Optium Conditions for Indoor Growing

[Oldmac]

I really wasn't sure where to post this. I usually get involved answering question's of this sort in indoor hydro since that's what I do and know best. Personal grow is 3 stage RW and a partnered grow is aero/fog. But this information pretains to all forms of indoor growing so I decided to place it here, "growroom design & equipment" since designing and specing a growromm should revolve around this info. You should also know that some of the information I will provide runs counter to a lot of info being passed around the boards as "gospel". I hope that this will help some people and get other's to rethink some strongly held but dubious beliefs.

I'm going to start with a research articule, that I've posted before at various times, and try to break it down into easier to understand english. This particular paper comes from the wonderfull folks at the National Center for Natural Product research, School of Pharmacy, University of Mississippi, MS (the folks who brought you G-13) and this study was funded by National Institute of Drug Abuse (NIDA), USA, contract # NOIDA-0-7707. (you can't make this shit up)
Yes this is US taxpayers' dollars at work, we might as well take advantage of it. Besides downloading it, you really should print out a hard copy, so that you can study it at your leisure. I'm sure it will seem a little overwhelming at first, but like I said Oldmac is going to try and break it down for you.

 

[Oldmac]

First let me give an overview. This paper was done to benifit other reseachers of marijuanna and to promote the concept that growing it indoors was a way to circumvent regulatory problems, sounds like some of us already. It was designed to figure out what are the optium conditions, for max growth and to insure genetic integraty and potency.

They used a mexican sativa mother and used clones for this study. They maintain a relative humidity of 55% thru out the grow, but they and I will address this at conclusion.

They start with amount of light, PPFD which is photosynthetic photon flux density, a fancy way to talk about light intensity that a plant can use. This is one of the more complex issue's since it is difficult to calculate all the various light sources that we use into that. They use umole m2 s-1 (micro-mole per meter sq per 1 second) and again other then KNNA, I don't know anyone who can take different light sources and come up with at least a close approxamation. But to put it into prespective if you were to take 2-600w high par HPS bulbs and put them over a 3'x3' area you can achieve 1500-2000 umole/m2s-1 depending on how high above the canopy they are placed. That's the best I can give you as a starting place.

This study showed that maxium photosynthesis (Pn) occurs at 1500 umole at 30C or 86F, and drops off after that, unless temps are at 25C or 77F in which case 2000 umole is peak. Obviously using the peak temp of 86F takes less light and the actual peak photosynthesis is greater @ 1500 then 2000 at 77F.

For our purposes, the sweet spot shoud be considered 80-85F since photosynthesis drops off sharply at temps above 86F and is much less below 77F.

This also leads to the plant's temperature dependence for dark period respiration. This is a fancy why to describe how well the plant uptakes water during the dark period using what they call WUE (water utilization efficency). Obviously a plant uptakes much of it's water needs during the dark period and that is dependent on how well it is doing during the photoperiod growing. Now this is a surprising temp for most people, it mirrors the day time temp needs, 77F-86F. While most of us believed that a plant should be cooler at night, that's just not neccessarily so. But the reality of a grow room is lights on is hotter, lights off cooler. Again looking to compromise but still be in a sweet spot dark temps of 77F to 80+F are ok, but again don't want to go below that min temp of 77F.

[since I'm pretty much computer illiterate, and not capable of "paste and copy" as much as I'd like to do this all in one post it is not possible] I will continue this a little latter with CO2 utilization vs temp.

OM

 

[rasputin]

Interesting. Not the first time I've come across information suggesting the lights on temp is ideal at mid 80's. Didn't know that bit about photosynthesis as it relates to different temperature, specifically. This has me thinking. I usually aim for low 80s in veg, mid 70's in flowering. I basically set-up the room in flowering to avoid temps over 80 at all. Looks like I should tweak that a bit.

You've piqued my interest on the suggestion about night temperatures. Bit different than the conventional wisdom that says to keep the day/night differential within 10 degrees. This is saying to keep it nearly at 0 degrees. I'll have to experiment.

 

[Oldmac]

Hey rasputin, slightly less at night or even 10 degrees below max (86F) is probably ok, here's why. They are using a mex sativa (probably landrace) that like all equatorial sativa's like hot and humid. Since nobody tries to grow pure sativa's and few have access to pure indica's indoors, we are all growing hybreds of sativa/indica. These are man made and do not appear in nature anywhere. It's man's attemp to exploit the best quailities of each to our benifit. Since indica's tolerate more extreme temps and are aclimated to less humidity or arid conditions...the cooler nite temps are actually OK. They do tend to have the same photosynthesis range tho, 77F to 86F, but again I like 80-85F.

