will LOW humidity hurt a plant?

Discussion in 'Basic Growing' started by Sjapp, Feb 18, 2008.

  1. Sjapp

    Sjapp Registered+

    If your humidity is around 20% or lower can it be bad for the plant? If it is I don't understand why it would be bad for the plant because the leaves don't absorb water do they?
    Last edited: Feb 18, 2008
  2. Mr. Clandestine

    Mr. Clandestine Registered+

    From what I've read - and this could be biased information, so take it with a grain of salt - but humidity levels lower than 30% prevent the stomata on the leaves from opening fully (or at all) and reduce the transfer of C02 and other gases to the cells of the plant. Slow growth and symptoms similar to nutrient burn can result, but aren't always typical. If most other conditions are optimal, excessively low humidity may not pose as much of a problem. It certainly beats excessively high humidity any day.
  3. Rusty Trichome

    Rusty Trichome Registered+

    Yes, water and nutes are absorbed thru the leaves.

    I live in the desert. During the summer the temps get to the 120's, and humidity get's up into the teens . You mentioned nothing about the heat, but excessive heat combined with low humidity, and the plant can struggle to keep upper zones hydrated.

    I veg indoors, and mist the plants once daily, as they much prefer the higher humidity. But I flower in an outdoor shed, where the A/C keeps the temps around 90ish and the plants have little to no problems adjusting. Keep an eye on them daily, especially when they start filling-out. The larger the plant, the more moisture they lose thru the leaves. (transpiration)
    Oversize pots will hold moisture a bit longer, but don't keep the soil wet all the time, will cause root-rot.
    It can be a fine balance between "ok...I need to water later today" and "shit...I should have watered yesterday."
    Last edited: Feb 18, 2008
  4. Sjapp

    Sjapp Registered+

    ha youre so humble, you sound like you know exactly what your talking about. so should i mist the plants or get a humidifier?
  5. Mr. Clandestine

    Mr. Clandestine Registered+

    I'm lucky enough not to suffer from extremes in humidity, but if I were needing to raise it just enough to keep the plants happy, I'd probably spray them with a fine mist like Rusty suggested. There's less chance of over-saturating the air and not noticing it. I wouldn't, however, do any spraying during the last month of flowering. Even though it's generally suggested that higher humidity is a good thing in flowering, I'd rather lessen the risk of mold. But that's just me. If you happen to go the route of a humidifier, it would probably be a good idea to use distilled water... especially if you've got noticeably hard water at your house. Take care.
  6. Sjapp

    Sjapp Registered+

    colorado water is pretty damn good. maybe im turning a blind eye to it, but i don't think it needs to be distilled. ill mist for now, until i can get a humidifier.

  7. melodious fellow

    melodious fellow Registered+

    Rusty, is it difficult for you to keep your temps down for your indoor grows in the summers with that kind of heat?

    And regarding the outdoor, the plants do not mind growing in 90 degree heat? Any strain?

    This would be great news for me, as I cannot keep temps in my grow cab under 85, as I can't seem to get the ambient temps under 75. I was dissapointed because High Times and many things I have read on these boards say growth virtually stops at 90.

  8. Sjapp

    Sjapp Registered+

    you should try and get more exhaust. if you have a greater exhaust than intake it creates a low pressure vacuum effect in the room or box which causes cooling. its how all cooling systems work, be it your fridge or your ac
  9. denial102

    denial102 Registered+

    Nobody has said it solely/specifically but I thought that the humidity effects plants a bit like heat does us.

    I thought the humidity specifically changed the rate of transpiration.

    It also effects the amount of fungus that can viably grow.

    Hope these simple pointers help,

    Peace Out + GL,
  10. Rusty Trichome

    Rusty Trichome Registered+

    Yes. But alas, I have no choice. Wifes friends (the church ladies) come over all the time, so indoor flower is out of the question.

    During the summer, I do have to stagger the 400w HPS' (12 to 14" or so from tops ) so only one is on at a time. I have it circuted to work like a light mover, light on the left is on for 6 hours, then just before it turns off, the light on the right comes on for 6 hours.
    It's usually around 110f outside, and the flower room is ok till that point. Once it get's up to the occational 120f, I raise the lights.
    130's outside, and I turn off the lights. Helps that I'm disabled, and am home most of the time.

    I also try to plan it so my Swazi Skunk (sativa) is the strain in line for the summer temps, but can't think of a strain I haven't finished in the heat, weather it's sativa, indica or hybrid.
    Instead of my usual 3 gallon pots, I flower in 5 gallon pots. Holds moisture a bit longer. Daily waterings up to around 3/4 gal per day per pot. Flushing the ladies is staggered over a week, too. Otherwise humidity skyrockets uncontrollably.
    Occational foliar for deficiencies, but keep it to a minimum, and only on the underbranches.

