Hi guys
Advance warning:
I've been busted for pondering while stoned more than once.

All the references I have say that MJ is a termed a high light plant, i.e. it's hard to give them too much light. They go on to list coverage areas for various wattage HID lamps. Some of the grow guides left it at that - xxx watts covers yy sq. ft. But Ed Rosenthal mentions illumination levels measured in lumens per sq. ft. ...footcandles... and he goes on to say that gardens should be lit with 1,000 - 3,000 lumens per sq. ft. to obtain fast growth rates and compact plants. He also says that gardens lit with 3,000 footcandles are more lush and compact than gardens lit with 1,000 footcandles.

The foot print of my grow space is approximately 2.5 ft x 2.5 ft. I chose a 400w lamp based on it covering a 3ft x 3ft area. And I'm happy with the results it has produced.

...pondering begins...
A 400w HPS produces about 50,000 lumens. Subtract about 20% lost to absorption, leaving about 40,000 lumens. My grow space is 6.25 sq. ft., so it gets lit with about 6,400 lumens per sq. ft. More than twice the 'ceiling' that Ed Rosenthal listed.

This is where my question comes in - there must be some point at which the plants are saturated with light, and CO2 availability becomes the limiting factor in photosynthetic rate. I don't want to supplement CO2, but I want to consume as little electricity as possible.

Do we know where this point lies?
Is it truly 3,000 footcandles?
Does this imply that I could still get decent results by downsizing to a 250w lamp? It covers 2.5ft x 2.5ft, and provides right around 3,000 lumens per sq. ft. And would be less heat to boot.
But surely a 250w doesn't have the penetration that a 400w has.

If sunlight at midday on a clear day lights the earth at 10,000 lumens per sq. ft., shouldn't that be my goal if I can keep temperature within reason?

There's a a lot of information on this around the forums if you take the time to search.

I remember reading a number of threads around a year or two ago discussing light potential and limits based on luminosity and PAR, as well as conversations on the benefits of other (non-visible) light types.

For flowering, cannabis requires upwards of 3000L and will benefit and even prosper at higher amounts 5-8000, or possibly higher, however --YES-- there is a cut-off point:

Many growers speak of "bleached" or whitened and "burnt" leaves. This is the result of intense light destroying the chlorophyll and other cellular structures, much like paint fading in the sun.

--I'm sure there's more going on than that, but I figure someone will come along with a link, or time to think about it.

good question. i know one guy who uses 250w and does killer. but you might be taking a loss at the final weigh in. how much was your last harvest weight?
now think about it a lil more.id really like t see this space and how youve utalized such a tiny space.

Right. But not related to what I'm wondering about.

I'm wondering about the point where the amount of available light in the room exceeds the amount of available CO2 to fuel photosynthesis, and therefore, becomes wasted electricity.A. My garden has been getting a little over 6,000 lumens per sq. ft., so I know Cannabis thrives under higher levels of lighting.
B. Surely. Do we know what it is?? My hunch is that it's somewhere above 10,000 lm/sq ft, because that's what plants in an open field outdoors receive.I grow strictly for personal use, my harvest measurements are rudimentary at best, lol....last harvest was 4 Afghaan Dream in 5 gal containers that were 2.5-3ft tall. By the time it got to qt sized mason jars, there were 6 or 7 full jars plus about 1/4 - 1/3 jar.

I've been thinking about a perpetual harvest rotation, so maxing yield becomes even less of a priority for me. If I can truly cut the illumination back to around 3,000 lm/sq ft as Mr. Rosenthal says, and not see a slow down in growth rate or diminished bud production, it'd be way cool. Not only would it save me electricity money, it would reduce heat and place the grow space within ideal temp range 24/7/365.

But my gut tells me that cutting the wattage in half is gonna have a visible impact on growth rate and bud production. And bottom line, my lighting isn't broken, so why attempt to fix it?There's not much to see. It's a closet that measures about 2.5ft x 4ft x 8ft. The door is offset to the left side, so reaching around the right side to tend plants is a PITA; this space houses the oscillating fan and other electricals. The grow space is situated right in front of the door and takes up about 2.5ft x 2.5ft. The light is enclosed in a 6" cool tube suspended from the ceiling on chains with S hooks. The exhaust fan connected to the duct on the cool tube pulls fresh air in through vents in the door, across the light, then pushes it out of the closet via the attic.

I don't know all that technical jibber jabber but did you come to the conclusion with that 6,000 lumens per sq. foot after using the inverse square law?


I'm willing to bet you'll see a noticable difference between a 400 and a 250 HPS and you'll soon go back to the 400 unless heat is a big concern for you.


It would be nice to have 10k lumens per sq. foot but if electricity is a concern like you say you'll never attain that am I right since you are looking to cut costs?


I think you should look at it like this, how can I have the most efficient setup using the most of the available Co2 I have available cuz I've seen the tops of plants 2 inches or closer to a Raptor hood with no bleaching what so ever cuz the Co2 levels were 1500ppm. I know you don't want to use Co2 and I wouldn't really try to use since you're using air from the room-cooltube-inline-attic and it won't get utilized before its sucked out.


The biggest difference between a 250 and 400 would be the penetration and available light so you would have to adjust your growing style perhaps as anything more than a foot in the canopy prolly won't do to good. I do know that less light= less bud and cannabis loves light and fresh air or a sealed room and Co2.


Here's a chart but you prolly seen it already

I just did the calculations and I'm sittin at 6000 lum per square. I was under the thought that 3000 lumens is where the base of healthy growth starts.

I really think it is improbable for one to think they can reach the limit with or without Co2. Besides the heat the cost would kill you.

As for going with less lumens, Im sure you can produce some killer weed with less lumens, people do it with cfl's, but you must realize that as lumens drop so does size and density.

I know a couple of guys that flower like crazy in the winter and shut it down in the summer. They have figured out that they produce just as much either way and figure that they save money by doing it this way.

I comes down to your situation, experiment a little, its fun.

I wouldn't put too much weight on stuff like lumens per sq ft. So many variables, I always say experiment too, and it totally applies here.

My own setup, in my veg area, about the same size, I went from 600w to 400, with no loss in growth rate. 81-82 degrees, dwc, about 800ppm co2. Very good growth rate, not affected by dropping to 400. (I wish I knew that at first!) Now I'm about to drop it even more, to 250w, see what it does.

You easily can have too much light.

Btw, I recommend you try a sun pulse 3k halide, 400w, (dig ballast) you'll probably be quite amazed at the result.

I had a similar thought, that's what spurred my thread about increasing light later in flower.
I noticed the 10 plants i had under a 400(digi) were looking as good (if not better) than then 10 i had under a 1000(mag) (both in veg).It made me doubt all the articles,books, graphs and seasoned growers who swear by pounding them with lumens equals the best results.

Of course.
Busted again.

Here's my previously mentioned veg setup that I started at 600w, went to 400, and now dropped to 250. It's about 3' wide x 2.5' deep. I don't really know how much less, if any less growth it's producing, it's just filling in like always. I can't believe I ever had 600w above this. I'm really considering dropping my flowering wattage to 400s now.