Water molecules at the surface of water in a container can have enough kinetic energy to escape the liquid and form vapour in the air above the water, creating a pressure within the container – the vapour pressure. As these molecules collide with the surface of the water, they return to the liquid phase by the process of condensation. When the number of molecules vaporizing and condensing per unit of time is equal, the pressure becomes constant and has a value that is characteristic of a liquid at a given temperature.

Developing and maintaining indoor growing and hydroponic systems all comes down the vapour pressures within a grow system and their direct impacts on plant growth and health. Like everything else, however, understanding these impacts starts with the basics.

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1.1 A few botany basics

1.1.1 Plant anatomy

Describing the vegetative growth process of plants starts with a look at their basic vegetative structure (a branch of botany – the scientific study of plants), given in Figure 1.

Water and nutrients travel from the growth medium to the roots (or sprouting seed) and up the stem through a vascular system that propagates water, minerals, and sugars to their intended destinations. Nodes are one of those destinations, where small clusters with high cellular activity and growth form buds that eventually turn into leaves, stems, or flowers. The entire process orchestrates the flow of water throughout the plant from the roots to the leaves, where it can eventually assist in photosynthesis or exit the plant via transpiration.

1.1.2 Leaf structure and function

Consequently, the leaf has a crucial role in these processes that allow a plant to breathe and grow. Within its deep green pigment lie several layers designed to carry out these specific functions, outlined in Figure 2.

The external layers of the leaf make up the upper and lower epidermis. Sunlight hits the upper epidermis, which is covered with a protective, waxy cuticle, while the lower epidermis contains stomata – tiny pores on the under-side of the leaf that are controlled by guard cells (for aquatic plant leaves, the stomata are found on the upper epidermis). Guard cells open and close the stomata in response to environmental cues, controlling the movement of water, oxygen, and carbon dioxide into and out of the leaf as a result. Between the upper and lower epidermis is the mesophyll, which contains the chloroplasts where photosynthesis takes place. At the root of it all enter the xylem and pholem, which are those key components of the vascular system transporting water and nutrients throughout the plant. Together, each of these constituents of the leaf drives plant growth via three major processes: photosynthesis, respiration, and transpiration.

1.1.3 Photosynthesis, respiration, and transpiration

Photosynthesis allows plants to create food from sunlight, carbon dioxide (CO₂), and water (H₂O). Carbon dioxide enters the leaf through the stomata, while water enters either from the stomata or, in higher quantities from the roots and vascular system. Sunlight then provides energy for the chemical reaction of photosynthesis (shown in Figure 3) to occur in the chloroplasts.

The products of photosynthesis are oxygen (O₂), which is released through the stomata, and glucose (C₆H₁₂O₆), which is either stored for later use (in low light conditions, for example), or sent where it is needed (such as to developing fruit).

Oxygen and glucose are necessary for cellular respiration, which breaks down glucose in a “reverse-photosynthesis” reaction to convert its chemical energy into a form the plant can use to grow and reproduce. Unlike photosynthesis, respiration can occur in the absence of sunlight, at any time of day.

At the same time, water moves into and out of the leaf through the stomata along with the carbon dioxide and oxygen for photosynthesis and cellular respiration. Transpiration occurs when stomata open to release water, thereby pulling more water and essential minerals up through the plant from the roots.

Optimal growth requires a delicate balance among each of these three major processes (outlined in Figure 4 ) to efficiently respond to environmental cues. In a particularly hot and dry environment, for example, stomata close to avoid losing excess water. Stomata also tend to close at night, when the absence of light means there is insufficient energy to power photosynthesis. In each case, plants retain water and, without access to carbon dioxide, must rely on glucose stores for energy via respiration to continue to develop and grow. Before too much water accumulates or glucose stores are used up, the stomata open up again. Similarly, when photosynthesis occurs at a significantly faster rate than respiration, the accumulation of its products will signal stomata to close and photosynthesis to slow or stop. When respiration occurs faster than photosynthesis can generate glucose, the accumulation of carbon dioxide and water from respiration will trigger the process to slow or stop photosynthesis. It’s all about maintaining a consistent environmental balance to control these responses.

