What is the ideal grow room temp and humidity? (Part 2)

If you’ve not already, you’ll want to check out my previous post: What is the ideal temp and humidity for a grow tent (part 1.) before  moving on to this post about ideal grow room temp and humidity.


Do have a read before you jump in here, because in it, in it I covered:

  • Measuring temperature and humidity in your grow tent.
  • What vapour pressure deficit is.
  • How your equipment dictates the ideal temp and humidity for your grow tent.
  • The ideal temp and humidity for clones & seedlings.
  • Ideal temp and humidity for plants in vegetative growth.
  • The ideal temp and humidity for plants in flower.


In this post, we’ll look at:

  • Whether it’s necessary to maintain ideal grow room temp and humidity.
  • What happens if you don’t.
  • Tips on how you can.


Don’t forget: Any information given on this site is for educational purposes only. Please ensure if you’re growing cannabis you’re doing so in accordance with the law and subject to appropriate permissions and licenses of the applicable country.


Ready? Let’s go!


Image of taking measurements to maintain the ideal grow room temp and humidity


Is it essential to maintain the ideal grow room temp and humidity?

Now that’s an interesting question… 🤔


If you’re in a position to be able to maintain ideal grow room temp and humidity, there are some distinct benefits:


  • You could prevent mould and mildew.
  • You can prevent pests.
  • You’re able to grow the healthiest, most robust plants possible.
  • You can get bigger yields.
  • You’ll grow better quality products.
  • Your plants will have a stronger fragrance.


But, dependent on where you live and what that climate’s like, controlling your environment to such a high degree can be pretty challenging.


In that case, it’s a good idea to match your grow room set up to your specific weather conditions and seasons.


For example if it’s really cold in winter where you are, and to grow, you want to maintain a hot grow room, you have to ask yourself:


  • Is this viable?
  • How much energy will it use?
  • Is it profitable?
  • Is it good for the environment?


Because whilst there are ‘ideal’ figures for VPD, temperature and humidity for a grow room if you want to grow to perfection, your set up does not have to be perfect for your plants to grow well enough. 🙂


If you have really cold winters and you’re thinking you need to heat your grow room to 28°C but in reality, it’s too expensive to do so – just don’t.


Realistically in this scenario maintaining 24°C will probably still do the job. You’ll be spending less, saving energy, and doing your bit for the environment.


That’s far preferable to striving for an unachievable 28°C, wasting energy and money and stressing about how hard that is to maintain along the way.


How does the Grow Sensor help maintain ideal grow room temp and humidity?


Hopefully by now you’ve got your copy of our ultimate VPD calculator & environmental timeline (<<<<< if not, seriously click this link and get it for FREE now!)


Whilst it’s already the most comprehensive resource out there, we’re making ongoing improvements to make it as useful to you as possible.


In future, you’ll be able to input whether it’s winter or summer where you’re growing and how much energy you want to save, then it will give you an updated proposed growing timeline! So watch this space! 👀


The Grow sensor app will also give you this data and feedback live. This will allow you to make changes to your environment that both improve your grow and reduce its impact on the environment.


It’s worth mentioning, that even if you have an environmental controller set up, if you can’t see your data live, hour by hour or minute by minute, or analyse your past data, you really don’t have clarity about what’s going on in your grow room.


Granular data is key. As an example, many hygrometers give you just 2 readings each day, a minimum and maximum within a given 24 hour period.


With something like that, you’re not going to have visibility of any temperature or humidity swings that happen, just the resultant average.


If you rely on just a minimum and maximum reading, your averages could look ok but you could be missing out on where things are going wrong, risking your plants.


A quick note on really small grow spaces:


Controlling temperatures and humidities in small grow spaces is usually harder because there’s less of an ‘environmental buffer.’ The space can heat up or cool down or humidity levels can fluctuate more quickly,


Think of when you open the oven mid way through cooking and how long it takes to get back up to temperature. Or how quickly your ice cold drink becomes warm when left somewhere hot.


