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Humidity: the wetter the better?





Humidity has a huge impact on plant growth, the guttation seen here as droplets on the tips of this leaf are a visible indication of an excessively high (night-time) humidity

There are many aspects to the environment that you can play a part in controlling. Temperature is usually always the first on people’s minds, we can feel heat so naturally it becomes quite a significant factor to us humanoids. If you hadn’t already guessed from the title of this blog, the second parameter on the hypothetical list should be humidity. The amount of water in the air doesn’t really impact people very much (other than say give us a tickly cough or make us overly sweaty) but for a plant it is a completely different kettle of fish.

Understanding your humidity means not just understanding humidity on its lonesome though. It is intricately linked to temperature and has multiple impacts on plant growth, so it takes a small amount of getting your head around to properly understand, but once you have the basics it will help you out leaps and bounds down the line.

So, what is humidity


Quite simply, it is the amount of water in the air. Strictly speaking, ‘Relative Humidity (RH)’ is what a grower measures though, and by relative we don’t mean uncle dickhead who turns up to ruin Christmas. The ‘relative’ bit is referring to temperature. The warmer the temperature of the air, the more water it can hold. What does this mean though? Well, imagine you had a sealed grow room that was 20°C, with a relative humidity of say 75%. Then, imagine you raised the temperature of the room to 28°C. As the warmer air can physically hold more water, the RH would go down to say 50%.

This graph shows exactly how much water air can hold at any given temperature

What humidity does


Humidity has a direct impact on how a plant functions, particularly in term of how much it transpires. If the humidity of the air is high, it is harder for moist air inside the of the stomata to get out, so the plant transpires less. This is because the pressure difference of the air inside the stomata, compared to the air of the outside world. In general, the lower the RH, the greater the pressure difference so the more air can diffuse from inside the stomata to outside of it - transpiration.

Inside the stomata, the relative humidity is always maintained by the plant to be 100%. As the outside RH changes, a plant opens and closes its stomata to various degrees to compensate for changes in humidity, to regulate its transpiration as is needed. Problems of course arise at the extremes of the spectrum. For example, no transpiration will happen when the outside air is also 100%. Conversely, the plant will transpire excessively with too low a humidity and end up wilting.

Let your humidity get too low and your plants will lose too much water, causing them to wilt. Sad face.


Humidity Control


You will need to control your humidity by being able to make it go up, or down. This means making use of a humidifier to raise the humidity and a dehumidifier to lower it. Ideally both units will either need their own individual controls or be able to be connected to a controller to operate them when your set points are triggered.

As a very broad and general rule, you will want a higher humidity through vegetative growth, and a lower one through the flowering period. The typical problems with humidity you will encounter is that it is too low for early veg (as there is little plant matter) and too high during late flower (as there is a lot of plant matter). On a more day to day basis, you are likely to need your dehumidifier during each night period, as the RH will rise with the temperature drop from when your lights go out.

Humidity being raised in a hydroponic grow room. Precise control will lead to bumper yields!



Knock on effects of humidity


Incorrectly controlling your humidity will have many knock-on effects on to how your plant will turn out come harvest time. Two highly significant things that humidity plays a big part in are: a plant’s nutritional uptake, and a plant's susceptibility to mould.

In terms of nutrients, a plant will only move calcium through it when there is water flowing through it. Water only flows through it when it is transpiring, which is dependant on the RH. If the RH is too high there is no transpiration, no water flow and therefore no calcium availability. Calcium deficiency is possibly the most common deficiency that a grower will encounter. Not because it isn’t there or the nutrient brand is the problem though, its usually because it has been locked out as a result of a poor environment.

Cal Mag products can help resolve the symptoms of poor environments.

Calmag ensures the plant has the additional minerals it needs to thrive.

In terms of susceptibility to mould, anyone with damp problems in their house can testify how mould like a high humidity. Botrytis (bud-rot or grey mould) basically needs two things to grow: something dead and a bit of water. In late flower, there are dead pistils all over your flowers and with them typically being quite densely set (if you know what you are doing) the microclimate around the flowers has an extremely high humidity. If you do not reduce this high humidity with a de-humidifier, you are quite literally inviting botrytis around for dinner. You can’t simply go from a high humid environment to a low one though, it’s not that simple, you need to employ another gardening technique: hardening off.

Control your humidity to avoid your fruits and flowers turning into mush


Hardening Off

You need to gradually lower your humidity over the vegetative period, so you are at the humidity you need to be at the end of flower, well before you even get there. As a plant grows each set of leaves, it creates a set number of stomata on that leaf. The number of stomata it creates depends on the environment at the time: it only makes the amount it needs for then. As you lower your RH for each new set of leaves, they will create more stomata to deal with those conditions. You gradually build your plant up to speed with each new set of leaves, until it is slowly climatised to living in a lower humidity environment. Suddenly dropping your RH in the middle of flower will result in a lot of stress on your plant, as the leaves physically aren’t built to cope with that environment.

Practice makes perfect

With the correct control over your humidity you really can get the most out of your plants. Without control, you can quickly see the in your plants. Hopefully this has been a useful introduction to humidity for you. If you are unsure of what sort of equipment might be best for you, pop down to our store or give us a ring for more info.


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