Not sure if my Gorilla Glue auto is right

she’s looking good

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Ever since flipping both lights on and bringing it slightly closer to the light I’ve seen it really grow. Wondering if I should have done that sooner. She might not have stunted as much

They are looking good. I try to keep an average of 81 in my tents. They seem to do much better than cooler temps here. Autos can be odd starting. They start a little slower. Then like hitting a light switch. They grow like mad. This auto here is 4 weeks old. When she was done. She would have filled a 3x3 tent. So I wouldn’t stress a lot over a slow start.

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Yea I wondered about that. This my first time growing autos so they’re completely new to me. Good to know she’s looking good

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They can be a pain to start. but once they are going. I have real good luck with them. I prefer them over photos myself

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@jetlag I am having some issues with humidity as my plants are in their fourth week of flowering. What was your RH average? Cannot run AC all day (wife will not allow it as I spent too much money already). The dehumidifier does not bring the RH down below about high 50’s to low 60’s. Any other suggestions?

Thank you.

I wonder if you could use bevita packs in the tent. Don’t know if that would help with the humidity. That’s been a challenge but heat has been my biggest issue. Especially this week it’s going to be in the 90’s and I have a single window unit.

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Ok, I am attaching two charts . They basically say the same thing, but will give you an excellent reference for what to look for. You didn’t say what your temps and humidity are, but look at the charts. One thing that can help a lot if humidity is too high for the temperatures is to get a decent fan to blow air on the plants. Had I done that to my WW Autos, I would not have lost part of the crop. I don’t have an average RH for me, since I am growing in a greenhouse in Eastern North Carolina. This is July, hot and humid as heck. My photos are nowhere close to flowering, so water and food are their main issues. My Gorilla Glue Autos are just over a month old, no flowering yet. Even in early flower the girls can handle humidity, but as flower progresses it becomes more critical. I did not have a humidifier by them, and was cutting the fan off at night, so I got what i deserved. I now have two fans in there, and will bring the dehumidifier in when in flower. Heck, will bring a second one if needed. A decent fan, not one of those little ones that barely blow air, will make a big difference. Moving air hampers the mold formation. @r3ys3r, I use the Boveda packs to maintain moisture in quart jars once dried and cured, but have no idea how much of that stuff you would need to use in a grow room . Very early on, I was advised to maintain moving air by the plants as the spring progressed into summer by more than one person here.

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@jetlag That is some very technical stuff! I had read to always have RH below 50% in flowering. However, according to the chart , at my current temp of 85F I should be at 67% RH. What gives?

Thankfully I have decent fans in the tent. The air flow is pretty constant so I at least have that going for me.

Article to help explain it. it will be in two separate posts

Vapor pressure deficit (VPD) in cannabis cultivation

What is vapor pressure deficit?

Often, marijuana growers who have their plants in indoor grow tents or rooms with artificial lighting, take into account parameters such as temperature and relative humidity to maximize plants growth and bloom, thus obtaining abundant harvest with top quality flowers. As we will see later, these factors affect different processes of the plant, such as transpiration or nutrient uptake .

However, we usually forget about another important factor that is related to the aforementioned ones: vapor pressure deficit or VPD . Broadly speaking, VPD is the difference between the amount of water vapor that the atmosphere is able to retain (which depends on temperature) and the amount of water vapor contained in it (relative humidity). It is usually measured in kilopascals (kPa).

Dew is formed when the environment is saturated with water vapor

Providing our plants with a correct relationship between temperature and relative moisture will keep us on the right VPD parameters, increasing the plant’s activity and thus improving its growth and showing its full potential during the flowering phase . The ideal would be to adjust both parameters (temperature and humidity) to get the best possible VPD value, so that the development of plants will be amazing.

VPD: Temperature and humidity in cannabis cultivation

Temperatures, humidity levels and VPD values recommended for the growth stage

In the chart above you can see the right temperature and moisture values to maximize the development of cannabis plants . The green zone shows you optimal values of VPD, in which it would be ideal to maintain the environment. The orange zone shows correct values, in which plants should not have problems to develop normally. However, red areas show values in which your plants will be very far from hit their full potential, since the relation between temperature and moisture is not optimal.

With relatively high values of VPD (between 5 and 12) plants tend to open their stomata and release a considerable amount of water vapor into the environment (increasing their transpiration). This increase in transpiration results in an increase in the plant’s photosynthetic activity, which will need more nutrients and will improve its overall growth, both during growth and bloom. The best range is between 7.5 and 10.5, which we have marked in green. However, we must be careful that VPD is not too high (dark red zone) since the plant would close its stomata to avoid releasing too much water vapor to the environment, which would result in fast dehydration due to excessive transpiration.

