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So, you have your mind set on growing a few of your own weed plants. To grow a good cannabis plant indoors, there are various things that you need to keep in mind, with temperature and humidity being some of those factors.
So, what is the best temperature and humidity for cannabis plants? Well, the ideal temperature and humidity levels for cannabis plant growth will depend on which stage of growth the plant is in (Seedling, Vegetative or Flowering stage).
The seedling, vegetative, and flowering states all require slightly different temperature and humidity levels. Yes, weed plants do need some humidity and fairly warm temperatures, but this will differ depending on how mature the pot plant in question is.
Best Temp & Humidity For Cannabis Plants
Something that you need to know about growing your cannabis plants is that they need pretty specific temperature and humidity levels to flourish.
Now, some people will say that the same constant temperature and humidity levels, from beginning to end, is fine, but this is not really true.
When it comes down to it, for the seedling stage, vegetative state, and flowering state, the 3 main stages of weed plant growth, the humidity and temperature levels will differ.
Let’s go over these 3 stages of growth right now, and what the ideal humidity and temperature levels for each are;
The Seedling Stage
When marijuana plant clones and seedlings are just beginning to grow and are still very small, they rely on transpiration in order to get the water they need to survive.
Clones and seedlings require a humidity level between 65% and 70% in order to flourish, and it’s because their root systems are not fully developed yet, as the high humidity level and relatively high temperature allows them to absorb water through their leaves.
A marijuana plant in the seedling stage should be kept in temperatures between 20 and 25 degrees Celsius (when the lights are on), and about 4 or 5 degrees less if the lights are off.
The Vegetative Stage
Now that your marijuana plant is no longer in the seedling state, and has now reached the vegetative state where the stems and leaves really start to grow, there are some changes that need to be made.
Here, the root system has developed significantly, and therefore your marijuana plant can absorb more water through its roots. This means that you can decrease the humidity level from 70% by about 3% to 5% per week.
The acceptable range from humidity in your grow room during the vegetative state is between 40% and 70%, so how much you decrease the humidity per week depends on how long you plan on leaving the weed plant in the vegetative state for.
In terms of temperature, you can crank the temperature up a little bit. For the vegetative state, anywhere between 22 and 28 degrees Celsius will do fine (with the lights on), and 3 to 4 degrees cooler when the lights are off.
The Flowering Stage
The final growth period of the weed plant is the flowering stage, which is where all of the buds, the THC, and those crystal trichomes develop.
You will once again need to make some changes in terms of humidity and temperature for the weed plant flowering stage. Here, you want to keep humidity fairly low.
The plants can handle it, they don’t need the air to be too humid, and it can actually cause problems.
A humidity level between 35% and 40% for the flowering stage will do just fine. Now, when it comes to temperature, for the flowering stage, you want to turn it down a bit when compared to the vegetative state.
If your weed plant is too hot during the flowering state, it won’t grow very well and it will negatively affect THC and trichomes production.
The temperature for a cannabis plant in the flowering stage should be no higher than 26 degrees Celsius at all times, with the ideal being somewhere around 23 degrees Celsius.
The Late Flowering Stage – 2 Weeks Before Harvest
This is a stage which many people do not consider, but weed plants to have an early flowering period and a late flowering period.
The late flowering period starts roughly 2 weeks before the weed plant is ready to harvest.
Now, this is not totally necessary, and you can by all means leave things the way they are, as we discussed in the flowering section above.
The temperature and humidity levels for the flowering stage, as indicated above, will do fine for the late flowering stage as well.
However, if you really want to get the best yields with the highest potency, you will want to make some more changes here. In order to increase THC quantity, potency, and the final yield, you should aim to bring the humidity level in your grow room down to 30%.
You do not want the grow room being very humid during the final couple of weeks.
The same goes for the temperature. Significantly decreasing the temperature here to between 18 and 24 degrees, with 20 degrees Celsius being the ideal, will also help increase yield and potency.
Here are 5 good tips to make your plants nice and bushy.
What Temp And Humidity To Dry Weed
Ok, so now you have finally grown your marijuana plant and have harvested it.
Now, it is time to dry your marijuana. Remember folks, marijuana needs to be dried properly in order to smoke it and use it in any way.
Of course, after drying, you will need to cure it as well, as this is one of the most important parts of the process.
However, for drying, keep in mind that the drier the air is, the faster the plant will dry. Yet, you actually do not want the air to be way too dry, and you do not want your weed plant to dry too quickly.
