I've grown autos and photos. I don't grow autos any more because I found they grow too large. I grow in a 2' x 4' x 8' tent and, since I can't flip them into flower, which stops their vegetative growth, they end up growing very tall. With photos, I can flip them at day 40± and that allows me to keep them smaller.
Two Gelato autos. These ones were pretty well behaved but I had to use my Vipar XS 1500 to light the "front row" of one of the plants.
Nice compact photo. 30" across, 20" front to back, and about 24" tall.
Light levels - autoflower cannabsi plants are cannabis plants, usually hydrbrids of cannabis indica and cannabis sativa, that have some genes from cannabis ruderalis which allows the plant to not need to be flipped go into flower. Astute readers will note that all of the plants are "cannabis" and they respond very similarly to light. Cannabis
is a light whore loves light.
I grow all of my plants at >1kµmol because there's a direct, almost linear, relationship between the amount a cannabis plant receives and crop yield. One researcher found that there's roughly a 4.5% increase in yield for every 50µmol above 600µmol that the crop receives. I created the table below from the paper, which is cited.
In Mitch Westmoreland's YouTube videos from earlier this year, he discusses yield, among many other excellent topics, and states that the amount of flower yielded in a cannabis plant is 0.2 to 0.3 gm/sq meter/mol. As the plant gets more light, it will be able to photosynthesize more which allows it to generate more glucose which allows it to grow more. I liken it to pressing down on an accelerator pedal in a car. The engine will run faster, up to a certain point. In an engine, it's the red line; in a plant, it's the "light saturation point". When growing autos, I hit the LSP at around 1kµmol but for my two photo grows, the LSP was 1100µmol±.
The difference between plants of the same strain should not be significant. The LSP is "800 to 1000µmols" but I've not done a grow that tops out at 800. Since I've only done 6 or 8 grows, that's not surprising but you might run into a strain that can't handle more than 800. So be it.
A big point to remember is that the PPFD for one spot not the canopy may well be 100µmol different that a location just a few inches away because a canopy is not perfectly flat and even the best lights don't have a perfectly flat PPFD map. That's why you can get a plant to, say, 1kµmol in some places but, in others, it will be 900 and 1050. And then you'll take a light reading in 5 minutes and that 1k spot is now reading 960 because you're holding the sensor 1" away from and 1" below the original spot.
Lighting on a canopy is "an area weapon", so to speak.
PAR meter? If you've got $300 burning a hole in your pocket, check out the Spot On PAR meter. Shane sells it and, while I have reservations about the light output of his lights (his drivers tend to be lower wattage than his competitors which is why his lights don't generate PPFD's in the 1k range) he seems to be a good company to do business with.
I use an Apogee and have a Uni-T lux meter. Unless you have to have an "unimpeachable" source for your PPFD readings, you can get close enough with a lux meter and a conversion factor. I've attached a document I wrote that will help with the conversion factor issue. In most cases, 0.015 is a good number.
Whether it's a PAR meter or a lux meter, the meter will just tell you how much light is falling on a plant but the plant will tell you how much light it can use. Those are, usually, not the same. The advantage of a PAR meter is that it makes it easy to sample a canopy and record the data. This is how I record PPFD for a canopy in veg.
With the data in a grid pattern like that I can add plant hangers to even out the canopy.
Re. DLI for autos - I've grown mine at 70-80 mols but photos only up to about 42, IIRC. Yup, 80. For the auto in the picture above, I think I turned the light down to 850±µmol for the last few weeks of the grow. One reason was that the tent was getting pretty warm. Second reason was that there was so much weed in there that it just didn't make a difference.
Yeh, the big plant was Chris (Evert) and
Wilma (Rudolph). The latter name was because the seedling was a lot smaller than the other one.
Re. bleaching a plant with an LED — Bugbee talks about flowers that are bleached. Bleached colas aren't, per se, a problem but they look strange. He's found that flowers get bleached under high amounts of light when the lights that are being used have very high amounts of red. I use a Growcraft flower light at PPFD's and haven't see that but there are other lights out there are have a higher % of red.
Re. giving a plant so much light with an LED that it damages the plant. That's
not easy to do. I end up giving my plants too much light almost every grow but the plants respond very quickly (canoeing/tacoing or rotating the leaf around its petiole like a Venetian blind) so I turn the wattage down a bit or raise it an inch. The only permanent change I've caused was that one large cola bent at the top and never straightened back out. Other than that if you set a new light level, check on the plants in 30 or 60 minutes and then an hour later and see how they're doing. If they're reacting negatively, just reduce the light level a bit and they should return to normal.
How quick to ramp up? There's no reason to gradually increase, not as far as the plant is concerned. Light is how plants make food. Feed your plants well.
In one of his video interviews, I think it was Mr GrowIt who asked Bugbee that question. There were two parts to the answer. In terms of mimicking the sunrise, there's no value in that. "Cannabis is ready to go to work as soon as it gets out of bed in the morning"- that's not an exact quote but it's really close. What Bugbee didn't say in that interview but he has published elsewhere is that, while cannabis is ready to "go to work", it takes a few minutes for photosynthesis to ramp up, so light schedules that alternate short periods of light and dark aren't quite 100% efficient because of the ramp up time.
The other issue was about taking a plant from 600µmol to 1000µmol. Per Dr. B, that's something that you should be able to do in two days. The key point is to watch the plant and see how it reacts. It's really hard to harm a cannabis plant with too much light from an LED. Sure, if you turn it uo to 11 and go away for the weekend, that's not too a good idea but if a cannabis plant is at a modest light level and you add 200 µmol, you should be in good shape. If not, go through the checklist and figure out why.
The only time I've seen plants not be able to handle 800-1000µmol was due to poor watering practices. In both cases, the growers had hygrophobic soil. With those being the exceptions, you should be able to get a grow to at least 800µmol. If not, the diagram below can help troubleshoot.
