Humidity either allows or prevents water evaporation in your marijuana plants. If the humidity level is on the lower side, marijuana plants actually absorb more nutrients and water, since the speed of evaporation is higher in drier climates. That being said, lots of dryness is not always a good thing. Oftentimes evaporation can become a strain on the plant if it goes at too high of a rate for too long – at which point the marijuana plant will close up its stomata and slow down the evaporation, so as not to lose too much water. This is less than ideal, of course since it will in turn slow down your plant’s taking in of water and, therefore, will stunt its growth.
That is why humidity levels are important to monitor. Different humidity’s coincide with different stages of growth better or worse. The flowering phase of your plant, for instance, requires a lower humidity level than the vegetative stage. Smaller roots need a higher humidity, so a general rule of thumb is that the younger your plant is, the more humidity it will need.
Use a hygrometer to measure the humidity levels in your grow room. It should be around 70% at first, and then every week take away 5% until it has reached 40%, then keep it steady at that level.
Read the article Structure and function of stomata for more information about evaporation.
In this article we will discuss:
Temperature & humidity
Temperature and humidity levels have a serious relationship. The temperature affects how much water the air is capable of absorbing. When temperatures are around 68 degrees Fahrenheit, for instance, the water can hold 7.2 milliliters of water at maximum. If it is holding that much water, the humidity level is at 100%. This would be so high that you wouldn’t even be able to see through the air. If the temperature level is at 32 degrees Fahrenheit, the air will absorb less water – closer to 5 milliliters of water. This is why winter air is drier than summer air.
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If you have the right kind of indoor setup, it will include frequent ventilation. Ventilation not only keeps your plants from having a buildup of moisture, but it also releases the humidity in the air. That means that you will need a consistent inflow of humidity.
Clones, like new seedlings, have tiny roots at first. This means they aren’t able to absorb much water, so what little water they do absorb you will want them to hold on to. In other words, you want to combat the speed of evaporation in these little plant. Keep the humidity high to achieve this. 70% is roughly the humidity level you should strive for when growing cuttings.
If the temperature is roughly 71.5 degrees Fahrenheit and there is plenty of humidity as well as fluorescent lighting, then your cuttings should grow nicely. With a 600-watt HPS lamp, try for 30% humidity instead. You need your plants to be strong and healthy to absorb enough water and nutrients.
Read the article How to make marijuana clones for more information
Seedlings function a little bit differently than clone cuttings. They actually absorb water quite fast when you take their root size into consideration. Whatever you do, don’t remove any of the seedling’s leaves, since they are crucial for water and light absorption. Make sure they do this by starting the level of humidity at 60% and decreasing it gradually to 40%. Click this link to buy marijuana seeds
The flowering stage of your plant’s life is when it is the most mature, so the roots are able to absorb high amounts of water and nutrients. Because mold can be a big risk to flowering plants, keep the humidity quite low. The older your plant gets, the more likely mold (such as bud rot) will infect it.
Read the article Flowering stage for marijuana plants for more info
You can follow this information exactly to make sure your plants grow as much as possible. Keep in mind that clones and seedlings require different levels of humidity.
Clones should start off with their first two weeks at 70% humidity, then start decreasing it once its flowering phase begins the following week (week 3). Start with 65% for week three, 60% for week four, 55% for week five, and weeks six and seven should be around 50%, weeks eight and nine at 45%, and weeks ten and eleven at 40%.
Download my free marijuana grow guide and learn to grow like a pro!
Seedlings, on the other hand, should start things off lower with their first two weeks of growth being at 60% humidity. In their third week (which is the first week of flowering), they should have a humidity level of 55%; weeks four through seven should maintain a level of 50%; weeks eight and nine 45%, and weeks ten and eleven at 40%.
If you are growing marijuana plants indoors, increasing the humidity level shouldn’t be too tricky. You can begin by just spraying water on the flowers and on the walls, then by moving the lights to a higher position, thus distancing them from the plants and slowing down evaporation. You can even fill containers with water and set them on the ground in the room to increase the humidity level.
As soon as your plants entering the flowering stage, you will need to start lowering the humidity level. An extractor fan will do the trick, or else just lowering the temperature by fanning cold air inside. To make sure it is consistent and reliable, it’s better to go with an actual dehumidifier. This will turn water from a gas into a liquid and either drain it out or hold it in its container. Larger ones are better so you don’t have to constantly check it and empty it out.
Even if you’re growing inside, the outside humidity will have an effect on your indoor grow room. If it’s warm and rainy, for example, it might be a good idea to turn off or slow down the extractor fan. Don’t let the temperature increase too fast.
You can use a hygrometer to measure the humidity of your grow room. Just place it above your plants in a place that is ventilated easily. They are inexpensive – ranging in price from $5 to $10 – and function best if you get one with a wire (so you don’t have to turn the lights on to see what the humidity level is). They will even keep track of the humidity level highs and lows so you can keep track of how you’re doing.
Watering & humidity
Predictably, water increases humidity levels. During your marijuana plants’ growing stage, this is no problem. When they are blooming, however, this can be a problem. To combat this, only water your plants when the lights are turned on so the water can evaporate quickly. Definitely do not spray your buds with water, unless you want a terrible case of bud rot.
Humidity & outdoor plants
If you are an outdoor marijuana grower, the chances are slim that you will have to worry too much about the outside humidity levels. There is naturally a higher humidity when your plants need it, and a lower humidity when temperature and light are decreasing and your plants are beginning to flower.
That being said, at the end of summer there could be plenty of morning dew that will sit on your plants and may cause mold. Therefore, when you go to your plants in the morning, wipe off as much of that morning dew as possible. If you are able to move your plants (i.e. if they are being grown in containers) then you can move them out of the rain when it is predicted ahead of time.