On the other hand, if VPD is too low (light red zone), the plant will also close its stomata for not releasing more water into the atmosphere, which in this case would be already saturated (it would have reached the maximum water retention capacity in a specific temperature). In this way, reducing transpiration will also reduce photosynthesis, which has a direct impact on the plant’s development and yield.

You can check the VPD chart for the bloom stage in this link.

That way, VPD is essential when it comes to offer the best conditions for the development of plants , also to understand its water requirements according to the nutrients used and thus adjust them correctly to meet the grower’s expectations. We have seen that the best way to keep a suitable VPD value is to control the temperature and/or relative humidity, which is something we have multiple options for, as we will see later.

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Low levels of VPD combined with high relative humidity (light red zone) may cause nutrient deficiencies, guttation phenomenon, different diseases or weak growth; while a high SPD value combined with low relative humidity (dark red zone) it could cause wilting, curled leaves, poor growth or crisp leaves.

How to control temperature and humidity in indoor grow rooms

If you usually read the indoor growing section of our Blog, you probably already know most of the systems to control the climate for this cultivation technique. However, let’s make a brief review for those who are not sure how to set up an indoor grow.

Grow lights used in indoor cultivation (usually sodium vapor lamps) produce a lot of heat, so an essential element to regulate the climate properly is the air extraction equipment. Thanks to it, we will be able to expel the heat emitted by the bulb and the moisture produced by the plants themselves, also to introduce new and fresh air into the grow room. Most modern air extractor fans include temperature and/or moisture controllers, which will operate depending on the values set by the user. Of course, there are also climate control units which can be connected to all kinds of devices to regulate the temperature and relative humidity inside the grow room.

Thus, we can connect an air humidifier in case you need to raise the moisture or fans if what we want is to decrease it. With the climate controllers we can also modify the extraction equipment speed and adjust it in order to expel the correct amount of heat and moisture. If the temperature is too high, we can use less powerful bulbs or LED systems, increase extraction and ventilation or use air conditioning.

Climate control unit

On the other hand, if it is necessary to increase the temperature, we can incorporate to the assembly some heating device like tubular heaters for greenhouses or heat mats . Reducing ventilation and extraction will also help, although you will need to control the increase in relative moisture. In short, climate control units allow us to set the temperature and moisture values that will determine the operation of all devices connected, which certainly represents a great help for the grower.

Dehumidifiers decrease environmental moisture and release heat, which can be helpful in certain cases like very moist and cool areas, such as basements. We can also add CO2 to the environment , although this is usually reserved for experienced growers who have their grow room fully optimized. As we see, there are numerous options in the market to meet all the needs and achieve the ideal environment for our plants, which has a direct impact on their performance.

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Air and humidity are very closely intertwined. The hotter the air, the more moisture it can hold, and the reverse is true. Notice 85+ degree days at home with little or no clouds. Humidity is very low typically, and you feel COOLER , or more comfortable than the temps would indicate. Same temps next day with big cumulus clouds, (thunderstorm clouds), and you are hot and sweaty. That you would call a very humid, sticky day. Once air reaches its saturation point, also known as the Dew point, water vapor turns into visible clouds in the atmosphere. Now stand in front of a decent fan blowing that hot humid air past you. You feel cooler, much cooler in some cases. It is still the same hot, humid air yet you feel a lot cooler. A weather front, cold front or warm front, typically bring moisture in front of them, and you get rain, snow etc. Notice once the front passes you usually have a very pretty day with low moisture. The front as it passes sucks the moisture out of the air and it is replaced with cold, or cooler air in the summer, dry air behind it. I know, getting deep here, that is the retired airline pilot in me. We lived weather day in and out, and your life depended on having a decent knowledge of what mother nature was doing. Another easier example to understand is BREAD. You buy it in a plastic bag, and you open and use and close again. If it sits in the bag long enough, it will get hard and moldy. Why? bread has moisture in it and in a plastic bag, moisture can’t get out. With the right temps, you get mold, which is why people refrigerate of freeze bread. .Ok I stop here, getting boring , hope this helps.

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Managing Humidity

Managing the humidity in your indoor garden is essential to keep plants happy and transpiring at a healthy rate. Transpiration is very important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. The temperature of a healthy transpiring leaf can be up to 2-6°C lower than a non-transpiring leaf, this may seem like a big temperature difference but to put it into perspective around 90% of a healthy plant’s water uptake is transpired while only around 10% is used for growth. This shows just how important it is to try and control your plants environment to encourage healthy transpiration and therefore healthy growth. So what should you aim to keep your humidity at? Many growers say a RH of 70% is good for vegetative growth and 50% is good for generative (fruiting /flowering) growth. This advice can be followed with some degree of success but it’s not the whole story as it fails to take into account the air temperature.

This humidifier releases a cool mist to increase relative humidity indoors.