Image showing what happens if you don’t have the ideal grow room temp and humidity


What happens if you don’t have the ideal grow room temp and humidity?

Not maintaining the ideal grow room temp and humidity is not a crisis.


As mentioned above, it’s totally acceptable to make the conscious choice not to strive for perfection and instead accept that good enough is totally ok given your current climate, equipment and setup.


Having said that, you do want to try and avoid extremes of temperature and humidity in your grow room where possible.


Here’s why:


If your temperature is too low in your grow room:

  • The speed that the plants metabolise at slows down.
  • This reduces the amount of nutrients the plant can take up and will inhibit development.
  • As temperatures lower, humidity levels rise (low VPD.)
  • Higher humidity = more risk of mould.
  • Extreme cold subjects your plants to high levels of stress.
  • Growth will eventually cease and your plants could die.


If your temperature is too high in your grow room:

  • The rate that the plants metabolise at speeds up.
  • This increases the amount of nutrients that the plant can take up which puts them at risk of nutrient burn.
  • As temperatures rise, humidity levels fall (high VPD.)
  • Lower humidity = more risk of pests such as spider mites.
  • Prolonged low humidity forces plants to close stomata to try to retain water.
  • Extreme heat subjects your plants to high levels of stress.
  • Growth will eventually cease and your plants could die.


If your humidity is too low in your grow room:

  • Clones and seedlings will struggle.
  • Young plants don’t have the root system in place to replace the water lost fast through transpiration at low humidity.
  • Low humidity forces your plants to close their stomata to retain water.
  • If subjected to prolonged low humidity, plant growth can be stunted.


If your humidity is too high in your grow room:

  • Clones and seedlings will also struggle at high humidities
  • Extreme humidity means transpirations slows.
  • They can’t take up CO2 and nutrients like they should.
  • With poor air circulation, your plants may rot or get mildew.


Overall want your plants to be comfortable and not stressed.


Avoiding extremes of temperature and humidity is far more important than trying to hit ideal grow room temp and humidity readings if your location and setup are against you.


Having said that, there are things you can do to get closer to and sustain better conditions for your plants’ life cycle.


Tips for sustaining the ideal grow room temp and humidity.


1. Create a buffer

When you’re growing in a grow tent, it’s advisable to control the temperature of the room that contains that grow tent, as much as possible.


Treat the room that houses your grow tent as a buffer between conditions outside and conditions inside your tent.


This can reduce the need for surplus heating or cooling, saving electricity and cost. It should also minimise large swings in temperature or humidity.


Man checking plants in his indoor grow



2. Choose your equipment carefully

It’s really important to use equipment that is appropriate for the size of your room or tent. You’ll need to figure out the volume of the inside of your grow space in cubic meters.


This can then inform what lights, heaters, fans, humidifiers, dehumidifiers or air con that you need to buy, or use to manage the grow space.


It’s also important to factor in the size of all of your pieces of equipment and how much of your grow space your equipment will occupy, ensuring you leave enough space for your plants!


Where appropriate, you can install some equipment inside or outside your grow room depending on the space you’re working with and how much your readings fluctuate.


Something that’ll have a large impact on temperatures inside your grow tent is the type of light you use and how much heat they give off.


The size of your grow space combined with your choice of lights will dramatically affect temperatures (and therefore humidity) inside your grow environment.


Hot lights like metal halides and high pressure sodiums generate more heat in a smaller grow space than LED’s, but LED’s do still give off some heat.


This is all part of the puzzle, if you use hotter lights, you’ll need to cool your space more efficiently.


Don’t forget if your set up allows, you can raise or lower lights to alter the temperature in your space at the level of your plant’s canopy.


Choosing appropriate equipment is a big way you can grow more efficiently and save costs AND conserve electricity.


3. Measure VPD for environmental stability

Try to create a stable environment inside your grow space.


VPD is a better tool for the job than measuring temperature and humidity separately. This is because it addresses the crucial relationship between temperature and humidity in one value.