Having your weed plant dry too fast will result in a decreased THC content. The ideal humidity level for drying your weed buds is between 40% and 45%, but absolutely no lower than this.
For the temperature, when drying your weed, you want to keep it at around 18 degrees Celsius, but not hotter.
On a side note, there should also be some airflow present for drying your weed, but just don’t point the fans directly at the plants.
What Temperature Do Plants Grow Best In
Ok, so now we have establish what the best temperature and humidity levels are for cannabis plants, but why is this important?
Let’s now talk about the problems which occur when weed plants are too hot and when they are too cold.
The fact of the matter is that there are various problems that can occur when there is too much or too little humidity, or when temperatures are too low or too high.
Problems With Weed Plants Being Too Hot
There are various problems that can occur when your weed plant is too hot, and this goes for all growth stages. So, what are the problems associated with your grow room being too hot?
For one, if there is way too much heat, you could just straight up kill your weed plant. Although pot plants usually will not die from being too hot, it will certainly slow down growth a whole lot.
If your weed plant is too hot during the flowering stages, the THC, crystals, and trichomes will not develop properly (here are some good options for viewing them), or will just not be produce in large quantities.
A weed plant that is too hot will not end up being as potent as it would otherwise be.
Heat also brings other problems with it. Weed plants that are too hot are susceptible to many issues such as root rot, nutrient burn, over stretching, increased susceptibility to mold, and increased susceptibility to the growth and multiplication of pests like nymphs and spider mites.
A weed plant that is too hot will have a lot of water evaporate out of the leaves, thus causing it to suck up excessive amounts of water and nutrients through the roots. This can cause structural problems and can put a real hurt on the root system too.
Problems With Weed Plants Being Too Cold
Yes, of course, there are also issues associated with your cannabis grow room being too cold. So, what happens when your weed plant is too cold during the various growth stages?
If it is too cool, as well as damp, mold has a really easy time growing. Mold will totally destroy your plants and they cannot be smoked when they get moldy.
Mold will stop plants from growing, it will kill them, and if you still manage to harvest, the end product will not be usable. This is something that the cold does.
Cold temperatures can also severely slow down, stunt, or even totally stop the growth of your cannabis plant. Simply put, you could just be freezing it.
If your weed plant is not warm enough, especially during the flowering stage, the active compounds will not develop as fast or be as potent.
A plant that is too cold will not end up being all that potent. The reduction of photosynthesis abilities is another related issue with the cold.
Issues With Humidity – Too Much Or Too Little
Finally, there are also issues associated with your weed grow room being way too humid or too dry. What happens if the air in the grow room is too dry or humid?
One of the biggest risks associated with weed plants that are in an excessively humid environment is mold. Mold can easily grow and multiply in very humid conditions.
If the air is too humid, a lot of that moisture will rest on the soil and the leaves, and this provides a great breeding ground for mold.
When the air reaches the dew point, moisture will condense and turn into liquid, and this is when you have problems.
The same is also true for pests. Pests like spider mites, nymphs, and all kinds of flies love damp and humid conditions.
All of these pests can end up ruining your crop in one way or another, so this is best avoided.
On the other hand, if the air in your grow room is too dry, well, all plants need water to survive.
If the water in the soil, the roots, and the weed plant itself evaporates way too fast and disappears into the air, you will have problems as well.
If your plant is too dry, it will become dehydrated, turn brown, burn, and will eventually just die.
Cannabis Growth Temperature & Humidity – Some Tips
Now, we have already established what the ideal levels of humidity and temperature are for each growth stage, as well as for drying your weed, plus we have covered why your weed plants need a certain level of humidity and temperature to survive.
However, problems can occur, and you need to know how to fix them.
You need the right tools for the job, and you need to know how to control temperature and humidity in order to achieve the ideal cannabis grow room parameters.
Having The Right Tools & Equipment
Something that you will definitely need to have in your cannabis growing arsenal is a thermometer and a hygrometer, or just a two in one hygrometer & thermometer combination.
We all know that a thermometer measures temperature, so the hygrometer must be the contraption which measures humidity.
Yes, you absolutely need these things if you have any hopes of achieving and maintaining the proper temperature and humidity levels in your grow room.
Properly Measuring Grow Room Temperature
So, first off, you need to be able to properly measure the temperature in your grow room.
Yes, this does mean that there is a wrong way to measure temperature in your grow room.
Getting a nice thermometer with a reader on the inside of the grow room, with the panel on the outside, for easy reading, is always a good idea.
