How to deal with humidity swings

Hydroponics and Grow Room Humidity

This tutorial is designed for indoor gardeners growing plants in grow rooms and grow tents using hydroponic nutrients

How does humidity affect my plants?

The humidity in your grow room or grow tent will determine how much your plants will transpire.

(Here is a link to a blog on transpiration if you're unfamiliar with the term)

In any indoor grow room where environmental humidity is very low (<RH%) your plants will 'drink' (uptake) significantly more liquids than usual, increasing the quantity of water and nutrients they consume. 

If the humidity isn't corrected then consumption of water and nutrients goes beyond the point of a safe level for the plant to handle. This can quickly lead to nutrient burn and lock out in a very short period of time, often within a few days.

When your room humidity spikes to a high RH% (75% and above), the opposite reaction will occur creating an environment that will force your plants transpiration to come to a halt, forcing a significant drop in uptake of liquids (relative to temperature, see our VPD tutorial below for more info).

This drop in uptake of hydroponic nutrients and water will now start to accumulate in the root-zone in the form of excess, which forms a toxic root-zone that will stunt new growth, cause nutrient toxicity and nutrient lock out.

For a further understanding of how humidity and temperature effect your plants, we recommend this tutorial on Vapour Pressure Deficit (VPD) found here

How to prepare for high humidity?

In a situation where your grow room doesn't have climate control then your grow room or grow tent will be subject to ambient weather conditions.

In these circumstances its important you monitor your local weather on a regular basis especially if you want to avoid potential nutritional issues, if you don't make adjustments to your hydroponic nutrient strength on these particularly humid days then your plant will suffer. 

To mitigate this growers use various types of equipment and solutions to control humidity;

When you are prepared for upcoming weather changes that will impact humidity, one course of action is to mix up a fresh batch of hydroponic nutrients that are heavily diluted with plain water, to roughly 50% of your current feed strength and flood your pots with this solution in replacement of their regular feed just for that day.

Flooding the media with a diluted nutrient solution will help flush out excess salts and helps maintain a well balanced nutrient solution in your growing media preventing the dreaded build up and eventual lockout of hydroponic nutrients within the root zone.

It is always a good idea to make sure you're aware of the next 2-3 days of upcoming weather to stay on top of any potential issues that humidity spikes can cause.

What is the right humidity?

  • For seedlings and cuttings - Propagation phase 75-100% RH%
  • For the Growing phase  60-65% RH%
  • For the Flowering phase 50-55% RH%
  • For the Drying Phase 50-55% RH%
  • For the Curing phase 55-62% RH%

Flowering plants and high humidity?

The problems that can arise to flowering plants that are subjected to high humidity can be disastrous for some species.

Those problems are:

  • Mould (Bud-rot)
  • Powdery Mildew
  • Root-rot
  • Pest infestation (Spider Mites)
  • Disease (Collar rot) 

If any of the above happen during the flowering phase for most indoor gardeners its game over, there are growers that will spray extremely harsh chemicals on their flowering plants to rid of these unwanted problems, but given that most of these crops will be consumed by people i'd personally advise against doing this, if its something like spider mites you can find a non-chemical (organic) option that wont cause any harm to the consumers of your crop.

If mould is the problem, cut it away and throw it in the bin in my opinion. There are plenty of threads online of growers saying soaking their flowers in H2O2 (hydrogen peroxide) and leaving them out in the sun and the mould will vanish, i highly doubt this i'm sure a test would prove the presence of mould. So, in this instance cut your losses and be better prepared on the next grow.

To avoid all of these issues completely, remember that prevention is a much better approach to cure. Integrate a foliar spraying program on a weekly basis during the growth phase and a lot of those problems can be completely avoided in flower!

We hope this blog is helpful to you. 

Follow us on social media for more tips, product info and tricks :)

#Melbourne #hydroponics



> Propagation
> Hydroponic Equipment
> Plant Nutrition & Health
> Pest & Disease
> Harvest
> How To...
> Geek Out With A-Grade
> Water Control
> Plant Training
> Grow Medium
> Troubleshooting
> Hydroponic Basics

Other Tutorials

What Should My Humidity Be?
Heat Stressed Plants
Understanding VPD - The key to hydroponics
Hydroponics and Grow Room Humidity
Grow Room Tip - Night Time Temperatures

premium hydroponic store

We're committed to helping communities in Australia grow
Shop now