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High how should my lights be from plants?

Posted by Hollywog z6soIL (My Page) on
Fri, Mar 11, 05 at 18:36

Hi all! I just bought some flourescent lights to put over my seedlings as some of my plants are getting a bit leggy in my portable greenhouses. Is there a specific distance over the plants that the lights should be? Is 18in too close? Need help ASAP!


Follow-Up Postings:

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RE: High how should my lights be from plants?

The closer the better. 18" is probably too far for most plants.

Here is a link that might be useful: Photo Albums


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RE: High how should my lights be from plants?

For seedlings, try to get the lights down to just a couple inches above the seedling tops. As a cautionary note, I wouldn't go this low with overdriven lights. I just did some temperature checks and in a room about 70-75 degrees after sunset (because of the small size and the lights on & overdriven), temps of the media used were in the 86 degree range about 2" below the seedling tops, and during the daytime with sunlight also entering the room, the temps were even higher, a couple times peaking over 90 degrees. (the house thermostat is set at 66 or 67.) Maybe it wouldn't be as bad in a cold basement or garage.

Hope this helps.

Mark


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RE: High how should my lights be from plants?

Fluorescent lights should be 1 or 2 inches at most, above the seedlings. Cool white (cheap) bulbs work fine.


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RE: High how should my lights be from plants?

I have mine about three inches above seedlings once there established, as far as getting leggy, I have learned over the years that pinching back any plant you can will make them much sturdier.


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RE: High how should my lights be from plants?

"pinching back any plant you can will make them much sturdier."

OWWW ! You're heartless !

Try a fan, it's less cruel.

I wonder if the ASPCA takes cases of plant abuse ???


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RE: High how should my lights be from plants?

I vote for the fan too.


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RE: High how should my lights be from plants?

I would just like to add a bit more information about how far away the lights should be. Placing the lights as close as possible to the seedlings isn't wrong but it isn't the best solution.

Fluorescent tubes emit light in all directions, up down, sideways, everywhere except out the end caps. It would be ideal if all this light reached the seedlings. Placing the tubes as close as possible to the seedlings means that nearly half the light could reach them. Most shop lights will include some sort of fitting above the tube and maybe even a bit more than half the light could reach the seedlings.

However, with a good reflector, you can direct almost all the light onto the seedlings, doubling your efficiency and removing the need to have the tubes right on the seedlings. A parabolic reflector correctly positioned directs all the light directly downwards and you can position the light as far from the seedlings as you like. It is just like the focussed beam on a searchlight. Simpler reflectors such as a flat white or shiny plate above the tube are less effective and should be combined with side reflectors because the light won't be focussed down as efficiently. Taking this one step further, you can enclose to the whole assembly of tubes and plants inside a white/shiny box and again almost all the light will reach the plants.

P.S. Stand back, I pinch!


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RE: High how should my lights be from plants?

Absolutely right. A good reflector will make all the difference.


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RE: High how should my lights be from plants?

This year was the first year I started seedlings under a 1000W HID lamp. Just for the record I didn't buy it specifically for that purpose as of course that would be a colossal waste of money-I grow citrus and tropicals too but started some tomatoes and peppers at the edge of the lighted area where they got about 1200 Foot-Candles and they have sprouted and are still small but appear to be happy. They're almost 4 feet under the light!!!


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RE: High how should my lights be from plants?

I keep a fan oscillating on low all the time while the lights are on. The fan is attached to the timer with the lights, it keeps them more sturdy and also keeps it from getting too hot.


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RE: High how should my lights be from plants?

I bought 2 20watt 24 inch fluorescent grow lamps with reflectors. the lamps emit 10000K of full spectrum light. I bought them to supplement the sunlight for my aquaponic's in a barrel system. I have tomatoes seedlings, marjoram, Basil, celery and Lettuce. At what distance should the lamps be from the plants to be effective? Currently they are set at about 30 inches above the grow bed and the plants are reacting to the lamps since I placed them above the beds.


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RE: High how should my lights be from plants?

Why 10000k?? Are you growing them under water?? 4100,5000 or 6500k would have been better.


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RE: High how should my lights be from plants?

Place you hand, palm down, over the tallest plant top. Wait a minute. If the top of your hand starts to feel too hot, then raise the lights. Otherwise you can lower them until you feel the heat. Quite simple.

Zink


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RE: High how should my lights be from plants?

A cheap reflector is one of these metal film coated mylar emergency blankets you can buy for around $2 at many sporting goods stores or online. I wrap my whole rack in them, and hold them shut with clothes pins. They serve several purposes.
1. They reflect light back at the plants that would otherwise be wasted.
2. They keep the heat in, no need for heat mats. Keep a thermometer inside the setup so you can open a flap if it starts getting too hot.
3. They keep the humidity in so you don't have to water as much. In fact the inside of the blanket sometimes is dripping wet from condensation.


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