WhAcKeD

I think if I read this thread every week for the next year I'd learn something new every time I read it!

What is the maximum allowable RH for late flowering? I've got a RH-controlled ventilation system setup, so it's just a matter of set point. Right now I'm running 55%, which in practice keeps it below 60%, and I'm in very dry environment (typically 30%) so I'm sure RH is lower near the intake.

Since knna posted the VPD info I've been monitoring my leaf temps, and instead of being 3 C lower, they are 3 C higher at the hottest point! This sends VPD to 1.76. The obvious conclusion: Is my 1000w too close? Would I get faster growth by backing the light off and increasing room temp?

I'm having trouble understanding this statement, knna:

Quote:

About CO2 at 1200ppm, the RH level to get 0.9-1KPa is over 70%. That is a dangerous level to deal with mold, so a logical conclusion is CO2 enrichment apport little in late flowering stage (2 weeks), because at reasonably RH levels (55%), VPD is outrange, and excess transpiration and probable stomata closure will lead to CO2 unused, without advantages over ambient CO2, or at least over slight CO2 enrichment (500-800ppm)."

My ambient co2 is never below 1000 ppm, and usually its right around 1200, without co2 supplmentation. I'm not sure why it' so high, but obviously I'm in the "slight co2 enrichment" zone. But considering I'm trying to set up the 'ideal' environment (self-contained cab, air conditioning, heating, bottled co2, lots of light) I need to decide whether I should (a) vent the cabinet and just use the ambient, co2-rich air to control RH, or (b) seal the cabinet and use a dehumidifier and co2 supplementation, which would allow precise control over co2 levels via a C.A.P co2 controller.

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Freakshow68

High Whacked. A high RH in flower will promote moldly buds. And that is a very bad thing.

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knna

As Freakshow68 noted, in late flowering is a must control humidity to avoid mold. As i dont use CO2 supplementation, i havent problems with it, as i can set humidity about 40% without mayor problems. But the highest humidity i would try in late flowering is 55%.

Have you measured it on the upper surface of the leaf? The right way is measuring it at the bottom surface, with the sensor well blocked from the light. Radiant heat from the light affect temp measurements strongly.

Anyway, its very usual having top canopy leaf temps near or over ambient temp, due to radiant heat and top leaves having the highest metabolism rate. Leaves from 2nd or third node (counting from top) gives less extreme figures and are more representative of the whole plant.

Link with info about devices to measure leave's temp (and other growing parameters, as soil water potential): USU CROP PHYSIOLOGY LABORATORY: RESEARCH: INSTRUMENT EVALUATION AND NEW TECHINIQUES
In the pdf about infrared termocouple sensors, it states that accuracy of those sensors (with sensor's body temp corrected) is below 0.2ºC.

If taking more accurate measurements, the leave's temp is still some degrees over ambient, then, yes, raising the lamp would help.

Do you live over a farm? That ambient CO2 is very high. I wish i have the same Maybe strong traffic ? (cars ).

I think is no sense supplementing CO2 with that ambient level. To have that rich CO2 air is a God gift for a grower! Just control the humidity in the late flowering to avoid mold. That way, your plants cant get clear benefit from the increased CO2, but they still grow awesome. Ensure there is enough water potential (not dry soil or whatever media you use) to avoid strong hidric stress. A bit of drough stress cost a bit of weight, but increase resin production.

When you outdoor air is dry, is very easy reducing humidity, you only need to increase exhausting flow.

Think that the VPD range provided is for optimize grow rate. Going out it dont kill your plants, only get a reduced grow rate. Only when VPD goes very outrange it creates problems (similar to an overfert or root damage).

knna

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seventynine

This information was great. I don't understand all of it and doubt that I have the ability to measure the temps of my leaves at present. However, I had a question.

My temperatures seem to get up to about 85F right now. My RH is usually only 35-40%. I've been having a slow veg. Should I try buckets of water or something to try to bring my RH up?

79


EDIT:

OUCH I just checked my cab. Temp was 83F and RH was 23%. I just put in a bowl with wet paper towels behind one of the fans. Maybe this will up it a bit. Could this be the culprit behind my slow veg?

WhAcKeD

But then veg always seems to go slow

That's too hot unless you're supplementing co2. Not a big deal now, but during flower that will cause your buds to become airy. Even if you get RH up to 60-70% (purportedly ideal for veg) you'd have to get your co2 up to 1200+ ppm for 83 to be okay... Try to get down to 78. Good luck.

PS, as an after thought, take a look at your nutrients and schedule - if that low of an RH doesn't cause your plants to suck up a ton of water and fertilizer and then burn themselves, than I'd guess that you're running a pretty low (ie weak) dosage of nutes. And what kind of light?

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swordfish

Didn't want to start a thread to ask a few questions related to this topic;
I am presently running a 1500w space heater in an enclosed 8 by 8 room I constructed inside a cold garage. I have a small 80cfm exhaust fan in the roof of this new structure. I also have a second exhaust port when flowering allowing me to run scrubed air out. This air in six weeks or so will be ducted out of the garage area as well outside my box.
Because of my cold temps which will be continuing until March and a harvest anticipated late April, I must run a space heater dureing both day and night cycles to achieve the desired results spoken about on this thread. Im doing a good job so far as long as I leave my exhaust off. Leaving my exhaust off in a sealed room must have negative consequenses. I am only in early veg at this time and have time to correct my inadequacies.
I regret the use of the space heater in the first place and have know other heating options. I further regret having to turn on an exhaust fan just to suck my expensive heating right up and out of the room. You say re-direct this air to a living environment. That would also be impractical with major logistical shortcomings.
My thought process was to have a small 80cfm exhaust fan running 24/7 pulling in passive air. Due to the very cold ambient conditions this doesn't seem practical. When it comes time to flower bringing online a 445cfm exhaust through 80lbs of carbon filter will further add to my expenses. All this info and I have still failed to ask a question. Just giving the backround. Im anticipating a whopping electric bill when and if I have to bring all this gear online when I flower. Any suggestions?

I was going to ask this question at the construction forum but you guys seem to have this stuff down. Thanks!

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swordfish

Just checked another heating exhaust thread and it was advised useing a dimmer or speed control device on my 80cfm exhaust. This would be a better option for me but still my space heater running 24/7 for the next 3 months just to achieve proper airflow seems $$ going out the roof. Insulating my room more efficiently will not solve my wastefull air movement problems however. I believe im stuck.

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knna

Some days ago, i suggested some tips to xblazedx421, with similar situation, in this thread.

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tripps

Don't use a light dimmer, it will smoke the motor, need a fan or motor speed control.

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First journal Soil
Second journal Hempy
Third Journal Ebb and Flow

LJB

Thought you might be interested in this:

Heat stress increases sensitivity of pollen, fruit and seed production in tomatoes (Lycopersicon esculentum Mill.) to non-optimal vapor pressure deficits.

and this: SpringerLink - Journal Article