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400 watt mh light height

Discussion in 'Grow Room Design & Setup' started by kodiak7, Mar 24, 2012.


    kodiak7 Member

    im getting ready to transplant my almost week old seedlings into 3 gal pots and im really needing some advice as to where to set the light height. i see so many different opinions on this issue and id like to get a few opinions of my own. i plan on doing this today so i need answers quick.

    Dubdeuce Well-Known Member

    There's no set answer to this due to a number of environmental factors. The BEST way to determine light height is to put your hand at the plant tops with the light at the desired height and then just wait a few seconds. Is it warm? Is it hot? If you were the plant in the container, would you want to sit under that hot of a light source for 12 hours or would it be uncomfortable? If you think it's hot, the light needs to go up. You want them to be well light, but warm and comfortable. Warmth is good, hot is bad. Layman's rule of thumb says probably about 12"-14" away from the tops, but I would always use the hand test method.
    CMKgreenthumb and grandpa 1949 like this.

    Stinkbait Active Member

    These are all under my 400w.

    +1 on the hand method! That's what I do.

    Attached Files:

    grandpa 1949

    grandpa 1949 Active Member

    Never seen it put this way! Good explanation. Dubdeuce you got a rep from me

    phillipchristian New Member

    You can have it as close as 6 inches if there are no heat issues and your plants do not suffer. Some strains, especially early on, do not like a lot of light. Also, if the temps get to hot on your plants then they will start to stress. I would stick it at 8"-10" and see how they react. If you see nothing negative in a day or two then just leave it there and your plants will grow toward it. They will let you know when you need to back it off. Keep a thermometer on your canopy level to make sure temps don't get too hot.

    Bernie420 Well-Known Member

    three feet away. They are little seedlings. You will burn them up if you put that light to close. You dont need to test this to see if they will burn up. Wait till they are around six inches tall to lower the light on top of them.

    kodiak7 Member

    ive got em at 2 feet right now and they seem to be fine except for one thats getting some leaf curl and i was wondering if thats due to heat stress.

    phillipchristian New Member

    Post pics of the plant. Curling up or down?

    kodiak7 Member

    curling down. i watered them yesterday so im thinking maybe overwatering cause they are all drooping a bit

    phillipchristian New Member

    That's normal. Next time try and wait till they start to droop before you water them. They will show you when they are thirsty.

    kodiak7 Member

    im going to be transplanting today. i know your supposed to water them after you do that so my question now is will they be fine if i transplant and dont water them?

    phillipchristian New Member

    Wait to transplant till they need to be watered again. You really need to water them right after a transplant and maybe even add a low dose of your roots nutrient (superthrive, jumpstart, etc..). Waiting a couple days to transplant won't hurt them. Plus, if it takes more then a couple days for the soil t dry out then the pot you have them in is still too big and you don't need to transplant.

    kentuckyboy Well-Known Member

    I am using a 400w HID MH/HPS setup, and when I my plants 1st popped out of the dirt from seed I kept my light about 12 inches away. I did that for the first week. Then I let the plants grow towards the light until they were about 6-7 inches away as the plants were about a foot tall by then and could stand the more intense light and heat w/o any ill effects. I never noticed any leaf burn by having them that close. Just let them grow into the light and when it starts getting closer just keep a good eye out for leaf burn. This methos worked well for me.

    kodiak7 Member

    its been 2 days since i watered and the soil is drying out but they arent ready to be watered so im gonna wait till tommorrow.

    phillipchristian New Member

    Sounds like a plan to me. Good luck.
    bird mcbride

    bird mcbride Well-Known Member

    Unless you're planning mass cloning a mh lightbulb is pretty worthless. There good in a pinch if your mh/hps, mv, or hps won't lite up.

    Destillat Active Member

    Why would you say a metal halide is worthless? Not only are they the preferred veg light, but studies show uv radiation found in halides, but not hps, aid in trichome production. Your post is seriously misinformed.

    As for the op, keep them at least 24 inches away until after the transplant. When the plants are acclimated to the environment, you can drop the lights inches away from the canopy. 4-6 inches if you have an oscillating fan between the light and the canopy.
    bird mcbride

    bird mcbride Well-Known Member

    If you check your spectrum a mh comes in at about the same as a cool white t5. A mv HID at 18/6 with 24hr daylight tubes are best for veggin'

    missnu Well-Known Member

    you put the light as close to the plants as you can without burning them...to figure this out you put your hand over the plants and then put the bulb as close as you think you can, and then wait and see if it starts to burn...if it does then move it up higher, if it makes you uncomfortable it will also make the plants uncomfortable.

    phillipchristian New Member

    I'm a little lost on this statement. First you say that an MH is too much for clone/veg and then you want him to use an HID and daylight tubes? Doesn't that seem like overkill. MH is the best spectrum for vegetative growth amongst fluoro, MH, and HPS. If you are vegging smaller plants or smaller numbers then I agree that an HID bulb is not needed but as soon as you atart vegging plants over 12-14" then MH becomes the most efficient and productive way to light them.

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