    I put an a/c unit in there, and it blows directly over the tops of the plants, and use a 12" fan blowing the cooler air up into the plants. I also have one of those breaker access panels (12" x 12") that is installed tword the top of my 9' tall room. I open it up, and have a dedicated exhaust fan blowing hot air out at all times. The intake is from the a/c.

    Alls I can say...it's a dry heat, and I strive to keep it dry. The first year, I misted heavy, and often, thinking they would like it. Burned the tops, and developed mold.
    Is it optimal...? No, but it most definatelly works. It can take an extra week or two to finish, but compared to my other options, I'll take it.
    Also, active trichome production starts a week or two later than usual, but come finish, they're just fine.:thumbsup:
  11. melodious fellow

    melodious fellow Registered+

    Thanks for all the feedback Rusty. You are a pal, bro.

    Regarding intake vs exhaust, I have heard, and I quote "nature abhors a vacuum." Now, if I can only remember the source and the respective credibility. I know theres a name the APA (psych association) has for that.... but as a stoner, I cannot recall that either :D:D

    I have also read that intake fans should be faster than exhausts so that air has time to circulate the cab before leaving.

    Not shore which is correct here, but I really appreciate the comments nonetheless.

    Maybe Stinky will drop by here. :Rasta::hippy:
  12. Rusty Trichome

    Rusty Trichome Registered+

    No problem, bro. Just payin' it forward.:thumbsup:

    Was it "Witches ride brooms, because nature abhors a vaccuum", or "Everything sucks, Nothing else matters"...?:jointsmile:

    Personally, due to my heat concerns, I'd trather have two exhaust fans at the top of my closet or shed, and an intake port (not fan) down low. The suction caused by the exhaust fans will bring in enough fresh air to keep the ladies happy.
    During the winter, I use one exhaust fan, and one circulation fan, but during summer I also use the a/c. so I guess technically it's an unintended intake fan.
    For me it's better to exhaust the hot air before it gets all mixed around the closet.
    Am I right? Shit...I don't know, but it kinda makes sence to me, lol.

    Likely she will be.:thumbsup:
    Last edited: Feb 20, 2008
  13. stinkyattic

    stinkyattic CultiModerVatorAtor

    Unless your intake and exhaust fan are perfectly matched, just run an exhaust. Two mismatched fans will actually inhibit air flow and waste electricity.
    Your exhaust fan should PULL rather than push air. The duct for the exhaust fan should be high in the room, preferably attached to an air-cooled hood unless you run lights on a separate exhaust loop. A passive intake should be LOW in the room.

    High temps + low humidity + intense light are dangerous to a plant.

    The plant transpires and photosynthesizes rapidly in intense light conditions, quickly using up water to form simple sugar by photosynthesis (sugar is a hydrocarbon; water contains a pair of H+ ions. The plant gets the carbon most efficiently from CO2 in the atmosphere).

    The plant also cools itself by allowing moisture to evaporate off leaf surfaces.

    The plant regulates its water loss and intakes atmospheric gases through the stomata, which are small openings concentrated on the undersides of the leaves. When the plant is trying to conserve water, and closes the stomata, it cannot breathe, and its metabolic systems start to shut down.

    What this all means is that when you are running BRIGHT and WARM, you must also add MOISTURE to keep the stomata open, and at temperatures above about 85 degrees, CO2 to ensure that CO2 is not the limiting factor in the plant's metabolism.

    When your conditions are COLD, and the plant is transpiring slowly, avoid overwatering, as moisture use/loss slows, and if you want to keep your plants fairly healthy even in low temps, you may want to decrease light intensity as well, because even though they may be able to photosynthesize under bright light, the chemical reactions that take place within the plant happen much more slowly. Below 65 degrees or so the plants become nearly dormant, and may be held in that state only under correct conditions.

    It's all connected. Hope that sorta helps.
  14. Rusty Trichome

    Rusty Trichome Registered+

    Well, not to cause any rift, I'd defer to Stinky's knowledge.

    Do I recommend this to those with better finances, that can afford the equipment to coddle their ladies? No...but I am saying you can still make it work with a bit of extra effort. I do stand by my post.