Knowing how photosynthesis, respiration, and transpiration respond to each other makes it possible to efficiently control the environmental cues that trigger them to collaborate seamlessly on plant growth and development. In fact, this is one of the main advantages of indoor growing and hydroponics.

1.2 Environmental control factors for growing

1.2.1 Light, air, and temperature

We’ve seen the importance of sunlight as a source of energy for photosynthesis. While a seed can germinate in the absence of light, for a seedling to continue to grow, light is essential. Plants in direct sunlight tend to grow to be more compact, while those in shady environments grow to be more elongated. The amount, intensity, and duration, of light all affect the quality plant growth.

Besides energy from light, photosynthesis relies on carbon dioxide. As a result, air – and what it’s made up of – is a key environmental factor to consider for plant growth. Our atmosphere is constantly fed carbon dioxide from plant and animal respiration, decaying organic matter, combustion fuels, and volcanic activity.

Wind is an important part of how plants are exposed to air. While it can assist in processes like transpiration by accelerating the transfer of heat from leaf surfaces, too much wind can lead to excessive transpiration from evaporation and potential structural damage.

Light, air, and wind can also all have an effect on temperature, which directly affects plant growth and development. The temperature of our atmosphere depends on the transfer of heat from the Earth’s surface to the air. As a result, temperature is naturally always changing. It also directly influences climate, which determines what types of plants can grow in a specific location. Plants that grow in colder climate have what is referred to as cold hardiness, while those that grow in warmer climates are known as tender.

Together, these factors influence that intricate balance of photosynthesis, respiration, and transpiration required for plant growth. Other important environmental influences, however, can make or break healthy and productive plant development.

1.2.2 Nutrients

Plant growth and development depend on 17 essential nutrients, or elements, which are divided into three categories. The first is made up of those macronutrients obtained from air and water: hydrogen (H), oxygen (O), and carbon (C). The other 14 elements, however, must come from the growth medium (soil, for example). These are split into the remaining two categories, termed soil-derived macronutrients and micronutrients. The divide simply refers to the amount of each element a plant requires; macronutrients – nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg) – are used in amounts above 0.1% of a plant’s total dry weight, while micronutrients are required in low concentrations, usually just a few parts per million (ppm) of a plant’s dry weight. These amounts are outlined in Table 1.

The 14 soil-derived nutrients each play an essential role in the different plant processes and functions, as outlined in Table 2.

Nutrient solutions are often used to enrich plant growth medium, thereby optimizing nutrient concentration availability and uptake by roots. As a result – like many plant processes- making these nutrients available really comes down to one vital resource: water.

1.2.3 Moisture and the hydrologic cycle

Water plays a vital role in plant growth and development through photosynthesis, nutrient transport and availability, and structure maintenance – keeping plants turgid. It also lowers leaf temperature through transpiration, drawing water from the roots to the top of plants and increasing nutrient absorption in the process. As a result, a large part optimizing a grow system involves optimizing water levels in the air, in the growth medium, and on the plant itself. In other words, while nutrients are essential to plant growth and development, how plants use these nutrients is just as important.

The moisture of a plant and its surrounding environment is central to its growth. In our environment, the constant movement of water between the oceans, land surfaces, and the atmosphere is called the hydrologic cycle, illustrated in Figure 5.

Water that leaves the atmosphere and falls down to Earth as precipitation joins surface water or makes its way to groundwater. Through evaporation and transpiration, that water then returns to the atmosphere for a full cycle.

Plants play a vital role in the hydrologic cycle, transpiring 5 to 10 times as much water as they can hold at once every day they grow. Consequently, the hydrologic cycle plays a vital role in plant growth ad development. Maintaining optimal levels of moisture and water vapour within a grow system encourages nutrient uptake and transport through transpiration for a stronger, healthier crop. So, how do we measure that water vapor among the various components of a grow system? That’s where VPD comes in.

1.3 VPD : A formula for success

1.3.1 What is VPD?

The Vapour pressure deficit (VPD) is the difference between the water vapour pressure at saturation (SVP) and the actual water vapour pressure at any given time and temperature. Air that is saturated has the lowest water vapour pressure, leading to the condensation of all that water on surfaces in the form of dew, for example. However, as temperature and humidity change, that vapour pressure increases with the evaporation of water from surfaces into the air. As a result, the difference between the two creates a deficit: VPD.

