In my experience, yes strains can stretch during the start of 12/12 under LED and HPS.
I find that it depends on the strain and other factors (like difference between daytime and nighttime temperatures, overall light intensity, what method you are using for growing, what you’re feeding, etc.)
I’m not positive but I think it was KIS organics podcast episode 127 “flushing and photoperiod research with Michael Alden” that they talked about what light spectrums cause more stretch vs more compact plants. Good episode either way though!
Look into what different light spectrums do, (more blue, red, far red, etc.), and that’ll answer your question better, because especially with LEDs, spectrum varies a ton. I think it was grandmasterled that put out some lights recently with full spectrum tuning capabilities
Ive not done heavy sativas under led but no stretch to talk of on hybrids or indicas. I am glad swapped from hps fo lots of reasons now. Temps, light spectrum for inspecting plants, yield, electric etc. I should say though I do run 12/12 from seed so the plants are not fully mature when they flower.
I have not found this to be the case at all. I get vegetative growth as fast and vigorous, and in most cases better than I did when I ran HID lighting. I wonder what factor is causing you to have a different experience.
I found stretching and node-spacing wasn’t great until I started accounting for PPFD. This and consistent magnesium supplement I found really helped anything under LED’s.
PPFD is photosynthetic photon flux density . PPFD measures the amount of PAR that actually arrives at the plant, or as a scientist might say: “the number of photosynthetically active photons that fall on a given surface each second”.
There are 86,400 seconds in a day (3600 x 24 hours) so the daily PPFD can be calculated directly from the instantaneous PPFD from the following equation: µmol m-2 s-1 multiplied by 86,400 and divided by 1,000,000 = mol m-2 d-1.
So: 1 µmol m-2 s-1 = 0.0864 mol m-2 d-1 with continuous light
These conversions are accurate only under electric lights. The integrated daily PPFD from sunlight changes during the day so a single measurement cannot be used to estimate daily PPFD.
"PPFD is photosynthetic photon flux density. PPFD measures the amount of PAR that actually arrives at the plant, or as a scientist might say: “the number of photosynthetically active photons that fall on a given surface each second”.:
Once you know your PPFD you can then dial in your DLI…
…which is:
Daily light integral (DLI) describes the number of photosynthetically active photons (individual particles of light in the 400-700 nm range) that are delivered to a specific area over a 24-hour period. This variable is particularly useful to describe the light environment of plants.
It’s more $ to spend but if you get a PAR meter you can then find out your PPFD and then adjust for your DLI. (PAR is photosynthetic active radiation. PAR light is the wavelengths of light within the visible range of 400 to 700 nanometers (nm) which drive photosynthesis)
It’s all very scientific to get the max out of a plant by ensuring it gets the highest amount of usable light before burning or wasting.
Here’s a crazy chart showing how someone mapped this all out:
As an example; in my 4x4 the lights are cranked at a PPFD of 1050 for flowering and at 12/12 lighting period. In my vegging tents they are dimmed down to 250 at 24/0.