VPD and DLI are prob the 2 most important things in a growroom to manage. And climate being the most important one (VPD), but don't get me wrong, even when you have a perfect climate you can easily fck up the plants by simply applying a wrong DLI.
Many growers nowadays make the mistake to use a high efficiency LED light, with about 2,7-3 umol/J that is made for growing photoperiods the same way for growing auto's. Simply said, using a 300w LED in a 80x80 or a 500-600watt LED in a 1x1 or 1.2x1.2 is prob perfect for a photoperiod, using the same wattage/light distance with an autoflower on 18/6 20/4 or even 24/0 will completely fck up your plants. The amount of photons is simply too high and will have damaging effects on the tissue/leafs, resulting in many many issues.
You would be surprised how many customers come back with similar issues related to light stress to us, simply because they are blasting them (young)plants with too many photons! It is the same as we North Europeans travel to the South of Europe or even worse, to the Equator and lay in the sun all day, I know when you're on holiday you can easily pick out the people from the UK/the Netherlands because those white skinned bodies cannot handle them high light levels!
An easy way to tackle this is simply by changing the power of the LED (if you have a dimmer) to match the extra hours that you use for an autoflower. For example, if a manufacturer says the LED light is perfect on full wack on about 60cm to the canopy, let's say 500watt in 1.2 x 1.2m tent with photo's during flowering = 12x 500w = 6000w on a day, growing auto's on 18/6 will result in +50% light hours per day (which is 3/3 of the
2/3 of photoperiod) resulting in = 6000w /
18 = 333w. so you run the lights at 2/3 of the maximum power, about 66,6% at the same light distance. And don't ever go higher in terms of wattage, period.
Most importantly, having in mind young plants need less light, you should start the power of the LED very low, prob at 10% with seedlings, and turn it up a bit every other day/week, depending on how big the steps of the dimmer are (an if you only have let's say 20-40-60-80-100% steps), you should play with the light distance a bit to increase/decrease light levels. At around 4-5 weeks in its lifecycle, maybe you can have around 40-50% power of the max power of the LED with plants handling that without issues. but rather go slow instead of wanting to give more light... because light can be very damaging during these early stages too, but it's harder to spot and often looks like nutrient imbalances...
During the grow when you see leaves starting to turn light green, with darker veins, starting on the leaves on top of the plant, you might be looking at the start of light stress. A light meter would definitely help so you can easily read the PPFD value plants are receiving.
Clones and seedlings can do with 50-150ppfd, veg plants are usually good between 250-450 (up to maximum 600ppfd for light feeders) and in flowering somewhere ranging between 600-900 ppfd. Around 1000ppfd, the higher you get the more the plants can be stressed, resulting in bleaching of the buds, I have grown strains also on 1200-1400ppfd without CO2, but these were lights with more green/orange/blue in their spectrum than full reds, because plant cells are easier filled/damaged with Red light than with 'white' light (more green/orange/blue diodes) that looks more like the spectrum of the sun.
On a sunny day outdoors (depending on where you live obviously) some days plants can get 1500-2000ppfd even, however, these wavelenghts are completely different to the LED's we often use indoor, no one can mimick the sun's spectrum yet.
Anyway, I'll leave it at that, because I can keep rambling on lol
I was a driving part behind an LED company we ran with a part of the DP team so I read my fair share about LED's and its technology in the past 10yrs. And all those manufacturers didn't have a clue about growing cannabis, or all the different variables people grow with, so we had to teach them actually...
Back in the day it was way simpler imo, oh you are using a 400w HPS? Have it around 40cm above your canopy, a 600w? 60cm, a 1000w, 100cm. Autoflowers weren't even part of the game. But now with all the new lights with that high efficiencies, different chips (cobs yes or no) and different spectrums & spread, different genetics with different light needs there are simply too many variables to come up with a one-fix solution.