Oldmac's translation continues;

Next up is co2. These researcher's used 250, 350, 450, 550, 650 and 750 umole mole-1. This is easy to translate, since micro-mole per mole is inherently a ppm value based on mole fractions. They used 350ppm as normal ambient co2 concentration (Ca). Obviously running 250ppm photosynthesis (Pn) suffered. I'll skip over the intermediate co2 levels and get right to 750ppm of co2. This surpressed transpiration (E) by 29% and also leaf stomatal conductance (Gs) by 42%, but increased photsynthesis (Pn) by 50%, water use efficancy (WUE) by 111% and leaf internal co2 concentration (Ci) by 115%. All of this occurring at max temp and light levels found above, and reflects the plants potential for better survival, growth and productivity in drier and co2 rich enviroments.

Now before everyone runs to thier keyboards and say hey they didn't go far enough, everybody knows 1200 or 1500ppm is the way to go, they did not because they cited previous research that has shown a simple doubling of ambient co2 (350ppm) will increase photosynthesis, at peak temp and light, and will increase plant mass by 40% or more. (this btw is way studies like this have at least a page of bibliogrophies, citing previous reseach)

Let me try and explain where the notion that you need 1200-1500ppm co2 levels comes from. As the canopy temps increase the pores of the leaves start to close (less stomadal conductance) and plant growth slows. Get the temps way up there and the plant stops growing, but if you add co2 you can offset this slowed growth and with enough co2 get increased growth. But this is simply covering up the problem of too high temps. If you have the optium temp and light and give the plant 1200ppm it will take what it needs and leave the rest so it becomes wastefull. Maybe if you have a heat problem you need more, but you and your plant is better served if the temps and light are correct.

I should mention that the temps are taken on the leaf surface not the room air temp. A hand held IR temp gauge works best for this measurement, I've been using one for years now and cost abt $40 or so dollars, well worth it for even hobby growers especially when using any HID light.

Here's what I aim for; temperture with lights on 80-85F (measured at leaf surface), night time temps a little cooler (this may vary depending on strain) indica dominate maybe 10 degrees less, sativa dominate 5 degrees less, co2 750-800ppm and light, well the more the better. In my personal grow I use a DIY LED/T5 vho hybred that has 420w LED red Cree's 635nm (660nm where not avaiable when I built it, tho I would use those today) and 320w of T5 2700K floro, for 740 watts total not counting ballasts, drivers or fan). Not a lot less then the 1000w HPS it replaced but outproduces and creates a better finished product. In the interest of full disclosure I also use, 2- 100w incandescent halogens (GE Revel's) for far red and are used 30mins before main light goes off and stays on 30mins after that. I also use them 15mins before main lights come on and they stay on 15 mins with main lights. I also use supplemental UVb in the form of Zilla "desert 50" reptile T5 floroescents that are NO tubes (18w each) but are driven to 40w VHO and are on 3 hrs in the middle of the photoperiod. This is all for flowering, which I do in a GI Grow rotating garden that's heavily modified, and does 144 4" delta blocks of single cola SOG size plants per run.

As always I'll entertain question's on or even off topic (I dare you Pinball Wizard).

OM

 

[sm0k4]

Another study stating low to mid 80s is ideal. Yet I will read 50 posts from people saying mid 70s is ideal. Gotta love the internet!

The night temps are also important for inter-nodal growth. Too large of a day to night temp shift will result in extra stretch. I try to keep my night temp no more than 10 degrees difference.

Good find Oldmac, thanks for sharing.

 

[Buddler]

Yes, since i run a humidifier at nite the temps stay around 75 at night and 82 during lites on i thought it was to hot but plants are thriving warm nites are supposed to reduce stretch as well. Good info thanks B

 

[PetFlora]

During my last grow of the season, where temps are usually moderate, close to ideal. I had just flipped to 12/12 when we had a 5 day heat wave where temps in the room shot up 10+ degrees (low 90s) and RH inside my pod was high 80s. It took me 4 days to figure out the best way to vent the pod, but the damage had been done. All 4 became males, where it looked as though 3-4 would be females.

hth

 

[OneTokeOver]

Thanks for the info, Oldmac!

Excellent timing for me as well as I am just finishing my flower and grow rooms.

One temp. day and night appeals to me. Less equipment to worry about, now I can simply set the mini-split to 83* and forget about a back up heat source for a diff. night temp.

This also opens up the possibility of controlling the air in both rooms with one unit. Both rooms are sealed, if I install an active intake and passive return from flower room to veg. I can keep both spaces at 83* 24/7 with one unit.

