Ventilation and Environmental Control

How To Raise Relative Humidity Inside a Grow Tent

Low relative humidity sucks -- quite literally. It sucks the moisture out of your plants. Dry air (particularly when combined with high temperatures) creates a vapor pressure deficit between the air and your plants' leaves. We take a look at how to take control of relative humidity levels inside a grow tent.

"Growing plants prefer relative humidity levels of around 60%."

You've probably read that factoid, or a variant of it, many times. It's repeated often, especially by indoor gardeners seeking to create the perfect environment for their growing plants.

But what do you do if your hygrometer (i.e. your instrument for measuring relative humidity) reads 30% or 40%? Your plants' leaves are curling upwards like taco shells. You keep spraying them ... but nothing seems to be working. So what can you do about low relative humidity in your indoor garden?

Broccoli seedlings exhibiting leaf curl (taco shells) in response to low relative humidity inside the grow tent.

Why is Low Relative Humidity A Bad Thing For Most Plants?

Plants suck up a lot of water—but very little of it (less than five percent) is actually retained for building new cells etc. The vast majority simply passes through your plant—sucked up by the roots, suffused through the vascular structure of your plant, and finally exiting through tiny holes in the leaves called stomata. Just like in a car's engine, water is primarily used by plants as a coolant.

Dry, low relative humidity air creates what is known as a high vapor pressure deficit (or VPD) - basically, the inside of a plant's leaves are full of moisture. If, somehow, you could measure the relative humidity inside a tiny pocket of air inside the leaf, it would read 100%. It's wet in there! And that's all good.

However, as we've already pointed out, plants need to breath (exchange gases) through tiny holes. This exposes the plant to the outside world. If this outside world is dry, the plant has to suck up more moisture to combat the drying effect—thus making it work harder, just to stay alive. Dry, arid conditions are also perfect for nasty bugs such as spider mites. Dessication (drying to the point of cellular damage) also opens up your plants to potential attack from mildew and other pathogenic molds.

Young Plants Need High Relative Humidity 

Humidity meter / hygrometer measuring relative humidity inside the grow tent via a remote probe.

Ever wondered why growers start their seedlings inside propagators? Wonder no more! The propagator seals in moisture creating a high relative humidity environment. This means your tiny, tender little seedlings can focus on the task of growing without their miniscule root systems getting stressed out with having to provide too much moisture to those young leaves. But what happens when your plants outgrow the propagator?

The Importance of Little Steps

Just like most humans, plants don't like big, sudden changes in their environment. Your task as a grower is to wean your plants slowly off their high-relative humidity incubation environment and prepare them for the "real world." Hopefully your propagator lid has small vents in it which you can open gradually, more and more each day. Eventually you might be able to take the lid off all together for an hour or more. Staging transitions between environments is key to mitigating plant stress. And successful growing is all about guiding your plants through their lifecycles with the minimum amount of stress possible.

Increasing Relative Humidity Inside a Grow Tent

Grow tents are a quick and easy way to create a "micro-environment" for your plants. Sure, you can dedicate a whole room to your indoor gardening hobby if you have the time, space and inclination—but a grow tent will make your life easier. Firstly, you have more control over your plants' growing environment. You can prevent bugs more easily, control your plants' light cycles (without having to black out windows) and, of course, it's far easier to raise (or lower) relative humidity levels in a smaller volume of air.

Small HOMEbox grow tent with supplemental humidification.

Here's a simple propagation set up. The grow tent pictured above is a HOMEbox Clonebox View—ideal for starting your cuttings and seedlings. An adjustable extractor with built-in thermostat controls the temperature by pulling out the hot air. It's vented into an airy loft space.

The blue bucket sat on top of the tent contains water. The yellow hosepipe is fed into the grow tent to a humidifier.

Humidifier in grow tent raising relative humidity 

Gravity feeds the humidifier with water (you may have to give the hose pipe a little suck first!) and the humidifer, in turn, generates a cool mist inside the grow tent, increasing the relative humidity.

Of course, if we leave the humidifier on full-power in such a confined space we could inadvertantly create the opposite problem—too much moisture in the air! Some humidifiers have a built-in humidistat which enables them to automatically cut in and out according to the user's desired setting.

Eos1 Humidifier / Dehumidifier Controller by Titan Controls.

Titan Controls' EOS1 can connect to either a humidifier or dehumidifier (but not both at the same time!) and it switches the device on or off according to your setting. It couldn't be easier to install—my only gripe is that the sensor is built into the unit itself. I'd prefer a remote probe so that I could house the unit outside of the grow tent. Instead, I have to hang it inside the tent.

I keep the humdifier on the bottom shelf, with plants growing on the top shelf. This means the spray does not come into direct contact with the grow lights. 

T5 fluorescent grow light with half the lamps switched off to lower temperatures.

Other Ways to Increase RH Inside Propagation Grow Tents

  • Use Victorian bell cloches over your plants.
  • Set your extraction fan to minimum.
  • Reduce temperatures by removing half of your fluorescent tubes - don't worry, young plants don't need much light!
  • Reduce temperatures by adding some A/C to your tent!
  • Introduce larger plants—they transpire more and produce more relative humidity!
  • Set trays of water near your air intake or passive air holes
  • Add a sponge to your trays, saturate in water - the sponge creates more wet surface area