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Humidity is the amount of water vapor in the air, expressed in percentages. Humidity is very important to marijuana plants, because it determines the degree of evaporation. The lower the humidity, the higher the vapor pressure, so the more water and nutrients your marijuana plant can absorb. But if the vapor pressure gets too high, the plant will protect itself from dehydration and the stomata will close. As a result it will barely absorb any water and the growth will slow down.

So it’s important to create the right humidity in your grow room. Marijuana plants require a higher humidity in the vegetative stage than in the flowering phase, because the root system of young plants is a lot smaller. You can start growing with a humidity of around 70% and go down by 5% every week, until you get to a humidity of 40%. You can measure the humidity with a hygrometer. Later in this course I will provide you with a schedule of the perfect humidity for marijuana plants each week. But let’s first talk about the relationship between humidity and temperature.

Humidity and temperature

Air always contains water vapor. This can be a lot or a little. The humidity will already be a bit higher in a grow room, since the leaves evaporate water through their leaves. Plants only use 10% of the water they absorb for growing purposes. The rest (90%) – is evaporated. Air can only contain a certain amount of water vapor. If this amount is exceeded, the condensation process takes place, during which water droplets are formed on the cold areas of the room or on the plants.

The amount of water vapor the air depends on the temperature. Warm air can contain much more moisture than cold air. The relative humidity (RH) indicates how many percent of the maximum amount of water vapor is already in the air. So if this is 70%, some more water vapor can be added before water droplets are formed. But 70% humidity at 77 degrees is different than 70% humidity at 68 degrees. Warmer air can contain more moisture. At 77 degrees, 2 pounds of air can contain 0.45 ounces of water vapor. At 68 degrees, 2 pounds of air with a relative humidity of 70% can only contain 0.32 ounces of water vapor.

How to measure the humidity
Measure the humidity

This also means that if the air cools off, it can contain less water vapor. This increases the relative humidity. If you cool 77 degree air with an RH of 70% to 73 degrees, the RH will rise to 80%. This is called the dew point, because dew will start to form. The air can’t contain any more water vapor, and the water condenses in the form of droplets (dew). The dew point temperature of 77 degree air with 70% RH is 66 degrees.

Since you’re constantly ventilating your grow room, heat from the lamps and the humid air are both sucked out of the room. So you have to keep your room moist at all times to increase the humidity. There are several ways to increase humidity, but I’ll first explain why cuttings, seedlings, vegetating and flowering marijuana plants all need a different humidity levels.

Humidity for cuttings seedlings and flowering plants

How to measure the humidity
Humidity for cuttings

When your cuttings are ready to be placed in the grow room, they have very few roots. They can only absorb minimal amounts of water, so you want them to evaporate as little as possible. By keeping the humidity high, the cuttings will evaporate less water and they also don’t need as many roots. You could also cut of half of the big leaves of your cuttings, so they’re unable to evaporate as much. An RH of 80% is perfect when placing your cuttings in the grow room.

Cuttings are rooted in a room with a temperature of about 70 degrees, a high humidity and fluorescent lighting. If you place the cuttings under a 600 watt HPS lamp in a space with a humidity of 30%, they’ll get quite a shock. It’s therefore important to keep the humidity up during the growth of the root system. The roots grow as long as the plant itself, so up to the second or third week of flowering. A strong root system is very important for the absorption of water and nutrients.

Marijuana seedling
Seedling

The rules are slightly different for seedlings, because they already have a fast growing taproot that can absorb moisture during germination. Don’t cut the leaves of seedlings, because the plant needs these to absorb light and evaporate water. It’s however not wrong to maintain a high humidity, because the seedlings can also absorb water and nutrients through the leaves. You can start with 70% and slowly lower it to 50%.

Marijuana flowering
Flowering plants

Once your marijuana plants start to flower, you have to slowly lower the humidity. The root system is now properly developed, so the plant can absorb the maximum amounts of water and nutrients. Another reason to lower the humidity during the flowering stage is because fungi flourish in a climate with a high humidity.

As the plant gets older, the chance of infection increases. The plant uses more water and the vapor pressure increases. If you have a high humidity, the water can accumulate in the buds, giving fungi room to grow. Bud rot is the most common and most annoying fungi.

Schedule for ideal humidity of weed plants

Here’s a sample chart you can use to make sure your marijuana plants experience the perfect humidity. Notice the difference between cuttings and seedlings, especially during the first couple of weeks.

