Marijuana and Humidity !

Tom’s facts on marijuana and humidity !                                                  rain-fog

Introduction – What is Humidity ?

Humidity is the amount of water vapor in the air. Water vapor is the gaseous state of fog water and is invisible. There are three main measurements of humidity: absolute, relative and specific.

Absolute Humidity ! Absolute humidity is the total amount of water vapour present in a given volume of air. It does not take temperature into consideration. A useful rule of thumb is that the maximum absolute humidity doubles for every 20 °F or 10 °C increase in temperature.

Relative Humidity !  Relative humidity is defined as the ratio of water vapor in the air to  humid window water vapor at a given temperature. Relative humidity of air is a function of both water content and temperature. The relative humidity of air depends on temperature and the atmospheric pressure. Relative humidity is normally expressed as a percentage. This percentage measures the percent of water in the air compared to the total amount of water the air can hold at a given temperature. The relative humidity will drop by a factor of 2 for each 20 °F or 10 °C increase in temperature. For example, an air temperature at 70°F with 50% relative humidity will become saturated if cooled to 50°F. Saturated is 100% humidity. A device used to measure relative humidity is called a hygrometer. Get one or two of these. Buy two hygrometers if you have separate vegetating and flowering rooms.

Specific Humidity ! This is the ratio of the mass of water vapor to total mass of the moist air sample, and is sometimes referred to as the humidity ratio.

What you NEED to know about Humidity !

  1. We are only concerned  with relative humidity (RH).
  2. Temperature and relative humidity are closely related and go hand in hand.
  3. The higher the temperature the more water vapor the air can hold.a hs_plant
  4. The lower the temperature the less water vapor it can hold.
  5. The higher the relative humidity the less water your plants will use.(drink) You will water less often in a high humid environment.
  6. The lower the relative humidity the more water your plants will use. (drink) You will need to water more often in a low humidity environment.
  7. High humidity does not mean your plants are more vulnerable to powdery mildew, as long as your growing space has good air circulation.
  8. High humidity does not cause mold, however it does make it easier to grow.
  9. Controlling humidity is not critical to growing organic marijuana.
  10. It is in your best interest and your plants best interest to monitor and to control the humidity.
  11. Marijuana will thrive in any humidity if your temperature is under control ( 65-80 degrees F), and you have good air movement with ventilation.
  12. Growers never seem to agree just what Relative Humidity values are best. This should tell you something.

What does this tell you ?  #12       

  • This tells you that the humidity range is rather large and no big deal.
  • This tells you that humidity will not make or break your crop.
  • This also tell you that humidity is the last thing you should worry about when growing organic marijuana.  Temperature, air circulation and ventilation are much more important. If your growing area has good air circulation, good ventilation, and the temperature is controlled within the proper limits, then humidity will take care of itself.  You can however improve on your crops performance by controlling the humidity. I’m just saying it’s not a critical consideration, where as air circulation, temperature and ventilation is a critical consideration.

What You should Know About Humidity !

  1. Seedlings will do best with a relative humidity of 50% to 70%. (Some say 70-80%) marijuana-humidity tray and dome
  2. Clones will root the quickest and easiest if their relative humidity is 70% to 80%
  3. 45% to 70% for vegetating This is the optimal range.(some say 50-80%)
  4. 40% to 50% for flowering This is the optimal range.  (some say below 40%)(Others say to drop the RH to 20% for the final week to increase resin production.)
  5. Transpiration is directly influenced by the relative humidity and because of this so is photosynthesis.

Transpiration !

What is Transpiration and why is it important ?  Transpiration is the process of water a Transpiration diagram movement through a plant and its evaporation from leaves but also from stems and flowers. Leaf surfaces are dotted with pores which are called stomata, and in most plants they are more numerous on the undersides of the leaves. Transpiration occurs through the stomatal apertures, and the opening of the stomata allows the intake of carbon dioxide gas from the air for photosynthesis. Transpiration not only cools plants, it also changes osmotic pressure of cells which enables mass flow of mineral nutrients and water from roots to all new plant growth. The rate of transpiration is influenced by the relative humidity, temperature, wind and sunlight. Transpiration accounts for most of the water loss by a plant, and serves to evaporatively cool plants as the escaping water vapor carries away heat energy.

Problems Low Humidity Cause !

What plants do in low humid environments. Marijuana plants use much more water ina heat stress low humid environments and this is because of transpiration. When the air is dry the plants leaves gives up water vapor thru the leaves pores or stomata. Drier surroundings increases this rate of transpiration and a temperature rise will also hasten the loss of water due to the increased rate of evaporation. During times of low humidity the stomata close to prevent loss of water thru transpiration. This closing of the stomata also slows down the intake of carbon dioxide which slows down photosynthesis. When photosynthesis is slowed so is the plant’s growth. If you add high heat to the low humidity, the stomata close even more slowing down photosynthesis more than before and could become critical to the plants health. Then if you add a brease or wind to this equation the situation can become extremely critical.

