How important is Relative Humidity.

Water is vital to every form of life. It influences the well-being of humans, animals and plants. The effects of too much or too little moisture can have disastrous consequences. Plants in particular are very sensitive to the right water balance. A deviation in the optimal water balance will lead to negative consequences for plants sooner than it would for humans or animals. Damage to the root system and plant, caused by too little or too much moisture, will in most cases be irreparable.

A common mistake is to overwater your plants. The soil becomes saturated and clogs up, and a lack of oxygen occurs in the substrate. The plant loses vitality and life force. Leaves yellow and growth visibly decreases. The plant and its roots may start to rot and eventually die off completely.

Relative Humidity. (RH)

In addition to the feed water, there is the Relative Humidity (RH). This is at least as important for the well-being of your green creation. Especially when growing indoors it is important to create an optimum climate.  To achieve this you need to understand the concept of relative humidity (RH). An incorrect relative humidity (RH) can lead to various problems.

Optimal relative humidity

What the optimal relative humidity (RH) is depends on the plant's specific needs and the stage the plant is in. Cuttings and seedlings have needs for a much higher relative humidity (RH) than plants that are in bloom. High relative humidity (RH) in cuttings and seedlings allows them to put all their energy into developing a strong root system. While high relative humidity (RH) in flowering plants can lead to rot in the flowers. 

Hygrometer

Vapor pressure deficit

The relative humidity (RH) of the ambient air determines the Vapor Pressure Deficit (VPD - an abbreviation of the English term "Vapor Pressure Deficit"). The Vapor Pressure Deficit (VPD) is the difference between the vapor pressure inside the leaf compared to the vapor pressure of the ambient air. This difference determines the water loss of the plant. Water vapor will escape through the stomates (pores in the underside of the leaves) if the vapor pressure inside the plant is higher than that of the ambient air

Based on the Vapor Pressure Deficit (VPD), plants adjust the openings of stomata. If the ambient pressure is too high, sap flows through the plant will slow down. Even though the stomata are fully open. As a result, deficiencies of non-mobile nutrient elements will occur first. Like Calcium (Ca) for example. So a Calcium(Ca) deficiency can be caused by too high relative humidity (RH). When the relative humidity (RH) is low, the vapor pressure of the environment is lower and the plant will sweat. The plant now closes its stomates to minimize water loss and wilting. As a result, photosynthesis decreases which inhibits the growth of the crop.

Air humidity in relation to temperature

humidity indoor grow
Hygrometer hygro en temperatuurmeter

The level of relative humidity (RH) is closely related to the prevailing temperature. Warm air can contain more moisture. As soon as the temperature rises, the relative humidity drops. The air becomes drier. When the air temperature drops, the opposite happens and the air becomes more humid. You can read the values on a hygrometer. This instrument should be used with every indoor culture. The hygrometer is used in combination with a thermometer. The relative humidity (RH) is influenced by the temperature, among other things.

As the temperature increases, the air becomes drier (RH decreases).
As the temperature decreases, the air becomes wetter (RH increases).

Below is a rough overview of relative humidity.

Relative Humidity (RH) 0% There is no moisture in the air. Even cacti will struggle to survive.

Relative Humidity (RH) 10% - 20% Cacti and succulents can survive. Most other plants will suffer damage in this dry air and eventually die.

Relative Humidity (RH) 20% - 30% This is the humidity level of an average home. Some plants can survive in this, including cacti and succulents.

Relative Humidity (RH) 40% - 50% This is ideal for plants that are in the flowering stage. 

Relative Humidity (RH) 50% - 60% This is ideal during the vegetative stage of plants.

Relative Humidity (RH) 60% - 80% This can be used for raising young plants and cuttings and for growing tropical plants, for example.

Relative Humidity (RH) 90% - 100% This is ideal for germinating seeds and growing some seedlings and cuttings.