This humidifier releases a cool mist to increase relative humidity indoors.
Photo credit: Aquaculture Hydroponics, UK.

Vpd _2degree

If your growing environment runs on the warm side during summer, like many indoor growers, a RH of 75% should be maintained for temperatures between 79-84°F (26-29°C.)

The problem with running a high relative humidity when growing indoors it that fungal diseases can become an issue and carbon filters become less effective. It is commonly stated that above 60% RH the absorption efficiency drops and above 85% most carbon filters will stop working altogether. For this reason it is good practice to run your RH between 60-70% with the upper temperature limit depending on your crop’s ideal VPD range, in the example it would be 64-79°F (18-26°C.)

The table also shows that if your temperature is above 72°F (22°C), 50% RH becomes critically low and should generally be avoided to minimize plant stress. Please understand that by presenting this information we do not want you to go to your indoor gardens and run your growing environment to within strict VPD values. What’s important to take from this is that VPD can help you provide a better indication of how much moisture the air wants to pull from your plants than RH can. If you want to work out for yourself the VPD of your plants leaves you can follow the steps below:

See how the vapor pressure deficit changes when there is a smaller gap between air temperature and leaf temperature.

Vpd _1degree

Measure the air temperature and relative humidity and look up the nearest vapor pressure figure on the above table.

Humidity’s Effect on Plants

Plants cope with changing humidity by adjusting the stomata on the leaves. Stomata open wider as VPD decreases (high RH) and they begin to close as VPD increases (low RH). Stomata begin to close in response to low RH to prevent excessive water loss and eventually wilting but this closure also affects the rate of photosynthesis because CO2 is absorbed through the stomata openings. Consistently low RH will often cause very slow growth or even stunting. Humidity therefore indirectly affects the rate of photosynthesis so at higher humidity levels the stomata are open allowing CO2 to be absorbed.

Thai basil leaves curling due to localized low humidity stress caused by T5 fluorescent lighting being too low

Thai basil leaves curling due to localized low humidity stress caused by T5 fluorescent lighting being too low

When humidity gets too low plants will really struggle to grow. In response to high VPD plants will try to stop the excessive water loss from their leaves by trying to avoid light hitting the surface of the leaf. They do this by rolling the leaf inwards from the margins to form tube like structures in an attempt to expose less of the leaf surface to the light, as shown in the photo.

For most plants, growth tends to be improved at high RH but excessive humidity can also encourage some unfavorable growth attributes. Low VPD causes low transpiration which limits the transport of minerals, particularly calcium as it moves in the transpiration stream of the plant - the xylem. If VPD is very low (95-100% RH) and the plants are unable to transpire any water into the air, pressure within the plant starts to build up. When this is coupled with a wet root zone, which creates high root pressure, it combines to create excessive pressure within the plant which can lead to water being forced out of leaves at their edges in a process called guttation. Some plants have modified stomata at their leaf edges called hydathodes which are specially adapted to allow guttation to occur. Guttation can be spotted when the edges of leaves have small water droplets on, most evident in early morning or just after the lights have come on. If you see leaves that appear burnt at the edges or have white crystalline circular deposits at the edges it could be evidence that guttation has occurred.

Tomato plants exhibiting the phenomenon of guttation due to excessively high relative humidity levels.

Tomato plants exhibiting the phenomenon of guttation due to excessively high relative humidity levels.

Most growers are well aware that with high humidity comes and increased risk of fungal diseases. Water droplets can form on leaves when water vapor condenses out of the air as temperature drops, providing the perfect breeding ground for diseases like botrytis and powdery mildew. If humidity remains high it further promotes the growth of fungal diseases. The water droplet exuded through guttation also creates the perfect environment for fungal spores to germinate inviting disease to take hold.

Powdery mildew takes hold due to poor daytime / nighttime relative humidity control

Powdery midlew takes hold due to poor daytime / nighttime relative humidity control

Quick reference chart:

Low VPD / High RH High VPD / Low RH
Mineral deficiencies Wilting
Guttation Leaf roll
Disease Stunted plants
Soft growth Leathery/crispy leaves

Words: Gareth Hopcroft and Everest Fernandez

Just4Growers - Comprehensive indoor gardening community


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I didn’t know I was back in school! :laughing: :laughing:

So much to learn. :woozy_face:

A big thank you for this amazing lesson. As I am learning from all the experienced growers here, there is no single correct way to grow; too many variables!

Let the learning continue.

Thanks again!


I second that. Thanks for all the info! Some of those issues I’ve been dealing with in my outdoor garden. Extremely informative.

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Amen, exactly what I said not long ago


You are most welcome, we are all learning. I am far from being any kind of expert. Great folks here that are willing to teach

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Here is a link to my GG grow journal If interested. New pictures yesterday

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