Humidity (water vapor found in the air) in a grow room or grow tent is great: it’s a sign that your plants are breathing healthily, and that there’s no lack of water throughout your grow. At certain stages of a plants life, different levels of humidity are needed so you can seed, veg, and flower the right way. But that grow room humidity level shouldn’t be more than 70% at all times, so if you’re reaching levels that high, you might want to consider where that moisture is coming from, and what you can to do prevent it from harming your grow.

Where’s all that grow room humidity coming from?

Illustration of leaves soaking in water
Humidity is a measurement of the amount of water vapor in the air. Water vapor's always in the air, but the temperature of your grow room effects the presence of humidity in your garden:
  • The higher the temperature in your grow, the more water vapor the air can hold. Heat also causes water to move faster through the air, covering more area, resulting in higher grow room humidity. That's why when there's lots of moisture and it gets hot (like near swamps) it gets sticky and muggy: there's warm water all through the air moving quickly.
  • The lower the temperatures are in your grow, the less water vapor air can hold. Without heat the movement of water vapor slows down, covering less area, resulting in less grow room humidity. Lower temps can be tricky because too much moisture can result in moisture build-up and/or freezing; not enough moisture and it can get bone dry like the desert in winter.
The carbon dioxide (Co2) that plants naturally give off can raise grow room temperatures, and that will result in moisture build-up which will raise humidity. If you use Co2 in your grow, that grow room humidity level will rise even more Grow equipment can also raise the temperature in your grow area like your grow lights. If it’s warm in your grow room and you turn the lights off, temps will dropdown. With enough of a temperature drop, you could see moisture buildup around your lights, which is a sign of humidity fluctuation

Grow room humidity can’t be that bad, can it?

A little more or a little less humidity in your grow room won’t hurt much as long as it’s within 40-70%.
Too much grow room humidity, though, and you’re headed for a world of trouble:
  • Moisture buildup and too much humidity in your grow room can result in rotting, like bud rot or flower rot. This is the worst, especially because the results can’t be seen until you’re harvesting, and at that point, it’s too late to fix any problems
  • White mold can collect on your leaves, preventing them from taking in light and water. White mold will feed off your plants, and if not detected early enough, can render your yield useless
Too little humidity in the grow room isn’t good for plants either:
  • Growth will become stagnant- If your plant feels it will become dehydrated, it can also result in the closing of the stomata, which means the plants won’t be able to take in much (if any) water
  • Nutrient deficiencies- Plants love to take in water vapor through their leaves, but when there’s not enough water in the air, your plants will need to get their water somewhere else: their roots. When your plants are thirsty and taking up lots of water into their roots, nutrients will follow. Too much nutrient uptake is bad for your plants, and you’ll start to see nutrient burn on the tips of your leaves
Humidity Levels for Each Stage of Growth
Several plants in a large indoor grow room
Like we mentioned earlier, plants thrive in various humidity levels during all stages of their life. Here’s an idea of where the humidity in your grow room should be: Seedlings/Clones: 70-75% Humidity- Because your plants need time to get some good roots growing, the majority of their water through their leaves. You’ve got to make sure there’s plenty of moisture in the air when your plants are babies, that way they’re fed properly. That’s why humidity domes and propagation trays are around: they build up lots of moisture in there so your plants can grow. Humidity in grow room for vegging plants (up to Flowering): 50-~70% Humidity -Now that your plants are pretty grown, you’re going to let the roots do a majority of the work. Plants will still need to take up moisture through their leaves, so you’ll still need relatively high humidity levels (60-70%) during the first part of your vegging cycle. After about the first 2-3 weeks of vegging, you’re going to prep the grow for flowering, which you will start by gradually lowering the humidity down to 55-60%
Humidity in grow room for flowering plants: 40-50% Humidity- Flowering plants will need a comfortable environment to live in, not too hot and not too cold. When beginning your flowering cycle you’re probably going to be starting around 50-55% humidity, which is right around where it should be. As you grow to harvest, you’ll gradually lower the temperature down to about 40% when you’re ready to clip, moving 5% down every 2-3 weeks. Humidity in grow room for harvest: Around 45-50% Humidity- Harvesting plants takes some finessing to get down pat. You want to make sure that the environment isn’t too humid to not create a molding issue. At the same time, if it’s too dry in there your harvest will dry out too much, and it’ll become brittle, harsh, and virtually unusable. Make sure when harvesting that your plants are in an area where humidity levels are at an even level- around room temperature and heat level- so your yield can dry without becoming stressed or contracting mold. Adding/Taking out humidity from your grow
Grow1 Humidifier
If you’re checking your humidity and you’re leaning toward being too humid or too dry, here are a couple of things to help you adjust those levels; Too much humidity: If it’s too humid in there, you can use a Dehumidifier to get rid of excess moisture in the air. Note: If the dehumidifier does not have an automatic shut-off, be sure to set a timer and regulate how much water it's collecting. If the unit also has no drainage system be sure to check to see when that humidifier is full and drain it when it is. You also have the old fashion way of getting the humidity out: ventilation. Whether you simply open a window, or vent a duct fan to push that moisture out of the grow area and outside. If you use this route, be sure that venting is a one-way street: prevent any pests or more moisture from coming back into your grow space
Using a humidifier in your grow tent
Not enough humidity: Increasing your grow’s humidity can be as simple as adding a humidifier to your grow. If it doesn't have an auto-shutoff switch, it’ll help to get a hydrometer or a digital humidity meter to measure the humidity level of your grow to know when to stop pumping the air full of moisture. Remember that humidity levels and temperature play off of each other, so to keep it humid in there, make sure your temperatures higher when you want a humid environment, and cooler when you want less moisture in the air.

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