Humidity 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. 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. 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. 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/hygrometer 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/hygrometer with a min-max feature.