Wherever possible, you want to avoid any large or fast changes in VPD, (temperature and humidity.)


I harp on about VPD a lot so checkout our other resources if you want to know more but i’ll move along swiftly now…


4. Match your grow to your conditions

When striving to grow the best plants, this is something that is often overlooked.


If you live in an area that has extremes of temperature (or humidity) as the seasons change, it’s something you should capitalise on or work with, not fight against.


When your grow room is too hot, but the ambient outside temperature is cooler, you can open up the zip or pull air in to bring the temperatures down rather than using air conditioning.


If the humidity is too high in your grow room, but outside, the air is dryer, bring in some fresh air to raise VPD and boost transpiration. Obviously the reverse can be helpful too.


When you get extreme heat in the summer, the heat generated by your lights if they’re also running during the day could create an environment that’s just too hot for your plants.


Try scheduling your light cycles according to outside ambient temperatures. e.g. running your lights at night when it’s cooler outside to try to sustain a more comfortable temperature for your plants around the clock.


Likewise if it’s really cold in winter (and therefore even cooler at night) your plants could benefit from the lights running at night to warm the grow room up.


Grow tents benefit from being portable so you could also move your tent to somewhere less affected by extremes if that’s an option for you.


If you have a space available that’s not too hot or too cold (i.e. loft or cellar,) you can use that to grow with the seasons and it’s less work if you’re just starting out, as you can lean on the conditions provided by nature.


Growing in synchronization with your local conditions is one of the biggest ways that you can reduce electricity consumption and grow greener.


Physical grow room sensor & data shown on iPhone app


5. Use a grow room sensor or monitor.

Both large and small grow tent or grow space setups can benefit from a grow room sensor or grow room monitor to consistently provide real time feedback on conditions inside your grow space.


When you have data to work from, you can make informed decisions about how to address any trends or fluctuations you may have.


Using an environmental controller can help you control your grow environment by switching on your heater, air con, dehumidifier or humidifier when a threshold is reached. But having the granular understanding that you get from grow room sensor measurements is better for managing your conditions long term.


Our Grow sensor will have the added benefit of AI that will allow you to optimise your grow room setup for more efficient energy consumption so you can grow in a more environmentally conscious way.


Using a grow room monitor can be of particular benefit if you’re new to growing and setting up a grow room. This is because you’ll get to see exactly what’s happening as it’s happening and learn why, which is super valuable and helps you improve.


If you just entrust your plants to an environmental controller, you don’t really learn about your optimal setup or how your appliances are interacting with your space.



Controlling temperature and humidity in grow tents and smaller indoor spaces is difficult. And the smaller the environment is, the harder it is to maintain steady environmental conditions.


The ideal grow room temp and humidity entirely depends on what plant or species you’re cultivating, what stage of the grow cycle your plants are in and whether it’s day or night… But your specific ambient environmental conditions where you’re located are also a big factor.


Finding balance is hard, temperature and humidity are intertwined. Decreasing the humidity of your grow room will increase the temperature in there. Then running air con to cool the grow room would increase humidity again.


Try not to fall into the trap of using every piece of kit possible to regulate your grow room, only for them to fight against each other and increase your costs and toll on the environment in the process, without any substantial improvement to the quality of your plants.


Remember: Whilst achieving the ideal grow room temp and humidity may seem like the smart thing to do, it’s not always possible and your setup doesn’t have to be perfect for your plants to grow well.


If striving for those ideal figures is too much work, uses too much power, is expensive and impractical to maintain, just don’t. Get as close as you practically can, stop worrying about it and get back to enjoying growing! 😃 🌿

Screenshot VPD chart zoomed out

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Author avatar
Lucy Starley
Co founder at Grow Sensor - the smart grow room monitor. Lover of art, animals, sewing, walking and mushroom hunting. Inbound marketing expert and expert tea drinker. Lucy is currently open for Guest posts.

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