How to Use VPD to Increase Your Yields
Many growers believe that the most important things in growing are the light and the nutrients.
Light and nutrients are very important, but plants can't use either of them properly unless the environment lets them.
About Transpiration
Plants need to lose some water through their leaves. This is so they can draw water (containing nutrients) in through their roots to replace it.
This process of water being lost by the leaves and replaced with nutrient rich water by the roots is called transpiration.
Plants cannot get the nutrients they need by the roots unless transpiration happens.
Why is VPD so Important?
VPD is a sort of measure of the environment. It combines temperature and humidity into a single measurement.
This tells us how well the air can dry things. A high VPD can dry things very well. A low VPD will not.
If the air is too humid or too cold then it will not dry things very well. Plants will not lose water quickly enough.
They may not be able to get the nutrients that they need to avoid deficiencies.
This could lead to slow growth.
If the air is too warm or too dry then plants may lose water too quickly.
This could lead to a water shortage.
If this gets too bad it could cause wilting.
Also warm, humid conditions may encourage mold
The VPD of the air needs to be in the right range so that plants can lose water out of the leaves.
This lets them get nutrients from the rootzone.
VPD is measured in Kpa or Bar. 1 Kpa = 10 Bar. (Kpa stands for Kilopascals)
A low VPD means that the air is cold/humid - plants cannot lose water through their leaves very well. There is a risk of mold.
A high VPD means that the air is warm/dry - plants risk losing water too quickly and could begin to wilt.
The Ideal VPD
The ideal VPD depends a lot on the stage of growth the plants are in:
Rooting Cuttings - These need a very low VPD as they don't have an established root system to replace water yet. A propagator holds moisture in and helps to keep VPD low.
Early Veg - The ideal VPD is between 4 - 8 Bar. The root system is still developing so keep things gentle with not too much water loss.
Late veg and Early Flowering - Ideal VPD is between 8 - 12 Bar. Now that the plants have a decent root system, increasing the VPD will increase nutrient uptake and fuel faster growth.
Mid - Late Flowering - Ideal VPD is 12 - 16 Bar. Now that plants are in peak flowering, raising the VPD a bit further will power a more productive flowering stage. The higher VPD also discourages molds to start.
Apart from the cutting and young plant stages it is generally best to try to keep the VPD in the green zone in the chart above.
If you have an extraction system, you may find it difficult to alter the humidity by much.
If you humidify or dehumidify the air, the treated air might quickly get sucked out and replaced by air with the same humidity level as outside. In other words, unless you have a sealed room system it can be difficult to control humidity.
Usually the temperature is easier to adjust than the humidity to get the VPD level right. This can be done by turning the extraction up or down with a fan speed controller.
For a more in-depth look at VPD, take a look at our full explanation in our sister blog here, entitled Vapour Pressure Deficit - Understanding how temperature and humidity affect plant growth.