I can sched. the lights so that the majority of the time I spend in the veg room is during flower room night (no CO2).

Off to the lab....

 

[crazybear]

I'm sure there is a beat temp etc. to grow anything! However if my cannabis plants were growing outside today they would be enjoying 100 degree day!?

 

[Oldmac]

Well even outdoors, when it gets too hot the plants growth will slow or even stop.
But that's a temp deal, and is more then offset my mother nature's sunlight when it's cooler.

OM

 

[Phaeton]

The first or second post mentioned the plant used in the test (sativa) and the humidity maintained (55%). I was given a sativa, short plant, and I have been growing tall plants for many years. This is the second time I have a short plant and it is doing sucko.

The summer humidity has jumped up to 30, that is a climb from 0-10 in the winter. Could this be part of why the short plants do not like me? I give them low humidity and cool lights?
I took a 300 watt halogen out of the budroom (had 2, overkill) and placed it 36" from the plant, replacing a 30 watt 2700K CFL @ 12".
With a mild breeze it holds 84* now.
Two 42 watt 6500K photo CFL's and a 30 watt 6500K daylight stayed in place at 16".
It is 8* warmer now with radiant heat and low red. I am unable to control the humidity. Am I helping?

Do the sativa and indica really grow that differently? or is it me stressing myself out?

 

[Oldmac]

Hey Phaeton,

Try not to stress yourself out. Indica dominates and sativa dominates grow pretty much alike when it comes to temps, tho indica's seem to tolerate greater day/night temps.

It seems strange to some people that indica's can do better in hot arid climates like desert regions where it is not just drier but have greater temp extremes. But they came from areas that were drier and have more temp extremes.
Sativa's even tho they originate from equatorial regions where it can be very hot these areas also have very high humidity levels.

So besides temp, relative humidity plays a role with how each does. If I lived in the SW desert area of the US I'd choose to grow a very indica dominate strain outdoors.

OM

 

[yesum]

Thanks for this info. I see for your own grow you hit all the bases with respect to spectrum. More important with a sativa than a indica imo.

I have an LED unit that provides 2 blue, 2 red, a white and a far red or infrared. In addition I use some 3000k pl=l flouros at 1 inch off the plants. I hope that provides some uvb and at least gives a balance of spectrum.

 

[spurr]

(note:
when I write "PPFD" below, I mean "umol/area^2/second", not necessarily "umol/meter^2/second, likewise "DLI" means "mol/area^2/day", not necessarily "mol/meter^2/day"; see the post after this one for my reasoning ... )

They start with amount of light, PPFD which is photosynthetic photon flux density, a fancy way to talk about light intensity that a plant can use. This is one of the more complex issue's since it is difficult to calculate all the various light sources that we use into that.

What do you mean by "difficult to calculate all the various light sources that we use into that"? Into what? Do you mean into micomoles per area squared per second within PAR range (400-700 nm), i.e., PPFD (when area^2 = meter^2), to quantify photons most useful to plants re rate of photosynthesis (Pn)?

I think you may mean when one tries to convert Lux into PPFD, no? In that case you are correct, but it's not only difficult, it's very difficult to do so accurately, when one does not have the raw spectroradiometer data. Ex., when one has to try and use KNNA's spreadsheet re "digitized SPD" from the lamp manufacture, this has to due with issues such as misinterpretation of irradiance % (re W/m^2) from digitized SPD, use of different ballast than when the SPD was created (which can change the SPD; and is also a problem with raw SPD data), running time (age) of lamp, etc.

Basically it's not possible to accurately convert from Lux (or FootCandles) to PPFD, without raw SDP data from high quality spectroradiometer, accurate radiance data from potential lamp, etc.

Because of the reasons above it's best to simply use a high quality quantum sensor to measure irradiance as micomole/~inch^2/second (i.e., Li-cor model "Li-190SA" with light meter "Li-250SA" or datalogger "Li-1400"; or an older version of same datalogger from Ebay, the "Li-1000"). That way you can see the real-time number of actual photons within PAR range striking quantum sensor (which is placed at canopy, for example). This method is far preferable than trying to use Lux and SPD to find PPFD, and far more accurate. Not only that, but if testing is done with the same brand and model of quantum sensor, comparisons between lamps are more accurate.

It's best to study light with real-world methods, as well a theoretical math, like the terrestrial quantum sensor I suggest (and use), the Licor LI-190SA (here). However, even with cosine correction the angle (degree) of photon capture/measurement is only 80, IIRC. Using an underwater quantum sensor from Li-cor, the Li-193 (here) is also good because the angle of photon capture/measurement is much greater than from the tessellate model (Li-190SA); that will better take into account reflection from walls, leafs, etc., as well as some photons from the reflector possibly missed by the Li-190SA.

