What Temperature & Humidity you need for your grow room.

When it comes to temperature, weed plants are abit like Goldilocks. One minute they are too hot, the next they are too cold . You’ll have to keep your eye on that thermometer to make sure the climate in your grow room is just right. The optimum temperature for growing weed is around 70-85 F (20-30C) when the lights are on and 62-72 F (17-22) C when they are switched off. Young plants prefer warmer temperatures with relatively high humidity, but as they get older they become more tolerant to colder and drier conditions. As a rule, colder temperatures with relieving high humidity, but as they get older they become more tolerant to colder and drier conditions. As a rule, colder temperatures slow down growth, especially below 15 degrees, if it’s gets that cold freezing temperatures can shock or kill a plant. Colder temperatures can also encourage some types off mold, especially when damp. Cannabis plants are fairly resistant to heat, but anything above 30 degrees can cause damage. High temperatures can also encourage infection and nutrient burn owing to increased water transpiration. During the first 2-3 weeks off flowering, some growers increase the night time temperature to match the day time to limit the internodal growth as much as possible, before reducing the night time temperature back to its original level. 

The best way to monitor heat and humidity is with a digital thermometer and hygrometer. The ventilation can be used to control the conditions with a humidifier/ dehumidifier introduced if further control off the humidity is needed. All strains off cannabis will have a varying preference of temperature and humidity, and it really comes down to experimenting and finding out your own individual requirements. However, the following table will give you an idea of what to aim for for a different stages of the grow. 

Growth stageTemperatureHumidity
Seedlings and cuttings70-77F (21-25C)70-80%
Vegetative stage75-82F (24-28C)50-80%
Flowering stage65-77F (18-25C)40-50
For Soil Grow

As well as the environmental temperature, hydroponic growers also need to monitor their reservoir temperature. The oxygen-retaining capability of water is reduced as it heats up, so maintaining the correct temperature is highly important in order to provide sufficient aeration to the roots. Ideally you want to aim for 65-75 F (18-20C degrees) as you can see from the following data, a ten degree rise can greatly reduce the oxygen level. 

Reservoir TemperatureDissolved Oxygen Level
10C (50F)13 parts per million (ppm)
20C (68F)9-10 ppm
30C (86F)7ppm
For Hydroponic Grow


Whatever stage you are at with your plants you need a constant supply off water throughout the grow. and the amount of water they require fluctuates with the humidity in your grow space. Once the humidity is high in your area the plants use their leaves to absorb moisture from the air which causes them to drink less water from their roots. When the humidity is low they will pull water in from the roots.

Because the humidity changes how much water your plants need, and on the other side the water you feed your plants with have nutrients in them, this means you have a lot off power and control over how much nutrients your plants are getting.

Perfecting the right amount off humidity encourages healthy leafy plants and encourages growth. Leaving your humidity too high gives your plants more off a chance to attract mold or bud rot which is not ideal at all if entering last few weeks off bloom stage it can really effect your final yield and what your buds will be like this will eliminate all big frosty buds that you may have created over the past few months and won’t be worth the effort you have had to put in.

During Vegetative stage

During the vegetative stage your plants may grow a lot of leaf mass in a nice cool environment with a humidity that should be between 50-70%. Plants aren’t only getting there water through the roots, plants can also pull water directly out of the air, with higher humidity in your grow space this allow the leaves to get more water straight to the plant. Until they are fully developed roots, low humidity in your grow tent can cause growth to be slower with your plants.

During the vegetative stage you’ll want to keep the humidity levels between 50% and 70%.

If you have got your humidity under control and lowered you need to make sure your plants are well hydrated or this could shock your plants if left too long.

Higher humidity levels:

  • (May) increase chances of diseases
  • High humidity slows down evaporation rate
  • Stomata close
  • Slows growth

Lower humidity:

  • Lower chances of diseases
  • Increase plant sweating (water evaporates easier)
  • Stomata’s open up
  • Growth increases
  • You’ll need to water your plants more often

Humidity During Flowering stage

During the bloom stage your humidity should be around 40-50% humidity this is something that needs to be lowered coming from the veg stage its a lot more important to keep an eye out off your humidity whilst you are in this stage as it can effect your plants a lot worse as it could make your buds rot and create mold within your plants. When entering the last couple weeks off flowering stage some growers like to get there humidity reduced to under 40% where this can create more resin.

At the start off this bloom stage your plants will off put together a large canopy which is relying on water to fulfil there needs. Keeping the balance off how much water the plants are receiving and also how much humidity is in the room will determine how the buds will turn out and also if they will attract mold, this can be very dangerous during this stage because mold can form on or inside your buds or colas without you even realising. If you find out that this is happening on your plants you should throw all off the rotten buds away so it doesn’t carry on spreading, if this isn’t dealt with sooner it can ruin your entire grow. Bud rot can end all off your efforts and hard work in a very short amount off time without paying the required attention.

If your humidity is still low keeping your grow room with a circulation off fresh air is vital this allows plants to receive fresh air and doesn’t allow any wet spots within your plants.

Flowering is the time when checking temperatures are crucial and should be kept relatively low to increase yield and bud size. Reducing the night temperature in the last few weeks off flowering(often whilst it’s been flushed) can also bring out purple, pink or blue shades in the buds. During these last few weeks, use a dehumidifier to reduce humidity as much as possible to encourage increased resin production in these final days. 

Temperature & Humidity is a big part off your operation depending on where you are in the world it will effect your grow differently places like England where it is mostly cold you may have to put the heating on a few times a month just to get the room back in-between the temperatures that they require. All round these are both things that are a lot more easier to balance now as they are so many different products out there that specialise in these issue such a dehumidifiers that reduce the amount off humidity within the room but what i would recommend is checking the reviews and making sure it is quite as a lot off these are known to be noisy and can sounds suspicious from a neighbours point of view.

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