    I've been forced to do what I do by necessity, and have found that, thru trial and error, low ambient humidity and infrequent misting during flower keeps my ladies from heat damage and mold. When misting or foliar feeding, the water drops act like little magnifying glasses, burning the leaf tissue. (which is why I only mist the lower sections of the plants)

    The below pix is link I posted from 2005 on another site, when I lived in Vegas. 100 to 110 degrees daily in the garage (where the flower room was, late spring temps) no a/c, and misting only once or twice a week, if that. 3 gallon pots, in soil, and I believe it was 37 grams average for the topped (shorter, 2 cola's) and 43 grams average for the un-topped. (single cola) Pix are in the link below. I can't really say what the yeild is on my spring/fall grows, as I don't have a scale any more. I truly see that it takes a bit longer to finish, but yeild seems approx. the same.
    Keep in mind, too...this is a 80% indica x 20% sativa mix. (ReeferMan's R&D #1 med. strain) Big, wide leaves. My Potent Purple and Swazi strains (both sativas) grow even better under warmer conditions. Don't know if it's the narrower leaf structure, the genetics, or both, but they handle the heat very well.
    Anyway, below is the picture and the original grow report link.
    (my username there was ThunderLungs)
    Reeferman R&D Strain #1 - Reefer World
  15. stinkyattic

    stinkyattic CultiModerVatorAtor

    Those look wicked good.
    How much did you find you were watering them?
    It stands to reason that strains whose heritage lies in more arid climes would handle heat better.
  16. Rusty Trichome

    Rusty Trichome Registered+

    Thanks Stinky. Hope you don't think I was attacking your knowledge base, as this wasn't my intent. I truly have learned shitloads of valuable info from you.

    In the summer, I water in the mornings, and can go thru 3/4 or so of a gallon daily in the 5 gallon pots. 1/2 gallon or so in the 3's. By the next morning, pots feel almost empty, and soil is sufficiently dry. Tiz one of the reasons I dont water to the point of run-off. Am afraid of flushing nutes unnecessarily. Also...I give half strength nutes, twice weekly instead of just once, to make up for volume of water being used. Flushes every few weeks.

    Wow...I just read my RM post again. I sure was inexperienced with terminology, back then.

    Anyway, I, thru a mid-winter series of catastrophies, will likely be growing the Pokerface and a Swazi or two come summer time. Will start a log, (including temps, lol) when they're ready for flower.:D

    I did a cursory search yesterday, trying to find out the elevations of Swaziland, Africa. (the assumed geographical source for the Swazi Skunk) Elevation maps say it's from approx. 50 meters, up to 1500 meters. So not sure if my theory about the skinnier leaved strains coming from hot, arid climates is valid.
    Last edited: Feb 20, 2008
  17. stinkyattic

    stinkyattic CultiModerVatorAtor

    Aw knock it off. I'm not worrying... :jointsmile:

    This is why you are so successful in low %RH. Your watering/feeding schedule is exactly what you should be doing to make up for the dry conditions. The plants are actually raising the humidity themselves with a lot of that water you are giving them, like leafy green humidifiers.
  18. Rusty Trichome

    Rusty Trichome Registered+

    Aww, Stinky...:computerlove1:
  19. melodious fellow

    melodious fellow Registered+

    That is pretty amazing dude, 110F! My plants stopped growing around 85-90 and my humidity is rather higher I think. At least the outdoor humidity sure is! I wasn't misting though... will try that

    How long did they take start to finish? I didn't see anything about them temps in your log..lol

    Regarding your theory, it may have some significance. In hot, dry climates, plants adapt to have a much higher volume to surface area ratio

    In climates with less sun and plenty of moisture, plants do best with very broad leaves, thin leaves, with a much greater surface area than volume


    O dude, and by the way.... from your thread, that grow was damn dialed in dawg!

    No wonder they decided life was peachy despite the heat.

    I was actually quite surprised you used two HPSs wth the garage already being 110F though.

    Last edited: Feb 21, 2008
  20. Rusty Trichome

    Rusty Trichome Registered+

    I had to put them into flower a bit early, as we were preparing to move. 28 days veg, plus germination. Flowered to the point in the pix for 40 days.

    85 - 90 degrees outside would cause 100-110f in the closed garage. Would get up to 130-140 in there, during the summer.

    During the summer months, I stagger the lights. Kinda like a light mover. Lamp 'A' on for 6 hours, just before it turns off, lamp 'B' comes on for 6 hours. (floro's on for steady 12 hours) Aggressive fans, low humidity, and raising the lights when need arisis.

    Still works for me where I am now, but now I have an air conditioned shed.:thumbsup:

    In regards to being dialed-in, I had a bunch of online help, and a local grower to get advise from. altho he was a hydro man, was still able to insist I get the knowledge, equipment and supplies I needed, and avoid the gimmicky bs.

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