Worth pointing out: (1) a lamp in a reflector is not a "point-source" of light, so the point-source law cannot be applied, re "inverse square law"; and (2) for a point-source light (ex., HID lamp not in a reflector) to follow the inverse-square law, the destination (ex. canopy) must be 5 times the size (area) of the largest dimension of the bulb (for clear lamps, not glazed lamps).
​

They use umole m2 s-1 (micro-mole per meter sq per 1 second) and again other then KNNA, I don't know anyone who can take different light sources and come up with at least a close approxamation.

I have four different studies all finding the same, that ~1,500 umol/area^2/second is close to ideal in terms of cannabis and highest Pn (before light saturation). However, in the study you are discussing, IIRC, the workers used a Pn meter from Li-cor (the Li-6400; the same unit I plan on buying). They used red light (660-675 nm) from the Li-cor Red/Blue LED lamp (model Li-6400-02) to provide near the listed irradiance. And IIRC the blue lamp was used to find the irradiance, which if I do remember correctly, means the actual irradiance from red would have been a bit lower than reported (because more blue photons reach point X then red in the same time frame when power output is the same, ex., one second, because blue photons have shorter wave-lengths to travel).

In that study, it was red light used, not white light, to study a single leaf at a time. And red light offers higher quantum yield (greater Pn possibility per photon as compared to blue and green photons), but lower energy per photon; a caveat is green light, which can have higher quantum yield than red and blue under bright white light (near light saturation levels).

To mimic the study, all one has to due is get a powerful LED array of red LEDs, and place it close enough to leaf to provide ~1,500 umol/area^2/second. However, that isn't gonna happen. Most every grower uses far less than 1,500 umol/area^2/second for all plants in the canopy; 1,000 umol/area^2/second is a better and more achievable goal for cannabis grown indoors in canopy at least as large (area) as the reflector (and if the reflector is pebbled, not mirrored).

Besides the point that most growers will be unable to provide 1,500 umol/area^2/second (of any wavelength) to the majority of their plants, providing that much instantaneous irrdaice over the whole day is not a good idea. I have tested up to ~1,200 umol/area^2/second for a few plants over a week or so, and it was too many photons per day, IMO. This is about the Daily Light Integral (DLI), see the link to the DLI thread below. Granted, I didn't use a Chl fluorometer or Pn chamber, yet, I only used visual evidence of plant growth and leaf phototropism as well as leaf morphology.

Here is a link to a good thread I think may be worth posting here because the topic of this thread, and that one, are nearly one-in-the-same. That and I go into much deeper detail about some of the issues above, than I will go into in this thread: "Daily Light Integral" (link).

In my next post I am going to copy/paste something from the thread I liked to above; it's about the limits and flaws of using PPFD for indoor growing ...

But to put it into prespective if you were to take 2-600w high par HPS bulbs and put them over a 3'x3' area you can achieve 1500-2000 umole/m2s-1 depending on how high above the canopy they are placed. That's the best I can give you as a starting place.

Maybe, maybe not. You are correct it depends upon the distance to canopy, but also the reflector size, use of (or not) a hot-spot diffuser such as the Equalizer, etc. If I where to place a bet, and the lamps were powered with good E-ballasts and the HPS were those that can make best use of E-ballast, I would agree. That said, I do not agree doing so would be wise, or helpful.

An important point from that study, and others, with respect to DLI, is the data in said studies was found after only 45-60 minutes of irradiation with said red light at listed irradiances (ex., 1,000 PPFD, 1,500 PPFD). So-called "steady state" Pn for cannabis is achieved after 30-45 minutes under high irradiance levels, so the workers from that study irradiated the leaf with stated amount of red light then tested Pn and other variables (Gs, E, etc.) after at most 60 minutes of light (irradiation). what this means is neither that study, nor any of which I am aware, have looked at long term rate of photosynthesis (Pnnet) under longer term high irradiance (hours/days).

If we used 1,500 PPFD all day, over 18 hours, the DLI (total photons over the whole day; def: mol/meter^2/day) would be ~97 mol/area^2/day! That is a lot of light considering the sunniest places in the world, on the longest days, rarely get over 60 mol/area^2/day. Providing too much light over the whole day (i.e., DLI) can be(is) just as bad as providing too much instantaneous irradianc (ie., 2,000 PPFD); for reasons such as photoinhibition and "midday depression of photosynthesis", (increased) photorespiration, stomata conductance (Gs), leaf temp, etc.

