulb) ea setup - that would be 128 total watts (or 11,400 lumens)The cost for both fixtures is around $76.00.
Another option - I can get a 3 bulb fixture from Gardeners Supply that are seconds (cosmetic scratches, dents) for $10.00 each. If i put two of them per shelf (6 bulbs)that would be 192 watts total per shelf (17,100 lumens). Which setup would be best or is there another option that is better to supply the needed light for a 4'x2' shelf of total growing space?

Another question i have is that I was thinking of using a 5000 K bulb in the fixtures. I know cool bulbs are 4100 and that "daylight" bulbs are 6500. They list the 5000 as a full spectrum bulb. I thought the 5000 would be a good all around choice for each bulb for the leafy vegitative plants that I'm growing. Is this right or do you recommend the 4100?


Follow-Up Postings:

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RE: Total light output needed for each plant shelf :T8 set-up

Wow, that seems more technical than it needs to be and I have no idea on the math. However, I just use two 4 foot shop lights per shelf and keep the flats of plants about 3 inches away and this works just fine. I use regular old cool florescent lights (T-8) and my plants are happy as can be. I bought some of those white wire racks from Home Depot that are 4 ft. long, and each shelf holds 4 flats of plants (16 flats per shelving unit using 8 total lights). I'm sure I'm not answering your question but I'd say to 'keep it simple'. Best wishes and have fun...Nancy


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RE: Total light output needed for each plant shelf :T8 set-up

If you are looking at the "Sunlite" T8 three-lamp system for $10 then I'd say snap it up! Six tubes on 8 square feet is perhaps more than you need, a nice system would be to wire only half to operate at the start and end of the day, with full intensity in the middle of the day. Just an idea, you can play it by ear and see how the plants are doing.

Only four 32W tubes is less than ideal, especially once the plants are more than an inch or two high. I run 4x36W on about the same area and I wouldn't want any less for starting veggies or annuals.

5000K is fine, 4100K is fine, 6500K is fine. I wouldn't pay extra money for "full spectrum" for raising seeds but there's certainly no harm in using it.


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RE: Total light output needed for each plant shelf :T8 set-up

  • Posted by watergal z6/7 Westminster, MD (My Page) on
    Thu, Mar 8, 07 at 17:49

Ooh, how do you get the fixtures that are seconds? All the $$ I've spent at that company, I deserve a deal! Please tell!


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