Understanding humidity and VPD

One of the biggest challenges for indoor and greenhouse farming is creating and maintaining a stable environment. Once you lose control of your environment, your plants will not perform optimally and the potential for WPM and botrytis will lead to reduced quality and lost revenue.

Understanding how water vapour and humidity affects your crop is critical. Relative humidity (RH) is a measurement of the amount of moisture in the air (as a percentage of what it can hold), at a given temperature. As humidity increases, the air will reach a state of saturation and water will leave the air as clouds, dew, or condensation. If the amount of moisture in a space remains constant and the temperature increases, the RH will decrease. Conversely, if the temperature decreases, the RH will increase. The water holding capacity of air halves with every 11 degree decrease in temperature. Air at 26C can hold twice the amount of water compared with air at 15C. One practical example in a greenhouse is that at night when temperatures drop, the RH increases and water moisture is no longer held in the air but as precipitation on your leaves and flowers and perfect conditions exist for disease to manifest.

Managing humidity is also an important tool for managing growth. Vapor pressure deficit measures the difference, between the air’s water content, and its saturation point. This pressure difference is what drives transpiration. As water evaporates from the leaf stomata, water is pulled from the roots through the plant carrying nutrients that support plant development. A growing space with low VPD results in low transpiration and metabolism. If VPD is too large, the plant stomata close and metabolism is affected by this stress.

In cannabis the optimal VPD is 0.8-1.0 kPa in Veg and 1.2-1.5 kPa during flowering. You can influence this vapor pressure deficit by managing temperature or humidity. With temperature, you can raise VPD by increasing temperature, or lowering VPD by decreasing temperature. With humidity, if you turn the dehumidifier on and decrease humidity, you raise VPD, and by turning the dehumidifier off, you increase humidity and reduce the VPD. You can also influence VPD by using good circulation. Moving stagnant, moist air near the leaf surface increases the VPD.

Improve your HVAC systems with Quest to reduce RH and optimize VPD. Quest is made in the USA, and they have set the gold standard in dehumidifier engineering, design, energy efficiency, durability and reliability. For Eskom power rating at 55HZ, Hydrobiz can only stock 3 quest units:
Quest 70 = 33l per day
Quest 155 = 73l per day
Quest 706 = 334l per day.

Contact Hydrobiz for your dehumidification requirements and together with Quest engineers we will support you with growing room design, equipment specification and installation support

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