IMO over 1,300 PPFD is too high, or the daylengths we use, ex., 12 hrs, 18 hrs, 20 hrs, 24 hrs(no night). In my testing 1,000 PPFD was better than 1,200 PPFD, over long daylengths we use. In nature there is a PPFD bell curve, it starts low, gets high, and ends low; peaking at around and above 2,000 PPFD in the sunniest locations of the globe.

This study showed that maxium photosynthesis (Pn) occurs at 1500 umole at 30C or 86F, and drops off after that, unless temps are at 25C or 77F in which case 2000 umole is peak. Obviously using the peak temp of 86F takes less light and the actual peak photosynthesis is greater @ 1500 then 2000 at 77F.

For our purposes, the sweet spot shoud be considered 80-85F since photosynthesis drops off sharply at temps above 86F and is much less below 77F.

I concur, to a point. Peak temp you listed is good if Co2 is also enriched (up to ~1,000 ppm; more is a waste and can be detrimental to plant growth). Otherwise peak temp is ~78-80'F, for highest Pn. Also, as you pointed out, lower temp and higher irradiance can give higher Pn, than higher temp and lower irradiance. This I assume is due to VPD (Vapor Pressure Deficit) and the effects ideal VPD has upon Gs, E (rate of transpiration), RUE (Radiation Use Efficiency), Pn, Co2 uptake and carbon fixation, etc.

VPD is found by using leafs temp (range as the temp per all leafs is not the same), canopy temp and canopy RH. A very good hygrometer and IR thermometer should be used to gather data; do not take canopy temp in direct light. Leafs are normally at least 2-5'F cooler than canopy temp, when canopy temp is < ~90'F and there is sufficient air movement and RH. Ideal VPD is 0.8-1.0 kPa, but up to ~1.25 kPa is also in the (close to) ideal range, that is, the A-Okay range

In that study the RH was kept at 55% +/-5, and if peak Pn came from 77'F, that means average leafs temp in a canopy would be ~72-75'F. However, in that study the leaf was in a clamp/chamber, so the leaf temp was probably not a lot cooler than the canopy temp (granted, the workers kept air-flow at a decent speed); so I will assume leaf temp was 75'F because it was under direct high irradiance. That means the VPD of the study was about 1.2 kPa (give or take ~0.1 kPa); which is damn close to ideal and is what I use in my flowering and grow room.

So, considering how VPD effects Pn and RUE, unless a grower uses the same (or close to the same) VPD used in that study (ex., < 1.35 kPa), even if they provided 2,000 PPFD of 660-675 nm red light at 77'F, the Pn would (most probably) be lower than in the study with the same plants.

FWIW, here is an easy and good VPD calculator: http://www.autogrow.com/vpd_calc.php

This also leads to the plant's temperature dependence for dark period respiration. This is a fancy why to describe how well the plant uptakes water during the dark period using what they call WUE (water utilization efficency). Obviously a plant uptakes much of it's water needs during the dark period and that is dependent on how well it is doing during the photoperiod growing.

One point, plants do most of their 'growing' at night (best conditions for growth) and during the very early morning hours.

... Now this is a surprising temp for most people, it mirrors the day time temp needs, 77F-86F. While most of us believed that a plant should be cooler at night, that's just not neccessarily so. But the reality of a grow room is lights on is hotter, lights off cooler. Again looking to compromise but still be in a sweet spot dark temps of 77F to 80+F are ok, but again don't want to go below that min temp of 77F.

This topic has long been studied, it called "DIF", the difference between day temp and night temp. A zero DIF (same day and night temp) is often employed to reduce stretch of plants (due to GA3 issues), as well as a negative DIF. I for one use a zero-to-negative DIF during pre-flowering to reduce stretch and internode elongation. Using a zero DIF long term is less than ideal, it can affect Chl A and B levels (leaf chlorosis), translocation of some elements such as Ca and N, the plants ability to carryout transpiration, etc.

I for one prefer to keep a positive DIF at time but during pre-flowering, a zero DIF during veg can also be useful. But a positive DIF is better for the plant in general, as well as it can help increase some flavonoids which are purple/red/plum colored pigments.

If Co2 is high at night (ex., > 500 ppm), dark respiration is reduced, which can reduce stretch but also cause leaf chlorosis.

 

[spurr]

---------------------
PPFD:

One point about PPFD as used below, the definition of PPFD is "Photosynthetic Photon Flux Density". That is a label for the number of photons (particles of light) that hit an area of a meter squared (three-feet squared) in one second; but only photons that are within a range that best drives rate of photosynthesis (Pn), called the PAR range (Photosynthetically Active Radiation) of 400-700 nanometers. However, when one measures PPFD one does not measure the instantaneous irradiance (as umol/area^2/second) over a whole meter squared, to find PPFD. To measure PPFD (i.e., instantaneous irradiance as micromoles per area squared per second) one uses a quantum sensor, but they are only about an inch squared.

