How to Manage the Humidity in Your Grow Room

We’ve just taken delivery of some shiny new dehumidifiers from Quest – a US manufacturer of high-grade air treatment equipment that’s utilised in the professional horticultural industry and also by the military. To celebrate, we thought we’d take you through the ins and outs of humidity in your grow room…

Water and nutrients that are absorbed from the rootzone are transported upwards via xylem cells and distributed to the areas that they are needed. Ultimately, though, 97% of the liquid that’s taken on board is lost through stomata – the tiny pores found on leaf surfaces that facilitate the exchange of gasses for the process of photosynthesis. Plants live in a perpetual cycle of acquiring and losing liquids, and for this reason, it is essential that the correct balance is maintained at all times. To complicate things further, this balance isn’t just a product of activity within the plant, but it is also dictated by the conditions within the room (or outside area).

  • If the surrounding atmosphere has moisture levels that are too high, then plants will hold onto water more easily.
  • If the surrounding atmosphere is too dry, then plants will transpire (lose water) more quickly.

An upshot of this is that your environment will have a huge impact on the way that your plant uses nutrients. When plants transpire too heavily, you’ll notice that the nutrient solution in your tank becomes more concentrated. That’s because the plant has used up the water from your nutrient solution and left behind an excess of minerals, increasing EC levels. If left too long, this will inevitably damage roots, stunting growth and causing irreparable damage to the top half of the plant.

Conversely, if your room’s humidity is too high, then your plant is going to have trouble uptaking the nutrients that it needs. That’s because, if the plant loses less water through stomata, then in turn, less nutrient solution will be absorbed by the roots, leading to deficiencies.

Further to this, high humidity levels massively increase the risk of encountering mould related issues like powdery mildew – a fungal disease that infects plants, slowing growth rates and compromising yields. Botrytis (otherwise known as ‘bud rot’) is another fungal pathogen that destroys fruits from the inside out, making it a difficult disease to spot. These two issues alone can completely wipe out entire crops and undo months of work!

So it’s pretty clear that humidity levels play an enormous role in deciding the fate of your indoor garden. Success or failure will depend on getting your humidity (along with temperature) within optimal regions. Get this sorted and you are well on the way to bigger and better yields!

So what are the right humidity levels to aim for? To truly perfect your environment, you’ll need to look at VPD (vapour pressure deficit) levels, but to keep things nice and simple, we’ll work with some rough averages, assuming that your room temperature is sitting at 25°C. If you want to skip straight to the more complicated stuff, you’ll find more info on VPD here.

  • 80% Clones
  • 65% during vegetative growth
  • 50% flowering

Achieving an 80% humidity level during the cloning stage is actually pretty easy. Keep your un-rooted cuttings in a propagator and use the adjustable air vents to allow moisture to escape as needed.

During vegetative growth, plants are still small in size, transpiring little. Yet this is also the stage of your plants’ life cycle that humidity levels need to be hitting 65% to facilitate peak growth rates. You’re probably going to need a bit of help at this stage, and this will come in the form of a humidifier.

The Ram Ultrasonic Humidifier is great for hobbyist growers, with a 5 Litre tank and a price-tag of only £47.95.

For growers operating on a larger scale, the HR-15 (£324.95) and HR-50 (£425.00) humidifiers do the job nicely. Both come with hygrostats, allowing you to add moisture as and when it’s needed. They cover 30m2 and 150m2 areas respectively.

It’s important to note that if you use plain tap water with your humidifier,  it will eventually leave mineral deposits on the surfaces of your equipment.  Below, you can see a Dimlux reflector that was given to us by a customer; we use it in-store to educate growers on the dangers of using tap water with their humidification equipment. You’ll notice that the salt deposits have completely corroded the reflective surface, promoting rust and decay. This reflector would have been practically useless long before the damage ever progressed to this stage… The issue can be solved relatively easily by running humidifiers with reverse osmosis water, which has an EC reading of zero. See our range of filters for more info.

During the flowering phase, plants reach full maturity, roughly doubling in size from the time they are first switched to a 12/12 cycle. To facilitate this growth, they will naturally use more nutrient solution. The overwhelming majority of the water that’s added to your reservoir tank is going to be released into the room as vapour. And this all happens at a time when you need to keep humidity levels down… Fortunately, this issue can be negated by using a dehumidifer. Dehumidifiers work best in closed loop systems because they don’t have to fight against the effects of your extraction fan(s), but they can still be useful to those growing without air conditioning. For instance, by treating the air in the area outside your grow tent, you can reduce moisture levels before that air is drawn into the grow area by your intake fan.

Many customers use dehumidifiers to keep humidity levels in the sweet spot when drying and curing large quantities of plant material. Doing so helps organic compounds, like chlorophyll, break down fully for a superior end-product. If the humidity is too low then plant matter dries out too quickly, affecting consistency and flavour. On the flip side, if humidity levels start to creep above optimal limits, then the likelihood of encountering mould is increased dramatically.

Whatever the application, our new range of dehumidifiers from Quest are up to the job! They’re built to military-grade specifications, and are used in major industries, on-board aircraft carriers and even inside submarines. They are trusted for use in high-intensity environments, and you won’t find a better constructed unit out there.

Quest 70 Overhead Dehumidifier

  • Pulls 26 litres of water per day from the air!
  • Built-in humidistat
  • Designed to be mounted at the top of the grow area (can be hung on chains)
  • Compact size – only 54cm tall
  • Economical on power – shifts 2.2 litres per KWH
  • Utilises gravity to send water directly to your nutrient tank (or to a drain)


Quest CDG 74 Portable Dehumidifier

  • Removes a huge 36 litres of water per day!
  • Features a built-in humidistat
  • Economical on power – shifts 2.1 litres per KWH
  • Easy to move around, thanks to its built-in wheels and handle
  • Comes with a purge function, allowing you to release collected water when needed


Quest CDG 114 Portable Dehumidifier

  • Pulls a truly stunning enormous 55 litres of water from the air per day!
  • Shifts 2.7 litres of water per KWH
  • Comes with built-in wheels and handle

So, which unit is right for me? This can be tricky because there are a few variables to consider, but an important factor to consider when choosing a dehumidifer is the volume of water that you add to your tank on a daily basis (on average) to keep it topped up. This will give you some indication of the volume of water that you’ll need to pull from the air in order to maintain optimal humidity levels.  Don’t forget that our staff are always happy to talk you through it if you need any help or advice. For more info, give us a call on 01782 749955 or drop us an email at