marijuana humidity
Growing Marijuana

What Is The Right Humidity Level For Marijuana Plants?

marijuana humidityHumidity and marijuana plants

Humidity is the amount of water vapor in the air, expressed as a percentage. The humidity is very important for a marijuana plant because it determines the degree of evaporation. The lower the humidity, the higher the evaporation pressure and the more water and nutrients your marijuana plant can absorb. However, if the evaporation pressure becomes too high, the plant will protect itself against dehydration and its stomata will close. Consequently it won’t be able to absorb any water anymore, and the grow will stagnate.

So it’s important to create the right humidity level in your grow room. During the growth period, marijuana plants require a higher humidity than during the blooming period because the root system of younger plants is a lot smaller. You can start the growing process with a humidity around 70%, and then lower it by 5 each week until you reach a humidity of 40%. You measure the humidity level with a hygrometer. Later in this article there’s a schedule with the perfect humidity for indoor marijuana plants for each week and some tips for outdoor growers. But first, the relationship between humidity and temperature.

Humidity and temperature in your marijuana grow room

It is important to know that the amount of absorbable water depends on the temperature. With a temperature of 68°F (20°C), the air can absorb a maximum of 7.2 ml of water. This air has a relative humidity of 100%. When that happens the amount of water vapor is so high that you won’t be able to see anything. Air with a temperature of 32°F (0°C) can only absorb about 5 ml of water. This is the reason why the air is a lot drier in the winter than in the summer. I’ve written an e-book that you can download for free, and that’s packed with tips about how to create the perfect climate for your marijuana plant. Download my free marijuana grow bible for more tips about growing marijuana.

Because you constantly ventilate your marijuana grow room you pull, aside from the heat that the lamps create, also the humid air from the room. Therefore you always have to make sure to keep your grow room moist in order to increase the humidity level. There are several ways to increase the humidity. But first I’ll explain why cuttings, seedlings, and growing and blooming marijuana plants all need a different humidity.

Perfect humidity for marijuana cuttings

When your marijuana cuttings are ready to be put in your grow room they don’t have a lot of roots yet. They can’t absorb that much water in this phase, so you want them to evaporate as less water as possible. By keeping the humidity high, the cuttings evaporate less water, and thus need less roots. You can also cut the large leaves halfway from the cutting so that they evaporate less. A humidity of 70% is perfect when you put the cuttings in your grow room.

Cuttings develop roots in a clone box with a temperature of about 71,5°F (22°C), a high humidity and fluorescent lighting. If you put the cuttings under a 600 watt HPS lamp in a room with a humidity of 30% it’ll get a great blow. During the growth period of the root system it’s therefore important to keep the humidity high. The roots grow as long as the plant itself, about until the 2nd or 3rd week in the blooming stage. A strong root system is very important for the absorption of water and nutrients. Download “The Marijuana Bible” for free, and learn how to create a strong root system for your marijuana plants.

Perfect humidity for marijuana seedlings

There are slightly different rules for marijuana seedlings because they already have a taproot during germination, which can absorb moisture and grows really fast. You should not clip the leaves of the seedlings, because the plant needs it for the absorption of light, and the evaporation of water. Yet, it’s not a bad idea to keep the humidity high because the seedling can also absorb water and nutrition via its leaves. You can start at 60% and then gradually reduce it to 40%.

Humidity during the flowering period of marijuana plants

When your marijuana plants begin to flower, you slowly need to reduce the humidity level. The root system is well developed now, so the plant can absorb the maximum amount of water and nutrients. Another reason why you should lower the humidity level during the flowering period is because molds thrive in a climate with high humidity. As the plant grows older, it becomes more likely that it will get infected. The plant uses more water and also evaporates more. When the humidity is high, water can accumulate in the tops where molds get the chance to grow. Bud rot is the most common and annoying mold. Download my free marijuana grow bible for more tips about growing marijuana.

Schedule for the ideal humidity for marijuana plants

Below is an example schedule you can use to make sure that your marijuana plants get the perfect humidity. So there is a difference between cuttings and seedlings, especially the first few weeks.

Marijuana cuttings humidity level

Week 1 growth: 70%
Week 2 growth: 70%
Week 1 flower: 65%
Week 2 flower: 60%
Week 3 flower: 55%
Week 4 flower: 50%
Week 5 flower: 50%
Week 6 flower: 45%
Week 7 flower: 45%
Week 8 flower: 40%
Week 9 flower: 40%

Marijuana seedlings humidity level

Week 1 growth: 60%
Week 2 growth: 60%
Week 1 flower: 55%
Week 2 flower: 50%
Week 3 flower: 50%
Week 4 flower: 50%
Week 5 flower: 50%
Week 6 flower: 45%
Week 7 flower: 45%
Week 8 flower: 40%
Week 9 flower: 40%

How do you increase the humidity in your marijuana grow room?

There are several ways to increase the humidity in your marijuana grow room. You can spray water on the floor or walls. Of course make sure to use pond liners or another waterproof foil. You can also hang the lights a little higher so that the temperature drops a bit around your plants so you don’t have to use the extractor fan as much. Also containers with water or wet towels increase the humidity in your grow room.

If you really want to do it properly you should buy a humidifier. This machine converts water into water vapor and sprays a continuous mist containing a high humidity in your room. They come with a water reservoir or some can be connected to a faucet. Some even have a hygrostat which measures the humidity level and kicks on if it goes below the set value.

How do you decrease the humidity in your marijuana grow room?

