Why the least?

It depends on a number of things, most importantly what kind of results you expect. While you could keep a plant alive with a 60W incandescent, why would you?

This question cannot be answered meaningfully without more information - how big, what kind of growth you want ... why you would only grow one plant? I would assume when someone says 'the least' they are concerned about spending a few bucks on light. I cannot really identify with that being my primary concern, as opposed to, say, rate and quality of plant growth.

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Lumens, lux, and adding it all up :: 16K HPS vs. CFL :: DIY Drip Irrigation :: Pack water transport

I think the question is more in terms of 'what is the minimum requirement for vegetative growth'.

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i think that's the same question, really.

You can grow a 2" plant under a 13W CFL just fine, but what kind of growth does that support when it gets to be 6", 12", or 18" ? Not much.

When I was a kid I grew a stringy plant under an incandescent. Would I do it again? Hell no.

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Lumens, lux, and adding it all up :: 16K HPS vs. CFL :: DIY Drip Irrigation :: Pack water transport

Ok, so let's complicate things a little more, from an outdoor grower's perspective. Surely sunlight has different qualities, as such, these qualities should be able to be quantified, scientifically. Is this an erroneous assumption?

What I am having a difficult task of determining is how important sunlight at time of day is to the plant's ability to photosynthesize. I am limited in that I can only grow on a southwestern-facing area, where square footage that is exposed to sunlight is becoming an issue. It gets direct sunlight, but later in the day.

If you like I will post pix to help illustrate what I mean, but essentially I've got some areas that are only getting direct sunlight at about 38.4*N-lat. (I think I've got that right) from anywhere between 2:15--7:30pm (guessing on the last number), and for the upper tier from 3:30pm--7:30pm. Is that going to be sufficient sunlight of sufficient quality? This is what I have been unable to ascertain with any precision.

I have just separated the plants based on perceived phenotype, in other words, what I perceive to be an Indica or Sativa dominant strain, or a dominant combination of the two. Completely arbitrary, other than based upon what I've read and seen now that I think about it, but, there ya go. I figure if there are differences in their needs based on that phenotype or preferential expression, maybe it will help me best meet them.

Maybe I'm looking in the wrong way (asking the wrong question), yes?

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If that's the only light it's going to get then no...you need at least 12 hours of sunlight a day for proper growth...put the plant inside and grow under floruro's or cfl's for the time it's not getting any sunlight. I utilize the sun everyday i can...but i'm lucky and have the sun shining on my porch from 8:30am till about 6-7pm, and then the remainder of the time i stick them under my flouro's. The reccomended is a schedule of 18 on, 6 off, i myself now am doing a schedule of 16/8.

Hm.. I've read in a couple of books and on this site that the minimum for direct sunlight, outdoors, is between 5-7 hours. Not 12.

Either way, I can't keep up the electricity on extra lighting, so that won't be happening. If I have to move the plants down below or reconstruct the shelving they're on, that's what I'll do. The light they're getting from 2:30pm on has been enough to give them some pretty good growth. It's the upper level that has me concerned (but also gives me floor space). I will be culling soon because I'm finally seeing some definite pre-flowers, some of which are inmistakable. If I end up having to cut out enough plants, then floor-space won't be a problem.

My question really has more to do with the quality of light during that time of day, as the more information I can tease out, the better I can adapt conditions for the plants I'm growing, and vice versa.

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Quality?...ok...it's the sun, isn't it the same all day? I would think that the coolest time with full sunlight would be best, as heat will stress your plants and stunt them, and it only gets hotter throughout the day, so by 4-5pm it's the hottest time...at least where i am it is like that in the summer time.

Yeah, that's exactly what I'm reading that most growers agree on, morning (eastern) light is better if it has to be cut up, so to speak. But, I can't do that, or almost all the neighbors would see what I have.

I don't think the sunlight's quality is the same throughout the day, though. You hit on one point that's coming up for consideration as things heat up, too. I've got to strike a balance between giving them the light they need, and protecting them from getting overheated (which, I've read, is one of the stressors that can lead to hermaphrodism in female plants, but no one's ever complained about their male going female).

Now, where I used to live, by 3-4pm you were getting the hottest part of the day (further south than I am now). Here, the days are noticeably longer, and at 5pm the sun is still pretty high (sometimes, so am I). On one hand I'm fairly certain it's good quality light (sufficient brightness, etcetera), on the other I am concerned that during this vegetative phase it may peter out too quickly before my garden is ready to flower. Btw, I've just begun the sexing of them, haven't been able to find pre-flowers before and now I am. If ONLY I could take pix through my jeweler's loupe.

Anyway! While the plants are growing I think about this shit and let it worry me.

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I think there are two major questions entangled in one another here. One of them I can answer, and one I can only speculate on.

1. What is the difference - if any - between receiving constant, weaker light, and the same total amount of light through some time-dependent intensity that depends on where the sun is in the sky?

From a photosynthetic perspective, this shouldn't matter. The flux of photons is so relatively sparse that at intensities we observe in nature, the plant's energy conversion machinery should not be overloaded. I've had to do the exact calculation for solar cell work, and there is very very little chance that two photons hit the exact same spot, so they don't physically interfere. If you jacked up the intensity, you would run into the problem that the supporting pathways for the carbon cycle simply could not keep up the rate of light conversion - assuming you had enough CO2 present to keep it running. As we have generally observed, however, along with the important wavelengths for photosynthesis, we also get IR, so before this happens we would get the leaf burning that we sometimes observe. If it weren't for these limitations, we could give a plant its day's worth of light in a minute! That sure make a light mover VERY efficient!

So assuming you have an intensity vs. time situation that looks like the attached plot, you can figure out the total energy you are giving your plants as being proportional to the area under each one of those curves.This again assumes that you're not spiking the energy so much that the plant can't digest what you're giving it. Given even total energy input, the plant has done the same amount of work to convert the light energy.

This is essentially the same principle light movers operate on: same total energy over a different amount of time. The short burst avoids the heating problems of the IR.

2. What affect do the differences in light distribution have on the plant?

This I can only speculate about. The fact that you're seeing preflowers doesn't surprise me. I've got outdoor right now that is in full-on flower. This alone defies the notion that if plants receive more than 12 hours of light - any light - that they will remain vegetative.

I would imagine that the response is a result of the decrease in total light that it has received, assuming it came from indoors. What makes sense is that the plant adapts to whatever environment it is in to grow vegetatively, and as the sun wanes - from whatever its maximum is - the plant responds by flowering. This would also explain why it is strain-dependent, as different varieties have evolved to thrive in completely different local environments.

I'll think more about this, but I think this is a case where - unless we have a botanist that knows what he/she is talking about - the experience of outdoor growers trumps for predicting how a plant will respond in terms of if it decides to flower or not.

Hope that helps.
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Attached Images

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Lumens, lux, and adding it all up :: 16K HPS vs. CFL :: DIY Drip Irrigation :: Pack water transport