The importance of humidity

The correct atmospheric humidity level is crucial for optimum plant development. This post explains in simple terms what we mean by humidity, why its important, and what you can do to make sure you give your plants the best environmentals.

High humidity: bad for houses , good for young plants
High humidity: bad for houses , good for young plants

What is humidity?

Humidity is the amount of water vapour in the air. Absolute humidity is measured by dividing the mass of water vapour by the mass of dry air in a volume of air at a given temperature .. and the hotter the air, the larger amount of water vapour it can contain. When growers talk about humidity they are referring to relative humidity: the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). As such it is usually expressed as a % in relation to the maximum amount of water the air will hold at the temperature of the growroom.

Monitoring your relative humidity and temperature is easy with an inexpensive thermo-hydrometer. We always recommend adding one to any grow room: they are an important tool which helps diagnose issues that may be effecting your plants. On numerous occasions we have been called into advise why plants aren’t “quite right” and the addition of one of these has shown the reason why, whether it be low temperatures, high temperatures or low humidity.


Why humidity is important to plants

Firstly remember that water – and thus water vapour – likes to find a balance. If there is an area of high % humidity next to one with low humidity, the water vapour will try to move across in order to create this equilibrium. Now the relative humidity inside a plant is close to 100%, so the vapour will always be trying to move out into the outside air.

Every time the plant opens its leaf pores (stomata) to take in CO2 it loses a little water to the air.

If the atmosphere in your grow room is at the correct %RH then all is well, because as the plant loses moisture through the leaves, it pulls up new water through the roots through the process of transpiration. Temperature control, nutrient uptake and intake of carbon dioxide all function in perfect rhythm. Seeds are activated in nature during the spring rains, move through a warm and humid summer and finish in hot dry conditions.


If the air is very dry, then the water in the leaves evaporates quickly, causing the plant to lose moisture quickly. If it does so at a higher rate than it can absorb new moisture through the roots, there becomes an imbalance and the plant struggles. In these conditions photosynthesis, transpiration and other processes are severely restricted and growth development reduced.  Increasing watering won’t help in this situation, and will only lead to overwatering problems.The plant is also likely to suffer from nutrient burn as the water is lost before the plant can process the feed thus making the nutrient solution more concentrated.

If the air is very wet, then there is a chance that molds and diseases can take hold and flourish in the damp conditions. HOWEVER this latter has become something of a bogeyman to many indoor growers,who are so afraid of these that they run their grow rooms at a much lower humidity than is beneficial to their plants. A damp, humid (and especially stale) air is the home-ground of many nasty fungal rots but these are only an issue from mid-flowering onwards and are usually prevented from taking hold in the grow room by having good ventilation and air-flow

Effects of poor humidity control:

sign cause
thin leaves and poor vegetative growth low humidity
nutrient burn – nutrient solution becomes concentrated as plant takes up more water relative to nutrients low humidity
curled leaves, brown tips low humidity
cuttings don’t root because they can’t take in water through leaves low humidity
white powdery mildew high humidity and bad airflow
bud rot high humidity and poor airflow

Plants need different humidity levels depending on their stage:

Young Cutting & Seedling Stage 70-80% RH high humidity is necessary because it allows plants to absorb water through the leaves, it slows down transpiration, and plants devote their energy into building a root system rather than struggle trying to take up water through a developing rootball. Use a propagator and a lid with ventilation openings to easily creates a high humidity atmosphere. The vents give you a level of control and as plants “take” you can expose them to lower %RH as part of the hardening off process.
Young plants (“maidens”) 50-80% As vegetative growth starts to kick in, plants still flourish in a relatively humid environment. . Keeping rH at 65-75% during this stage will have noticeable improvements on overall growth rate and general plant health.
Regularly replaced damp towels in the area from which intake air is brought from works as a short term solution to this problem. Better still invest in a suitably sized humdifier for the area.
Vegetative stage 50-80% Leaves are wider providing a larger surface area for transpiration to take place with less stress to the plant, but there is still a risk of problems if air is too dry and transpiration rates become too high. However larger plants will be adding to the humidity level in an enclosed grow room and you shouldn’t need to use additional equipment to make this level.
Flowering Stage (1st few weeks) 40-50% By now plants should have a fully developed root system capable of transporting water around the plant. Avoid high humidity at this stage as a damp environment allows molds, fungi and diseases to spread within the enclosed buds. Increase ventilation rates to pull out hot, humid air and replace it with cooler, drier intake air more frequently
Flowering Stage until harvest 30% If you can get your humidity levels down this low, many growers believe it encourages higher resin production as plants use the resin like a “sunscreen” Use a dehumidifier in the last 2 weeks of flower
Drying 45-55% The drier the surrounding air, the quicker the plants will dry. If your normal air is itself hot and dry, you will have to put into curing jars to keep a level of humidity in, and frequently turn the buds for a slow cure process. If your normal air is too damp, then there is an increased risk of bud rot If necessary use a dehumidifier in the drying area

Products to help you get the right %RH:

Leave a Reply