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light intensity for "full-sun" plants

Posted by joeschmoe 6 (Ohio) (My Page) on
Tue, Jan 8, 13 at 9:50

Measured in footcandles, is there a good rule of thumb (using, say, metal halide) as to how intense light should be?

I realize they say full sun is "10,000 foot-candles" but that most plants max out at a lower number as far as what they can use.

So is it maybe 3000 or so FC that I would shoot for if I were growing "full sun" plants?


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RE: light intensity for "full-sun" plants

This is hard to know precisely because there are many variables that come into play (distance from light to plant, light coverage, reflection, etc.), not to mention the imprecise definition of the word "full-sun" which refers to an uncertain intensity of light energy (varies by angle of sun in sky, cloud cover) over an uncertain period of time (usually a minimum of 6 hours but as many as 12).

With indoor growing, you can make up for a lack of light intensity vs. the sun by extending the period of time you provide light energy because energy is cumulative over time, though some plants require a cyclical dark period for blooming, etc.

I am not familiar with the footcandle measure but just to give you a ballpark estimate in lumens, full sun conditions are roughly approximated by 30-40K lumens trained perfectly on a plant (with limited light diffusion).


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RE: light intensity for "full-sun" plants

10,000 is too much - strong mid-day sunlight actually INHIBITS photosynthesis - 3000FC should be good. if you use PAR - 1000 is above as high as you want to go, which is about 5000FC. The plant is actually more efficient with less light x longer time, but it needs a certain minimum, or your internodal distances start to stretch.


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RE: light intensity for "full-sun" plants

I used to grow citrus under a 1000W MH lamp. At around 4000 foot-candles of light on the topmost leaves (per my cheap lightmeter), I got blooms on citrus, so that was likely enough for them (kept on 15 hrs/day).

For what it's worth, since citrus are tall plants, the lowest leaves got around 1000 FC or so, so I worked to "overkill" a bit on the topmost leaves. A low growing plant would have probably been fine at 2500 - 3000 FC if it had similar light needs to a citrus.

To get 4000 FC, the light was maybe three feet away from what I remember. This is with a rather standard reflector on my light.


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