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Grow lights for greenhouse

I'm looking to supplement sunlight in my greenhouse as it doesn't get full sun. Ideally I want to put a light or lights up on the roof joist but everything I've seen online shows grow lights a few inches from the plants.

I want them up high out the way, this is what I was looking at. 

Is that strong enough to reach from about 3-4 feet away?


  • Hi Kweegly

    I have a little experience of growing tomatoes under artificial lights but I am no expert.

    Unfortunately, from what I understand, the LED lights that are of interest to you are too weak to be deployed anywhere but very close to the plants.  Perhaps as close as 15 cm or less. 

    This is because the brightness of a lamp decreases according to the square of the distance from the target object. In other words, the strength drops off really fast in a non linear fashion. 

    As a result, even a powerful 1000w bulb  would still need to be close to the plant, at which point heat then becomes a problem. 

    One option would be to use several large bulbs and hang them up high, but the cost of running them would be sobering and you would be also be wasting most of the lumens because you would get the same result by using a lower output light and mounting it much closer.

    In that case, you could mount the lower wattage lights on pulleys and lower them when needed and raise them when not.  Whether that can be done in practice would depend on the strength of your greenhouse frame and whether there are suitable places to anchor them.  Such fixings need not be very fancy as those LEDs probably wont weigh much. 

    Also, electricity and damp environments is something that needs careful thought, so that is also another issue to be considered in a greenhouse.

    I used to love the challenge of growing plants under artificial light and only stopped because it dawned on me how much each individual tomato was costing me in electricity, fun as it was!

    To my mind, if you want to grow things under artificial light, you would be better moving all the plants indoors and doing it there, where you will have better control over other factors that influence plant growth, such as temperature and humidity.

    If all that sounds like too much hassle, perhaps it would be easier to simply move the greenhouse :D




  • KweeglyKweegly Posts: 101

    Thanks for the detailed reply Tim.

    The greenhouse is two big and heavy to move, this is my first growing season so I guess I will see how it goes. Direct sunlight only lasts until around 1pm then its cast in shade, its not the best place for the greenhouse but it was my only spot free.

    I might put the lights lower down on the wall and give it a go, I looking forward to the build as I also want to make a solar battery bank to power them for a few hours.

  • Pete.8Pete.8 Posts: 11,126

    You can always use fluorescent T5 Hi output tubes.

    There are many systems available for tropical fish tanks. I use 4 x 50W T5's for my aquarium.
    The tubes no not get hot like a light bulb so they can be placed ~10cm above the plants. A good reflector is important.

    I get mine from here

    The choice of tube colour is up to you. Plants are not really fussy (with the exception of green light), but prefer light in the red/blue spectrum. I have a combination of Tropical and Blue 50:50 which gives good growth. Avoid the UV colours as these are for corals.

    Billericay - Essex

    Knowledge is knowing that a tomato is a fruit.
    Wisdom is not putting it in a fruit salad.
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