Humidity for plants in a grow tent is perhaps the most underrated growth factor in the hobby cultivation. We use relative humidity RL as a unit to measure this and to make adjustments.
A dry air will accelerate the evaporation of the moisture from the leaf and a very moist air will delay this. A plant must always evaporate some water so that there is room in the plant for new water from the medium in which new nutrients are dissolved.
The plant is like a pump that works on solar energy. However, if the relative humidity around the plant is too low, the plant will evaporate more than it can absorb and the plant will experience a growth disorder and may eventually dry out.
If the humidity around the plant is extremely high, the plant will eventually grow little because it can vaporise less.
A balance in this is therefore very important.
Humidity schedule
Small cuttings still have very little root or no root yet at all and therefore cannot absorb much water. To make a strong cutting, you need a 80% humidity in the cutting room.
If the humidity is to low around the plant cutting, the cutting will vaporise quick it inside water without able to absorb new water.
In the first 3 weeks of growing you have to have a relative humidity for plants of about 70% - 80%.
After that, up to and including approximately the 6th week, the optimum relative humidity for plants is 50% - 60%.
In the 7th week and later we bring the relative humidity for plants down again but not lower than 40%. Below 40% the evaporation goes very fast, making it very difficult for the plant to get new moisture into the plant in time. If there is also a breeze from a fan blowing on the plants with a relative humidity of 30% in the air, the plant will have a very difficult time absorbing enough water, especially if the EC in the medium is also high.
When humidity is in the cultivation room to low the plant will close the stomata at the bottom side of the leave. This prevents the leaves from evaporating moisture, which means that there is no more negative pressure in the leaf that sucks new moisture from the medium via the root, which stops the growth and flowering of the plant. And downtime is always at the expense of the proceeds.
The humidity temperature is also important in a cultivation room. There are systems that can easily blow cold water into the air. However, this cold moisture will naturally also slow down the plant development. The cold will bring a deficiency of phosphorus as explained in this blog here. It is best to use lukewarm water.
How to increase humidity for plants ?
To increase the humidity the best is to use a humidifier that is connected to a tap water system. A humidifier with a tank can be suddenly empty with disaster result for small cuttings. The humidifier have to placed strategic so that the fog can get everywhere in the cultivation room and at certain distance from the air exhaust.
Be sure that the water is not cold to avoid getting a phosphorus deficiency.
How to increase humidity in a grow tent ?
For smaller cultivation rooms like a grow tent you can increase the moisture by a small grow tent humidifier that have it own water reservoir. Such humidifier is perfect for a small grow tent.
Hobby growers who do not have a grow tent humidifier at their disposal can best spray the leaves regularly during the first 4 weeks.
Also a transparant foil between the light and the cuttings will work well. The foil you make for 1 week just above the cuttings before you spray them.
The humidity you can easy measure with a hygrometer. A hygrometer also named moister meter or humidity meter you can buy at any garden centre or growshop and cost not expensive.
Most of the hygrometers can also measure the temperature at the same time. Be sure that you get a hygrometer where you can see the minimum and maximum moisture and temperature in the air. On that way you can also measure what is happening with the moisture and temperatures in the nights.
The best place to hang the hygro and temperature meter is at the same height as the flower buds.