When the concept of measuring (quantifying) photons within the PAR range (per second; instantaneous) that best drives Pn, was being developed, it was (mostly) developed to be used for outdoor and greenhouse growing, when used for whole-plant application. AFAIK, the designers did not take irradiance foot-print of a horizontal lamp in a reflector into consideration, only an ideal situation of point-source light from the sun or greenhouse lamps far from the plants. Granted, the scientists who came up with PPFD used single leafs, at times, when measuring effect of irradiance on rate of photosynthesis.

The calculation of PPFD is based upon at least one big assumption: all spots (ex., plants) within the area of a meter squared get the same instantaneous irradiance. Ex., all the plants in a meter squared outside in a field the get the same 'amount' of sun light (photons; if they are no shaded); so one measurement in any spot within the meter^2 will show the PPFD of the meter^2. That means to find PPFD outdoor or in a greenhouse, one measurement is taken within a meter^2 with a quantum sensor and the assumption is made that all other areas within the meter^2 will have the same instantaneous irradiance as the original spot. Thus one measurement of a small area (about an inch or 2 squared) is used to find PPFD for a whole meter^2. But indoors under a reflector with a horizontal lamp, not all plants in a meter squared get the same 'amount' of lamp light (photons). That means using PPFD is not representative of the real-world irradiance per smaller-than-a-meter^2 area, within that meter squared. Also, the PPFD from a horizontal lamp in a reflector can't be found by just a single measurement, many measurements must be taken to account for un-even light distribution over the meter squared (and light side the meter^2).

Along with the reasons above, un-even irradiance over many areas of measurement within a meter^2 is another reason PPFD is not ideal for our use case; IMO anyway. By Definition PPFD is fixed at a meter squared, which works fine for outdoor and greenhouse growing, but indoors canopies are often smaller than 3'x3' or not shaped in a square, as well as the flaws/limits above.

What all the means, in my opinion, is using the term PPFD is not ideal, mostly because the area is fixed. I think it may be better if PPFD was not fixed in area, but required one to report the area measured for irradiance; no assumption made. So PPFD could mean umol/1"^2/second or umol/12"^2/second; as long as area is defined.

Anyway, in the post below when I write PPFD I mean "umol/any-area-^2/second"; not necessarily "umol/meter^2/second". Likewise, DLI means "mol/area^2/day"; not necessarily "mol/meter^2/day".
---------------------

 

[spurr]

I'm sure there is a beat temp etc. to grow anything! However if my cannabis plants were growing outside today they would be enjoying 100 degree day!?

No, they would not enjoy the weather. Look into the phenomenon called "midday depression of photosynthesis" and "multipeak photosynthesis", re photoinhibition from heat and RH, as well as reduction in stomatal conductance, rate of transpiration, etc., re VPD.

Then we have the issue of "Rubisco activase", which is used to turn 'inactive' Rubisco into 'active' Rubisco; which in turn effects Pn because lots of active Rubisco is needed to keep Pn high. Thus, under 100'F, Pn will be hindered not only due to photoinhibition and VPD issues, but becuase of reduced amount of active Rubisco because 100'F will hinder Rubisco activase; it's a chain reaction ...

Can you put shade cloth over them?

 

[spurr]

Thanks for the info, Oldmac!

Excellent timing for me as well as I am just finishing my flower and grow rooms.

One temp. day and night appeals to me. Less equipment to worry about, now I can simply set the mini-split to 83* and forget about a back up heat source for a diff. night temp.

I would suggest against using a zero DIF from seed/clone to harvest. I have used a zero and negative DIF during veg and pre-flowering more than a few times, and I find zero DIF is just as good as negative DIF (granted I do not go lower than -5'F DIF). Using zero DIF all the time can/will lead to loss of Chl A and/or B, more extreme leaf chlorosis, slow growth, reduction/loss of other pigments (such as the purple flavonoids), reduced Pn over time, etc.

I would suggest using zero or slightly negative DIF for pre-flowering only, and possibly zero DIF for veg if you're growing lanky plants.

