VPD or vapor pressure deficit measures the amount of water present in the air and compares it to its saturation levels.This data is very handy to know how fast are plants hydrating and how often you need to irrigate and obtaining a correct analysis can substantially improve the yield.
Calculating VPD is an advanced technique that requires specific devices that measure temperature and humidity, two essential values to carry out this process.
Thermometer: Measure temperature, essential to know VPD values.
Hygrometer: It measures relative humidity, the second most important parameter to calculate VPD.
Infrared thermometer: Used by professional growers. It measures the exact temperature of the leaves, improving VPD’s precision.
How is measuring VPD helpful?
This process measures the level of plants’ transpiration, which refers to the speed at which water moves inside the plants. H2O enters the plant through the roots, goes up by the stem and comes out through the pores of the leaves, which are called stomata. This process hydrates the plant, transports the nutrients all over its structure to promote its correct development and also cools it down.
To make it simple, the leaf of the plant is completely humid (100%) but not the air. Therefore, when the plant releases water, it does so in vapor form, and once the plant needs it again, it absorbs it through the roots. This process can be measured and manipulated to maximize plan’ts development by watering them at the exact moment it needs it.
However, this cultivation technique is a bit risky if not done by professionals because an error in calculations can ruin the crop. If the plant doesn’t transpire enough, it won’t absorb the nutrients efficiently and if it transpires too much, stress will kill it.
What parts of the plant are affected by VPD?
VPD levels have a direct influence over the following parameters:
- Transpiration: When VPD levels are high, the plant transpires very fast, therefore, it needs hydration often but it will grow faster.
- Stomata opening: Increased VPD levels forces plants to close down their stomata to prevent water from getting out, thus staying hydrated.
- CO2 absorption: When VPD levels increase, stomata close down. This maintains hydration but decreases CO2 absorption, delaying photosynthesis.
- Nutrient absorption: Higher VPD levels mean more transpiration, therefore plants absorb more nutrients at a faster rate.
- Stress: All of these processes increase plants’ stress when VPD levels are higher than average. To prevent this, the recommendation is to use fertilizers that strengthen the immune system during growth and flowering; which is when VPD levels increase.
VPD levels depending on the phase of the plant
Plants don’t need too much water or fertilization during the first days of life; but as they grow, it’s important that they acquire the necessary nutrients in an optimum way. Therefore, VPD levels are progressive and will increase as the plants grow.
- Germination to the start of the vegetative phase: 0.4-0.8 kPa.
- From the end of the vegetative phase to the start of the flowering period: 0.8-1.2 kPa.
- From mid of the flowering period until the end: 1.2-1.6 kPa.
- Cuttings and clones: 0.4-0.8 kPa
Depending on the needs of the plant, temperature and humidity values should be modified to change vapor pressure deficit. To increase it, you need to increase the temperature or decrease humidity and to decrease it you need to do the contrary. It’s also very important to take into account lighting intensity, as it also modifies temperature.
VPD parameters chart
Calculating the recommended VPD values depends on the temperature and humidity of the grow room. However, there are charts that display the suitable values; they are quite handy to not lose time with complicated calculations. In this case GB The Green Brand has created a chart to calculate the perfect VPD if the temperature of the leaves is two degrees lower than the temperature in the air; which is very common in cannabis grows.
Therefore, if you don’t feel like calculating exactly what you need, you can check the chart. Follow the guidelines according to the temperature and humidity of your grow room.
As the chart indicates, the ideal is for VPD levels to be low during the germination phase. Plants don’t need too much water during this stage and they need to grow properly. Once they reach the growth phase, the ideal is to increase VPD levels to something between 0.1-1.2 kilopascals; which will increase transpiration and nutrient absorption speed. We’ll maintain such levels until the start of the flowering period.
When the buds reach the fattening phase and they run the risk of being attacked by fungi; it’s essential to raise VPD values to increase transpiration to improve the final yield.