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green house plant lights

Posted by veggiecanner Id 04 (My Page) on
Thu, Mar 2, 06 at 15:37

I have a 1000 watt plant light that I want to use in my 8x12 foot green house. I am trying to find out some idea on how to use it. What I need to know is how far above the plants and how many square foot of space I can expect for it to work on. There are many cloudy days here right now.
This is the light with the blue spectrum. I thought it would be marked on the bulb wether it is metal or sodium but it is not.
I also have a 400 watt yellow lighted one. I was told it was for flowering is that right?

Any help you can give me would be appreciated. I am growing standard garden veggie plants and a few marigolds, nothing fancy.


Follow-Up Postings:

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RE: green house plant lights

blue spectrum are metal halide or mercury vapor, mercuryvapors are worthless for growing. the yello is the sodium.

generally folks can expect a coverage one square foot for every of 30 to 50 watts, so with a 1000 you can shoot for 20 to 30 square feet more if a light mover is installed and a good reflector.

do the "back of hand test" to find the distance. usually its 1 foot to 2 but depending on reflector design you could be limiting coverage the closer you go.


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RE: green house plant lights

If it's sold as a grow light, the blue one is probably metal halide, and the yellow one is probably high-pressure sodium (HPS). As a supplement to daylight, HPS is a good choice, since the light is a bit more efficient and daylight tends to provide plenty of blue light. But either one will work well. For a 1000-watt bulb, 3 to 4 feet above the foliage is a good distance; you may want to start it even higher (or the plants even lower), until the plants get used to the light.

These lights give off a lot of heat -- as the weather warms up, make sure you vent the greenhouse so the plants won't bake. You'll also lose some of the light through the greenhouse walls. You might want to consider lining the north wall with a reflective material like Foylon or mylar.

As the days lengthen and the weather warms, you can rely increasingly on natural light to grow your seedlings until they're ready to plant out after frost. Please let us know how it goes sounds like a great solution for getting the most out of a short growing season!


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RE: green house plant lights (cont'd)

By itself, a 1000-watt light with a good reflector will cover about 30 to 40 square feet. As a supplement to natural daylight, as in your greenhouse, it should cover most or all of the greenhouse's interior. Mount it so that the edges of the light reach nearly to the edge of the greenhouse walls at plant-table level. You'll get best results if you put it on a timer and run it about 16 hours per day.


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RE: green house plant lights

I prefer HPS as a supplement to natural light, more light from a bulb and no worries about spectrum because you are getting half your light from daylight.


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RE: green house plant lights

I was reading over on the tomato forum and I got the idea that my 400 watt HPS was only good if I wanted my tomatoes to flower. I got the idea that it would make immature plants flower. Like a young cabbage go to seed. I thought I had it figured out till then. I used the HPS last year and some of my cabbages did go to seed early. Not sure if it was related however.
Another problem I am having is I cann't get my 1000 watt bulb to work with my timer. I bought a heavy duty one at Home Depot but the ballist only hums and there is no light at all coming from the bulb. The same timer works well with my 400 watt HPS bulb. I had to also make sure the 1000 watt set up was plugged into the wall with out any extension cord or the system did not work. That may have been my extension cord, not sure.


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RE: green house plant lights

I've used them both, and both work fine as a supplement to sunlight (if you have cloudy spring weather or want to get an early start, or if your greenhouse doesn't get full sun). As you say, metal halide seems to be a better choice for indoors, though some people have used HPS exclusively and have gotten good, stocky plants.

There's no reason a heavy-duty timer (rated for at least 1500 watts, or 15 amps) shouldn't handle a 1000-watt bulb and ballast. You do need a heavy-duty, three-prong extension cord as well, like the ones sold for use with air conditioners.

If you get good sunlight by April, then you may not need an HID light at all. You can start your tomatoes under fluorescents in early March, and by they time they've outgrown the fluorescent lights, daylight in your greenhouse will be enough to grow them on. You might ask some local commercial greenhouses that produce their own vegetable starts what they do.


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