Either way, for one, always measure the temperature in the shade, not right under the lights, and then in several places in your grow room, such as 1 foot from the floor, feet up, and so on.
Measuring the temperature directly under the lights is more or less useless, as it is always going to be a few degrees hotter right under the lamps.
This does not really matter as much, as long as your plants are not too close to the lights.
Either way, take a few readings from different areas in the grow room, and then take the average.
Properly Measuring Grow Room Humidity
To measure humidity in your grow room, you will need to use that hygrometer which we talked about before.
Here, keep in mind that warm air holds more water than cold air, and warm air rises, so generally speaking, your grow room will be a bit more humid along the roof or ceiling of the room than at the floor.
To get a good humidity reading, you should aim to take it from the center of your grow room.
Also, keep in mind that you need to use your hygrometer to measure the relative humidity in the grow room.
How To Raise Temperature In The Grow Room
If the temperature in your grow room is too low, there are a number of methods at your disposal to increase the temperature by a few degrees. How do you raise the temperature in a grow room?
If you get bigger and better grow lights that put out more watts and lumens, they will produce more heat. HID grow lights are known for producing a lot of heat.
Get a little heater. Make sure to get a space heater that has great safety features.
Don’t cheap out here because you don’t want end up burning your plants, or your whole house down, because of a subpar space heater.
Many people choose to use heat matts to put under their plants.
These work well too, but once again, remember to get high quality heating matts, as low quality options come with some pretty big fire risks.
You can also decrease the air flow and ventilation a little bit.
The more hot air gets sucked out, and the more cool air replaces it, the lower the temperatures will be. Now, this is tricky, as weed plants do need some ventilation, but it is an option.
You can use a grow tent, which keeps heat in really nice, or if you are using a closet, using insulation and reflective materials will work too.
On a side note, you should already have reflective materials in place anyway, as it does wonders for increasing light intensity, and yes, temperature too.
If you need some good grow box suggestions, check out our buyers guide here.
How To Lower Temperature In The Grow Room
The next problem you might have is that your cannabis grow room is too hot. So, how do you lower the temperature in your weed grow room?
You can always increase the airflow and ventilation in the grow room in order to decrease temperatures.
Turn on a fan (if you need a good fan, then we have covered them here), cut a little hole in a wall, open a door, or whatever else. Increased airflow will decrease temperatures.
If you have HID lights, while they are best for weed growth, they do produce a lot of heat.
You can switch to LED lights which produce less heat, or you can get a special ventilation system for the HID lights. Either way, a ventilation system will go a long way in decreasing grow room temperatures.
Naturally the day is hotter and the night is cooler. You might be tempted to have the grow lights on during the day and off during the night.
Yes, this would seem natural, but for an indoor grow operation, it really does not matter. Therefore, to decrease temperatures, have the lights on during the night instead of the day.
How To Raise Humidity In A Grow Room
So, now the problem is that the humidity level in your grow room is too low. How do you raise the humidity level in your grow room for your cannabis plants?
You can use a simple spray bottle and mist your plants and the soil to increase humidity, although this is a very temporary and short lasting solution which will need to be repeated several times per day.
Just make sure to provide your plants with enough water.
You can insert open water containers into the grow room, or hang up wet towels. Really anything that is wet and will allow water to evaporate into the air will do fine here.
You can always get a good old humidifier. Yes, these do cost some money, but they do the best job because you can accurately set and control a specific humidity level with a half decent humidifier.
We have covered a separate article on raising humidity, you can find it here.
How To Lower Humidity In A Grow Room
The final thing we want to go over quickly is how you can decrease the overall humidity in your grow room.
Water your plants right after the lights are switched on. The lights will cause water to evaporate more quickly.
You can always use a dehumidifier to decrease humidity (here is our buyers guide if you need help), but be careful not to overdo it, as some dehumidifiers work very fast to suck moisture out of the air.
Try adding more some ventilation to the grow room or find a way to let more cool air in. Cool air holds less moisture than hot air, so fresh and cool air should lower humidity levels as well.
**Do keep in mind that there is a relationship between temperature and humidity. Generally speaking, hotter air can hold more gaseous moisture than cold air, so the hotter the air, the higher the chances of excessive humidity, and vice versa.**
There you have it folks, literally everything you need to know about the best temperature for growing marijuana, as well as humidity levels.
As long as you keep all of these things in mind which we have covered here today, you should have no problems achieving and maintaining the proper humidity and temperature levels in your cannabis grow room.