 

[spurr]

Oldmac's translation continues;

Next up is co2. These researcher's used 250, 350, 450, 550, 650 and 750 umole mole-1. This is easy to translate, since micro-mole per mole is inherently a ppm value based on mole fractions. They used 350ppm as normal ambient co2 concentration (Ca). Obviously running 250ppm photosynthesis (Pn) suffered. I'll skip over the intermediate co2 levels and get right to 750ppm of co2. This surpressed transpiration (E) by 29% and also leaf stomatal conductance (Gs) by 42%, but increased photsynthesis (Pn) by 50%, water use efficancy (WUE) by 111% and leaf internal co2 concentration (Ci) by 115%. All of this occurring at max temp and light levels found above, and reflects the plants potential for better survival, growth and productivity in drier and co2 rich enviroments.

Now before everyone runs to thier keyboards and say hey they didn't go far enough, everybody knows 1200 or 1500ppm is the way to go,

I wholeheartedly disagree with that claim, too, even though I knows it's the prevailing wisdom. I have written about this a few times, there is a Co2 thread in the science sub-forum with more info I posted, as well as test by another grower here (re lower Co2). I also have references I have yet to post in that thread, and other info with respect to Co2 levels, such as NO3 vs NH4 and how plants given only NO3 show much reduced benefit from Co2 after weeks.

Basically, the Co2 satiation point for higher plants (C3) is ~1,000 ppm; just like the irradiance saturation point for cannabis is ~1,500-2,000 PPFD. Providing Co2 over ~1,000 ppm is either (most assuredly probable) a waste, and/or detrimental (if Co2 is high enough).

Co2 saturation point of ~1,000 ppm has been found to be accurate for most higher plants studied, and cannabis should fall into the category, even though I know of no studies looking at 'luxury' Co2 levels of 1,500 ppm and cannabis. When higher C3 plants where evolving into what we see today, the ambient Co2 was not much higher than 800-1,000 ppm, at the highest point. That is one reason I think the saturation level of Co2 for many/most higher C3 plants is ~1,000 ppm.

Once Co2 level reaches and surpasses ~1,200-1,300 ppm Rubisco activase can be/is hindered, just like when canopy temps exceed ~90'F. When Rubisco activase is hindered via high Co2 the same end result of reduced Pn, and Pnnet will occur.

As shown in that study, when Co2 level increase Gs and E decrease, this is because higher Co2 signals the plant to reduce stomatal opening size (re Gs). And because high Gs is best for high Co2 fixation, it's a good idea to try and hinder the reduction of Gs. One way of doing so is by preventing Co2 from getting very high (ex., >1,000 ppm). It's kind of a catch-22: the higher the Co2 goes past a benchmark (ex., 500-600 ppm) the less efficiently the plant can use said Co2 with respect to Gs ...

What that means is keeping Co2 below 1,000 ppm is a good and worthwhile goal; it's better than 1,500 ppm. I have tested the claim and yeild nor growth was better with 1,500 ppm. At least one other grower at Icmag is testing lower Co2 vs higher Co2 (he used to use Co2 at 1,500 but IIRC he dropped it to 1,000 after he and I chatted), and he wrote he noticed no decrease in growth or health.

Neither I, or anyone else who has looked, could find scientific validation for the oft used claim of 1,500 ppm Co2 for cannabis. IMO Mel Frank, et al., read it somewhere from a source based upon anecdotal evidence/conjecture/assumption or hearsay, then they repeated it and it stuck in our culture ever since (kind of the like the high P paradigm).

FWIW, I am open to the possibility 1,500 ppm Co2 could be better for cannabis, but so far there is no scientific or otherwise valid evidence suggesting so. I plan to study this issue in the future, along with similar topics, with the same Pn chamber used in this study (the Li-cor Li-6400-kit); or maybe the cheaper and less ideal option(s) from Qubit Systems (here: http://qubitsystems.com/featured/plant-soil/).

If anyone is interested I have great lab manuals using Qubit System Pn meters setup, step-by-step setup and use, so anyone can study Pn of cannabis with respect to irradiance, etc. All for less than ~$7,000; as of circa 2004 one could buy the Qubit System tools for ~$5,000! Granted, the method used by Qubit Systems is less ideal than that of Li-Cor, but the Li-6400 kit (with Chl fluorometer) is ~$45,000 IIRC. At least one University plant science course uses Qubit Systems equipment and Qubit Systems designed labs (such as studying Pn of plants). Qubit Systems was created and is run by a respected Ph.D plant scientist out of Canada; Qubit Systems focuses on lower-cost options, ex., in plant sciences, geared to Universities with plant science degrees. So Qubit Systems is a great option for those on a budjet, just not as ideal as Li-Cor and the like.

... they did not because they cited previous research that has shown a simple doubling of ambient co2 (350ppm) will increase photosynthesis, at peak temp and light, and will increase plant mass by 40% or more. (this btw is way studies like this have at least a page of bibliogrophies, citing previous reseach)

Let me try and explain where the notion that you need 1200-1500ppm co2 levels comes from. As the canopy temps increase the pores of the leaves start to close (less stomadal conductance) and plant growth slows. Get the temps way up there and the plant stops growing, but if you add co2 you can offset this slowed growth and with enough co2 get increased growth.