Humidity cutting plant
Humidity for cuttings
  • Week 1 growing: RH 80%
  • Week 2 growing: RH 70%
  • Week 1 flowering: RH 65%
  • Week 2 flowering: RH 60%
  • Week 3 flowering: RH 55%
  • Week 4 flowering: RH 50%
  • Week 5 flowering: RH 50%
  • Week 6 flowering: RH 45%
  • Week 7 flowering: RH 45%
  • Week 8 flowering: RH 40%
  • Week 9 flowering: RH 40%
Marijuana flowering
Flowering plants
  • Week 1 growing: RH 70%
  • Week 2 growing: RH 65%
  • Week 1 growing: RH 60%
  • Week 2 flowering: RH 50%
  • Week 3 flowering: RH 50%
  • Week 4 flowering: RH 50%
  • Week 5 flowering: RH 50%
  • Week 6 flowering: RH 45%
  • Week 7 flowering: RH 45%
  • Week 8 flowering: RH 40%
  • Week 9 flowering: RH 40%

How to increase the humidity

There are different ways to increase the humidity in your grow room. You could spray water on the floor or on the walls. Just make sure you place pond liner or other waterproof foil beforehand. You could hang the lamps a bit higher, so the temperature at your plants is lowered a bit so you won’t have to extract as much air. Placing wet towels and bins of water in your grow room also increases the humidity.

How to increase the humidity
Increase the humidity

If you really want to do it properly, you should get a humidifier. This device converts water into water vapor and spouts a constant mist with a high humidity into your room. Some humidifiers have a water tank, some have to be connected to the tap and others even have a humidistat that measures the humidity and is turned on once the humidity drops below a set value.

How to lower the humidity in your growing room

Once your plant starts flowering, you want the humidity to go down a bit, so you have to dehumidify the room. You can do this by turning up the air extractor or by blowing cold air into the room. The best way to lower the humidity in your grow room is to buy a dehumidifier. This device removes moisture from the air and transports it to a drain or stores it in a tank. Don’t use a tiny one that people use for basements, because these are very quickly saturated if you place them in a grow room.

Lower humidity
Lower the humidity

The humidity outside can also influence the humidity of your grow room when sucking in outside air. If your humidity greatly increases during rainy days, you could turn off the fan that sucks air into the room for a bit, or put it on the lowest setting. Make sure the temperature doesn’t increase too much, since you’re not sucking in as much cold outside air.

Humidity during watering

The humidity increases enormously when you’re watering your marijuana plants. That’s prefect during the vegetative stage, and you could spray the floors and wall (if they’re covered with waterproof foil) to increase the humidity. But the humidity could become too high during the flowering stage after watering. Once your lights go out, the temperature usually decreases and you won’t have to extract as much air, causing the humidity to rise.

Humidity during watering
Watering

You should therefore always water your plants right after the lamps come on, to let most of the evaporation take place during the day. If you’re growing on hydro, make sure you don’t spill any water and that you keep the room dry during the dark period. Never spray water on your buds if it’s not necessary. This greatly raises the humidity and increases the chance of fungi.

Humidity for outdoor marijuana plants

If you’re growing outdoors, you can’t do much to manage the humidity. During spring and the beginning of the summer, a high humidity is no problem, and the plant won’t have buds yet for the water to accumulate in. You’ll see that the plant is very wet early in the morning from the dew, but once the sun comes out and the day passes, this all evaporates without any problems.

But after the summer, the actual flowering of the plants starts, and the climate changes. It becomes colder and it rains more often, increasing the humidity. The dew can cause problems, because the sun doesn’t always show and the temperatures aren’t high enough to sufficiently evaporate it. Large amounts of rain can also cause problems.

humidity outdoor marijuana plants
Outdoor marijuana

It’s definitely not the case that your plants will develop bud rot after a bit of rain. But it is however something to keep an eye out for, because it would suck if you see your buds start to rot after 6 months of offering them your love and care. You could shake the dew off the plants in the morning, during the last month of flowering. It’s good to shelter your plants from the rain if very rainy days are predicted.

How to measure the humidity

A plant can experience heat stress when it gets too much warmth. Heat stress doesn’t usually occur outside, because it won’t get warm enough for that. We’re talking about temperatures above 105 degrees. Photosynthesis is greatly inhibited during heat stress. Furthermore, the activity of enzymes decreases and the production of proteins slows down. Certain proteins could break. If heat stress goes on too long, the plant will die.

How to measure the humidity
Measure the humidity

In this way, you can also check the humidity in the room when the lights are off. These hygrometers often have a memory feature that will remember the highest and lowest measured values, so you can tell whether you stayed within the margins. I always use a thermometer/hygrometer with min-max feature. Check this one on Amazon.com

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