For cannabis plants temperature and humidity are almost as important as the grow light hanging above them. Use vapor pressure deficit (VPD) to control the right temperature at the right humidity, during every stage of your plant's life.
The moment you start working with Vapor Pressure Deficit, you will see that your plants grow and flower considerably better. With the right VPD value, leaves are nice and green, firm and pointing up proudly. It also eliminates curling or dried-out brown leaves in the flowering phase, and keeps flowering hairs fresh and white for as long as possible!
This means it is very rewarding to use VPD for your marihuana plants and regulate the optimal temperature and humidity, at every stage of life.
Humidity is directly related to temperature, which is why we talk about relative humidity. After all, warm air can hold more moisture than cold air. Therefore, when warm humid air cools down, moisture can precipitate from the air. Think of condensation on windows for example, or dew on the grass early in the morning.
Vapor pressure deficit is the difference between the amount of moisture in the air, and the maximum amount of moisture the air can hold. For cannabis plants, that difference is important because it determines how much moisture a plant evaporates at a given temperature. Evaporation then also determines how much moisture, and therefore nutrition, is absorbed at the roots. Humidity and temperature must be balanced. If it's too hot and dry, for example, the plant closes its stomata (as pores on human skin, plants have these also) and evaporation eventually comes to a halt. If it's too cold and wet then not much happens either. The plant would like to evaporate but the air is saturated.
So it is important to keep humidity and temperature in a good balance. Not only that but each life stage of the cannabis plant also requires different values, but the humidity should always be in balance with the temperature. The vapor pressure deficit gives you a safe margin within which your plant performs optimally. We made it visible by a chart as shown in foto above.
How to read the VPD chart for marihuana:
The left column shows temperature and the horizontal line shows humidity. For each temperature and humidity, the map shows the VPD values in hectopascals.( Note: sometimes maps also show the values in kilopascals which actually means that for each value the comma moves one place to the left. In fact, 1 kilopascal is 10 hectopascal.) The VPD chart shows the optimal VPD values within which you should stay in green. The yellow areas indicate the minimum and maximum recommended VPD values, and at the VPD values in the red areas, the air is either too humid or too dry for marihuana plants. For example, if the temperature in your grow room is 27 degrees, the humidity should ideally be between 85 and 70%, according to the chart.
Optimal VPD value per life stage
Marihuana plants have different VPD needs at different life stages. (Also, humidity above 70% is not recommended at all, in relation to the operation and life span of your carbon air filter.) So we must not only handle the chart blindly but also know what the optimal VPD values are for each life stage of your plants, and circumstances such as using airfilters with carbon. On average, the optimal VPD value for cannabis plants is somewhere between 5 and 14 hectopascals (0.5 and 1.4 kilopascals). As said earlier, plants go through different phases of life, and higher humidity is desired during the growth phase and lower humidity during the flowering phase. For each phase, we can recommend the following VPD values:
VPD value for cuttings and germinated seeds:
Because cuttings have no roots yet, you must prevent a lot of moisture evaporation by means of a higher humidity. For this you ideally aim for a low VPD value between 5 and 7 hectopascal. This also applies to seedlings that are not yet rooted. Once a seedling is planted, such high humidity is no longer necessary because the seedling can then absorb water through the rootlet.
VPD value for cannabis plants in the growth phase:
In the growth phase, marihuana plants have roots and the humidity level no longer needs to be as high as for cuttings. And so the VPD can go up a bit to about 7 to 10 hectopascals (0.7 to 1 kilopascal). By doing this, you will find that cannabis plants will also absorb a little more water and nutrition. Be careful not to increase the VPD too much because then the stomata (pores of plant) may close and your plants will be able to absorb less CO2.
VPD value for cannabis plants in the flowering phase:
In the flowering phase, your plants can have a fairly high VPD of 10 to 14 hectopascal (1 to 1.4 kilopascal). However, the buds of cannabis plants are sensitive to high humidity and can develop bud rot. In addition, low humidity stimulates the production of trichomes, terpenes and THC. Therefore, try to keep the humidity as low as possible while still increasing the VPD value.
Adjusting the VPD values:
This can be done by adjusting temperature and by humidity, and also by adjusting light intensity. By warming the air, it can hold more water which causes a higher VPD value. By using a dehumidifier the air will contain less moisture and the VPD will increase. By using a humidifier or put buckets with water in your cultivating room, it will decrease the VPD because the air contains more moisture. To adjust VPD by light instensity, this can be done using the dimmer on your growlight if this in incorperated on your device, or by simply raising or lowering the lamp. If you lower the lamp or increase the power, the leaf temperature also increases which results in a higher VPD value. To lower the value do the oposite. Measure temperatures at leaf hight and in between the plantes and leaves, and measure humidity in the middle of your cultivating room and at leaf level. Keep your thermometer out of direct light of lamps. In larger cultivations or rooms its recommended to measure at several places to be able to detect differences quikly, the values should be almost equal everywhere.