It is indeed species specific and has been studied extensively. See Cannabis Sativa

quirk said:

It is indeed species specific and has been studied extensively. See Cannabis Sativa

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Conversion

Very nice. Do you have a chart with the various species? Sativa, Indica, Ruderalis? We might be able to infer the perfect number from that.

quirk said:

It is indeed species specific and has been studied extensively. See Cannabis Sativa

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ComfortablyNumb said:

Newbies hear it all the time, if you want taller plants, raise the light, or dim it. Why?
Light Intensity. Light is made of photons. Photons all have the same amount of energy.
The farther away the light is, the more it spreads out. The more spread out, the less intensity.

Your plants have an intensity that they really like. When you are below that level, they grow taller to get more intense light.
When you are above that level, they slow way down so they don't grow out of the plants favorite intensity zone.
When you are way above that level, the plants begin to burn and suffer.
When you are way below that level, the plant will almost stall and grow just enough to stay alive.

What is that magic intensity level for my plants?!
Nobody knows yet. I'm sure there is ongoing research on this because if you know the number you can have your plants on the bleeding edge safely all the time. Dr. Bruce says it can change depending on how much you feed or water.
The number is likely species dependent with strain variations.

So, if you want taller plants, lower the intensity.
Shorter plants, raise the intensity.
Your dimmer switch changes the amount of photons.

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Hmm…

Light travels in straight lines (unless you're talking about it moving through curved space). Photons, which have different energy levels, lose energy any non-vacuum medium.

Plants in low light environments will tend to stretch. Plants in high light environments will tend not to stretch. Plants don't stop growing because they don't want to leave a "zone" - plants will grow into a light.

I'm not sure about "magic intensity" is but the light saturation point for cannabis is 800-1000µmols in a non-CO2 enhanced environment. That value varies among different strains and it varies with environmental conditions.

The light compensation point for cannabis is 64µmols - below that, a cannabis leaf consumes more glucose than it generates and, if the entire plant is receiving only that level of light, it will die.

If you want taller plants, you can increase your light levels or use a light that does not generate blue photons. Blue photons inhibit cell elongation which is why "veg lights" are used - they grow plants that are short and bushy. If you reduce the amount of light that you're giving your plants, plant quality and plant yield will suffer.

If you want shorter plants, use a veg light which has a high percentage of blue photons. However, the more blue you use during flower, the more your diminish crop yield. Per Bugbee's research, for percentanges of blue photons between 4% and 20%, crop yield was reduced by 0.77% for every percentage of blue photons above 4%.

Characteristics of cannabis plants that are not exposed to "enough" light are that they are tall (stretching for the light), they don't have a lot of leaves and branches, they have significant internodal space, and they suffer from diminished yield.

Conversely, plants that have been given "high light", which Bugbee says starts at a DLI of 45 moles, tend to be more dense (more leaves and branches), are shorter, have shorter internodal space, and have superior plant quality and crop quality.

I've done my three previous grows using a Mars SP 3000 or a Vipar Spectra XS-1500, each one with a blue LED puck, for germination and using the Mars for veg. In all cases, the plants grew well because they got good light. For my current grow, I had to use the XS-1500 + the puck for the first week of seedling but, after that, the veg light has been a Growcraft X2. The strains are different - Gorilla Glue previously and Strawberry Pie for this grow. The difference in the density of the leaves and branches is amazing. The plants under the X2 are so dense that it has been very hard to get "into" the canopy to top the plants and do LST.

There are only so many photons emitted from a grow light. The further away, the more spread out they become because, a) the emission surface is not perfectly flat, b) atmosphere can and does deflect, or dare I say, reflect photons. Intensity is a measurement of the number of photons hitting the plant. The further away, the lower the intensity.
Atmosphere = All the components of the "air" including any airborne particulate matter.

ComfortablyNumb said:

There are only so many photons emitted from a grow light. The further away, the more spread out they become because, a) the emission surface is not perfectly flat, b) atmosphere can and does deflect, or dare I say, reflect photons. Intensity is a measurement of the number of photons hitting the plant. The further away, the lower the intensity.
Atmosphere = All the components of the "air" including any airborne particulate matter.

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Agree completely.

I'm trying to get my canopy to be flat. That would make things much easier. :-)