From the moment your marijuana plants begins to flower you want to have a somewhat lower humidity, so you’ll need to dehumidify. You can do this by putting the extractor fan on a higher setting, or by blowing cold air in the grow room. The best way to lower the humidity in your grow room is by getting a dehumidifier. This device extracts moisture from the air and either drains it or stores it in a reservoir. Do not use one of those small ones, meant for in the basement, because those fill up in no time if you put them in your marijuana grow room.

The current outdoor humidity can also affect the humidity in your grow room. If the humidity level rapidly rises on rainy days, then turn off the extractor fan or put it on the lowest setting. Make sure the temperature doesn’t rise too quickly, because you’re getting less cold air from outside. Download my free marijuana grow bible for more tips about growing marijuana.

How do you measure the humidity?

You can measure the humidity with a hygrometer. Hang the hygrometer just above the plants, in a well-ventilated area. You can get an analog one for $5, or a digital one for $10. It’s better to get a little more expensive hygrometer. Preferably one with a wire so you can hang the display outside of your grow room. This way you can also monitor the humidity when all the lights are off in your grow room. Often they also come with a memory that keeps track of the highest and lowest values, so you can see if you stayed within the margins. I always use a thermo/hygro meter with a min-max feature.

Humidity while watering your marijuana plants

When you water your marijuana plants, the humidity rises tremendously. That’s great during the growing period, and you can spray the floor and the walls (only if there’s waterproof foil on it), to make the humidity rise. But during the flower period, the humidity levels may get too high after watering. When the lights go off in your grow room, the temperature usually goes down and you don’t need to extract as much hot air, which causes the humidity levels to rise.

So always give your marijuana plants water when the lamps go on, so they can evaporate most of it during the day. When you work with hydroponics, be sure not to spill any water, and keep your space dry in the dark period. Never spray water over your buds if it’s not necessary. This greatly increases the humidity and increases the chance of mold.

Humidity for outdoor marijuana plants

If you grow marijuana outdoors you have little to do with humidity. During the spring, and the beginning of summer a high humidity is no problem, and the plant doesn’t have any buds yet where moisture can accumulate. In the early mornings you’ll notice that your plant is totally wet from the dew and when the day goes on this evaporates without any issues.

But after the summer the actual flowering of the plants begin, but the climate changes as well. It becomes chillier and rains often which causes the humidity to rise. The dew may also pose a problem, because the sun may not always break through and the temperatures aren’t high enough to evaporate it. A lot of rain can also be a problem.

Your plants won’t get any bud rot after a little bit of rain. It’s just something to keep an eye on, because it would be a waste to see any rot after so much love and effort you put in your marijuana plants for six months. If you can, shake the dew off your plants in the morning during the final bloom month. If rainy days are forecast, it’s recommended to put your plants under a shelter or another dry place.

How do you measure the humidity

You can measure the humidity with a hygrometer. Hang the hygrometer just above the plants, in a well-ventilated area. You can get an analog one for $5, or a digital one for $10. It’s better to get a little more expensive hygrometer. Preferably one with a wire so you can hang the display outside of your grow room. This way you can also monitor the humidity when all the lights are off in your grow room. Often they also come with a memory that keeps track of the highest and lowest values, so you can see if you stayed within the margins. I always use a thermo/hygro meter with a min-max feature.

If you want to start growing, download my free grow guide and order some marijuana seeds. All top quality marijuana seeds are available in my marijuana seed shop. We ship seeds to the US, CA and many other countries. For any growing related question please visit the marijuana support page.

Source: ILoveGrowingMarijuana.Com

 

  • It should be added that 40% is safe, and that RH below 30% will increase tric production, I like to be in the mid 30’s

    • budz253

      Is 30% were I Wana be? Is there anything I can spray to control the PM? I’m in week 4 flower. Thanks

      • Mix one part skim milk to 10 parts water and spray em

        • budz253

          Is 62 degrees to cold for night temps? I was using a baking soda mix for the PM but it’s leaving a white coat on the leafs and still showing up. I increased air flow. I had to take over this grow on emergency reasons.it’s a 10 k garden. Pm is the only problem

          • no, but it’s close, PM is caused by humidity being to hign

          • budz253

            Ok thanks humidity is 38 to 50. One the dehumidifiers broke in week 2 so the humidity really spiked up. I’m gona get a bigger one. It’s real rainy were I live

          • click link in my avatar I have disqus there you can ask stuff there :)

  • budz253

    So for bloom I Wana be around 30% RH or lower. I’m having trouble with PM. Please reply thank you

  • tweed

    What kind lights do you need??

  • Nadav

    Hi,

    I wonder if the moisture of the seeds or the moisture of the plant during the growing process can be measured (not the atmosphere, but the plant it self) and how important is this test for the quality of the desired outcome?

    Can the plant moisture be measured online and is it a better indication for quality then any other measurement tool?

    Do you know of any systems like that?

    Thanks a head for helping me!

  • Danbo

    Hi. Can u help!!?? I am a first time grower and have experienced sum problems with setting up but thanks to a little knowledge acquired via ur site I solved them and now have an nft sytem fully operational with no upsets but there is one bit of info I cant source and that is wen I turn off the lamp for 6hrs dark, do I also turn off the pump and stop the flow of nutrients or keep it running continuously which I am doing at present? Please advise. Thank u

  • tj

    in the seedling to first three weeks do I need to have a fully closed area or can it be a half closed with a/c always on it like a custom made work bench all side covered but open in the front for a/c

  • Can I run two 6500k 18w and two 5000k 23w bulbs for one seedling 3 days old. In a 4.75 square foot area 30 inches tall?