Sort of, I think that is a bit to simple of an explanation. Ex., once temp passes a benchmark addition of Co2 won't help; and vis-a-verse-a.

Gs will decrease with respect to (A) VPD or (B) E (rate of transpiration) as affected by VPD; it's either A or B depending upon whether the plant has ever been in drought conditions. Gs will also decrease under high canopy or leaf temp, but Gs will decrease due to A or B above, before canopy and leaf temp by themselves (irrespective of RH and water-status).

Many environmental factors affect Gs (re openness of stomata), ex., (1) blue photons vs green photons; and red photons (blue increases Gs, green reduces Gs and some areas of red can reduce Gs); (2) VPD, if too high or too low, granted too high is what most people use, rarely too low; (3) Co2 level because high Co2 ppm means reduced Gs; (5) E, if too high (often with respect to VPD), will cause Gs to decrease; and (6) etc.

But this is simply covering up the problem of too high temps. If you have the optium temp and light and give the plant 1200ppm it will take what it needs and leave the rest so it becomes wastefull.

Often more than wasteful, often actively detrimental; ex. temp > ~88'F, Co2 > ~1,200 ppm, irradiance > ~1,600.

Here are the specs for my room, it's optimal in all areas, save irradiance and RH. There is nothing I can do about either now: at peak the irradiance is ~930-950 umol/~4-6"^2/second and at low end is ~500 umol/~4-6"^2/second; and I use carbon filter to remove smell, so RH can't go above 65% and I like to keep it < 60% (it says ~55%).

I use a very high quality hygrometer [analog with synthetic hair, NIST certified calibration and I re-calibrate weekly] and IR gun thermometer. Every single hygrometer I have tested, normally used by growers, is either far inaccurate, or very far inaccurate; ex., the CHHC-4 is ~10 points too high, when it reads 70% RH it's really only 60% RH.
​

• Canopy temp day: ~75-78'F
• Canopy temp night: ~68-70'F
• Leafs temp: ~70-76'F
• RH: ~55-60%
• VPD: ~1.15-1.3 kPa
• ADT: ~72-74'F (above 70'F is a good goal)
• Canopy: ~5'x3.5'
• Co2: ~750 ppm
• Irradiance:

• peak = ~920 umol/~4-6"^2/second (10 sec avg. to account for flicker)
• outside edge = ~400-500 umol/~4-6"^2/second
• core = ~800-920 umol/12"^2/second

• Lamp: 1,000 watt HPS Digilux with Galaxy select-a-watt ballast set to 'turbo' to overdrive the lamp; along with BlockBuster 8" reflector and Equalizer hot-spot diffuser.

I should mention that the temps are taken on the leaf surface not the room air temp.

Are you sure? I thought in that study the workers used the Li-cor Li-6400 when measuring/controlling temp, and I thought they measured temp via air, not IR on leaf; am I wrong? Being that "canopy temp" = "temp of air surrounding leafs".

edit: I just re-scanned the paper and saw no mention of leaf temp specifically, can you point out where you read the workers used leaf temp? Thanks.

A hand held IR temp gauge works best for this measurement, I've been using one for years now and cost abt $40 or so dollars, well worth it for even hobby growers especially when using any HID light.

I agree. And I like to use a good IR temp gun with a white piece of paper to get canopy temp, instead of using a traditional thermometer. It's more accurate using an IR gun, IMO and AFAIK. I just place the white paper out of direct light and point laser beam at paper, done and done

Here's what I aim for; temperture with lights on 80-85F (measured at leaf surface), night time temps a little cooler (this may vary depending on strain) indica dominate maybe 10 degrees less, sativa dominate 5 degrees less, co2 750-800ppm and light, well the more the better.

Ah, a man after my own heart Nice specs, right in the sweet post, but what about RH and canopy temp (with respect to VPD)?

 

[rives]

Spurr, I think that you should consider an alternative career in politics or law. It appears that you used in the neighborhood of 3800 words to agree with Oldmac's 1100 word discourse! Great information here.

 

[spurr]

rives,

Ha, thanks, I think But I didn't agree with Oldmac in many areas; that is why I was so verbose. These topics are esoteric and complicated, so I tend to try and over explain than under explain. If what you took from what I posted, to be that I agree with Oldmac on all points (or even most points), I did a poor job of expressing my thoughts, opinion, and facts.

Let me know if something I wrote was not clear.