Transpiration increases with low humidity even with the closing of the stomata a heat stress (2)trying to prevent this evaporation. When high heat is added transpiration continues to increase. There comes a time where the humidity is so low, coupled with the high temperature, that the plant is unable to keep up with the water loss. The transpiration is so great that the plants roots can not take up water fast enough to replenish the water loss. When this happens the leaves have no choice but to give up the water in their cells. These leaves then lose their turgidity and wilt. If this condition continues the leaves may never recover their health. In very dry, hot conditions the leaves may even become dry and crispy with no chance of recovery. Imagine what happens if wind is involved. With wind added to this condition the problem can become critical very fast.

Problems High Humidity Cause !

Transpiration slows down in high humidity(80 + %) situations even though the stomata a Ganja_Fogare wide open. With the stomata wide open you would think that carbon dioxide absorption would be at its highest along with photosynthesis but that’s not the case. Slow transpiration limits the plant’s ability to cool its self but more important it limits the transport of minerals and nutrients to the plants tissues. With the ability to transport nutrients hindered the ability of carbon dioxide absorption is also hindered and thus so is photosynthesis. With transportation of water to the plants tissues slowed down or stopped the roots can not take up water. If the soil is wet and the humidity remains high for an extended length of time the roots may begin to rot. That’s right, extended times of high humidity coupled with a wet root zone can begin rotting the roots of the plant.

The Grow Room and Humidity !

What really happens ? In almost all grow rooms small and large the relative humidity iswater drops going to fluctuate a fair amount. The main contributing factor to your growing space is the relative humidity outdoors wherever you live, except in sealed rooms using CO2. If you live in a climate that is usually around 60% RH, that will in most cases be the starting RH for your grow space. I live in Colorado and our RH is usually around 35 %. My two grow rooms (one for vegetating and one for flowering) are also usually at 35%. If your rooms are ventilated as mine is, then you are pulling air from outside, so you are filling up your room with the same RH as outside. Now add this to the mix. The humidity outside constantly changing. Colorado Springs average morning(5am) RH is 62% but by 2pm the average is 35%. As the humidity changes outside so goes the inside. What happens when it rains outside ? The grow rooms RH also goes up.                                                                                                                           rain and sunshine

But Wait Thats Not All.  If your grow starts out at 35% RH, what happens when you water your plants ? If you are watering properly, water is coming out the bottom of your pots. Where is that water going ? If you are lucky you have a drain, but for most of us we have trays under the pots to catch the water or the water just runs on the floor. This will also increase the RH of the room fairly quickly. My rooms go from 35% to 50% RH within an hour of watering. The RH then slowly goes back down over the next several hours.

The Point ! The point I’m trying to make is the Relative Humidity is changing all the time. marijuana crop Keeping an even humidity level can be an impossible task without some type of equipment like a humidifier and a dehumidifier. Then you have to justify the cost. Is the extra expense of the equipment and the operating cost worth what you will gain in the crop ? In some climates, a humidifier or a dehumidifier may be a must have. I know I must have an air conditioner which also serves as a dehumidifier. At the dispensary grow we use a swamp cooler which also acts as a humidifier. However relative humidity remains hard to keep constant. I don’t even try. I monitor the humidity but I have never done anything about controlling it. Although I grow a fine crop, who knows, it might be better if I did control the humidity.

Conclusion, The last Word !

Humidity! When growing organic marijuana humidity is best when it is no higher than water on marijuana leaf70% with good air movement. 45% to 70% humidity is the perfect range, however it is not critical for growing good organic marijuana. If your humidity is out of this range it’s really not a problem if you take a few precautions and your temperatures are controlled. Humidity lower than 45% is not a problem for growing marijuana. I have grown organic marijuana in a low humidity environment for a few years now, usually around 35% and have not had any problems.  High humidity will need fresh air and a lot of air movement. ( oscillating fans work well) This is important to prevent fungus and powdery mildew. Just because your climate is dry  with low humidity does not mean your plants are not susceptible to powdery mildew. All growing spaces need good air movement and ventilation. Always monitor your rooms temperature and humidity. I suggest a digital thermometer and hygrometer that keeps track of the high and low temperature and humidity.  Go to Page : Marijuana & Temperature, Page : Powdery mildew and Marijuana !

If a plant is sad, do the other plants photosympathize with it ?

Author Tom D.

humidity cartoona tufail



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