Not so sure, really not so sure about this because I already tried with HPS - and it does work. But it's not as clear-cut as should and this is mostly due to the plasticity of living organisms - I'll try to explain in detail:
What is needed is a distinct separation of the "day" & "night" phase - this is usually reached by simply shutting the lights out completely. In this case it doesn't matter much whatever light you use because a plant can easily distinct between (light vs no light).
However, this is even false for long-daytime-flowering-plants (=sativa-genes) because the relative lightstrength is "weighed" also against a plants physiology esp. age & height.
Or to be blunt: If a big sativa doesn't receive enough lightstrength it'll flower because it "thinks" autumn/monsoon has arrived - even if said light is on for 18 or 20 hours. There are multiple examples present in various hemp forums which I could cite as proof (happened half a year ago to a guy growing a "Santa Marta Colombian Gold" sativa strain...)
Secondly, IF there is light during the nightphase then the day HAS TO BE stronger in luminosity - but I'm not so sure by what factor. Your quantum boards are obviously much stronger than the 660+730nm combo. Especially since you leave them on during the day, too - isn't it?
[snipped]
Let's swiftly examine the HPS & S-HPS spectra:
HPS:
S-HPS:
The amount of UV & blue is much bigger in the S-HPS while the HPS has green instead, and no UV at all. And I guess the tiny amount of blue in HPS isn't much noticed (or superceded by the other wavelengths....) esp. since one needs to keep a HPS fairly away from a plants tops. So the distance alone is going to deprecitate that unwanted blue... (
still, it may pose a risk !!)
There are also other things to consider, esp. that some strains need different lightcycles - and I have to be especially thankful to
@Dr. Who who explained some of this in great detail to me:
1) Indica dominant strains may need a shorter daytimephase than the nighttimephase so 11/13 or 10/14 should be the target
(just a sidenote, actually I found that the transition from veg to flower happens much swifter if one goes 10/14 right away.)
2) Sativa rather need a continous decrease in the daytimephase, 18/6 --> 16/8 --> 14/10 --> 12/12 --> 10/14 (and so on... most likely taking an hour or half an hour off each week - depending on the estimated total flowering time....)
(on a sidenote, this behaviour is sort of also integrated into the Gas Lantern Routine as well, "Flower Cycle"
Here's an example of one of the self-fashioned lamp I use for a combo MH/HPS dual combo:
(costs were around 250€ for 2 sets sporting both Reflectors & Digital Ballasts, plus two new bulbs.)
Lesser powered bulbs can be bought for even less and it's godsend these digital ballasts can drive different strengths, so one can save some electricity duriing the early phase of plant develoment)
(still needs to work a bit on it - reflection at the sides to decrease lightloss....)
***
Then going from here there are many possible light-scenarios that could all work, e.g.:
Indika or short-daytime plants: (tested and works)
HPS - 24/0 (always on)
MH - 16/8 (veg) and 10/14 (flower)
(keep in mind we already give the plant more light due to the nightphase to hit up the Daily Light Integral; DLI)
Sativa or long-daytime plants: (this is
untested)
HPS - 24/0
MH - 20/4 then gradual weekly reduction - starting well before flower (mid veg)
(there are strong genetic factors at large indicating in order for this to work the growroom would need to be
cooler in ambient temps than during the veg phase...! This could for example be artifically created by heating up the tent during the nightphase in veg with
IR heat lamps - and their shutdown would indicate the beginning of flowering as then the nightphase would be relatively cooler...)
***
Also tested & found working:
HPS - 20/4
MH - 12/12
(I did veg plants with this and the whole tent went into flower, small 3 week young plants! Multiple strains different strains)
This is actually a pattern which I would recommend to use for most
hybrid 50/50 strains...
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Another thing to consider is maybe to still give the plants, at least, a short *true* nightphase in flower, like the last pattern illustrates; either ending with a 730nm End-Of-Day treatment or leaving the 730 monos on but shutting the 660nm off.
With EOD the plant should get the same
*respiratory* phase than a 18/6 veg phase like when not ending with FR. (4h true respiration)
https://de.wikipedia.org/wiki/Photorespiration
I've ran the last pattern in different fashions:
1. Starting the day with MH, after 6h powering the HPS, after yet another 6h cancel the MH, after again another 6h terminating the HPS as well, then 6h of total darkness. (for a total of 12h MH + 12h HPS light usage)
OR:
2. Starting the day with HPS (dawn-simulation), after 3h powering the MH (full daylight), after 12h terminating the MH, after 3h also terminating the
"dusk-simulating" HPS, then 6h true night. (for a total of 12h MH
but 18h HPS light usage)
Both work, but the last pattern is more true to what's happening in nature, also delivering more photons. I consider the 2. a bit more gentle on plant physiology because the heat emmisions from the HPS may warm up a plant and prepare for the rough bright blue/UV.