The VPD within an environment tells us how a plant will transpire. As we’ve seen, plants release water from their leaves via transpiration and absorb it from their roots so that the rate and control of transpiration directly affect water-nutrient delivery and growth. The whole process is dependent on temperature and humidity, which greatly affect the flow of water (think, hydrologic cycle) throughout an environment. An increase in temperature, for example, causes the evaporation of water from surfaces, which increases water vapour pressure in the air (or, the level of moisture in the air), to a certain extent, thereby increasing the VPD. The same is true for higher levels of humidity.

While the humidity of a plant’s environment does directly affect transpiration, it’s dependency on air temperature, leaf temperature, stage of growth, and the time of day makes it a difficult variable to set specific targets for at any given time. VPD, however, combines each of these factors into one reading, allowing growers to maintain ideal temperature and humidity levels for optimal transpiration and growth.

1.3.2 VPD targets for growth stages

When plants transpire too quickly, they can lose too much water and either be deprived of nutrients, or absorb a surplus of water in response, eventually leading to toxicity from an increased concentration of nutrients in solution; plants that transpire too slowly may not have access to sufficient nutrient levels. The balance of temperature and humidity required to achieve optimal transpiration and VPD is shown in green in Figure 6. 

Environments with high humidity and low temperature, or with low humidity and high temperature are in the red zone; they lead to transpiration that is too slow, or too fast, respectively. These red and green regions (yellow for warning zones) show the boundaries to healthy plant growth and development. The highlighted sections then change slightly with the different growth stages and associated needs.

Generally, younger plants and seedlings are still fragile and require low-strain environments to reserve energy for the development of strong roots. As a result, plants in this growth stage thrive in conditions of low transpiration and VPD from about 0.4 to 0.7 kPa. With more roots and some leaves, plants are then ready for a higher VPD to help boost nutrient uptake and delivery for growth. The ideal VPD for this group ranges from 0.7 to 1.1 kPa. For plants already flowered, a slightly higher temperature with lower humidity  is best with VPD values from 1.1 to 1.4 kPa. This allows them to take up more water while still being dry enough to avoid any rotting buds or fruit. 

Every stage of growth brings unique requirements. By adjusting temperature and humidity to meet these requirements and reach optimal VPD values, growers ensure the most efficient and productive environmental conditions for their crop – a major advantage of indoor growing and hydroponics. Achieving these target VPD values begins with understanding how they are calculated.
 

1.3.3 The VPD formula

The VPD in a growth environment can be calculated in 3 steps from the temperature  (T^o in degrees celcius) and % relative humidity (%RH) within a system. Because VPD depends on the water vapour pressure in the air at saturation (vp_sat), determining that value at any given temperature is the first step to VPD calculation, and can be achieved using in equation 1.

Expressed in kPa, vp_sat represents environmental conditions of full moisture (100% RH), where transpiration is eventually slowed or stopped with lack of evaporation. As the %RH decreases and varies, the water vapour pressure within a grow system (vp_air, in kPa) can then be determined using equation 2.

The vapour pressure difference between these two environments gives us the final VPD value (kPa), as in equation 3.

Together with the VPD values from Figure 6, these equations give system growers a chance to create and modify a prime environment depending on the various factors that affect plant growth and development. But to keep it simple, a calculator never hurts.

1.3.4 DiscountHydro VPD  calculator

Our quick and simple VPD calculator helps growers establish the optimal VPD target for a system given its various temperature and humidity conditions. Try it today!

Enter your system’s air temperature and %RH to obtain a VPD target value as well as your system’s associated vp_air and vp_sat values.

For a VPD value more representative of what your plants are experiencing, the canopy temperature (that of the vegetative covering they form) can be entered in place of the air temperature.  It’s all about establishing and hitting that VPD target.

Welcome to DiscountHydro: All the hydroponic equipment you need, online, at a discount. It’s that simple.

Check out our environmental controls today to optimize your VPD and get the best out of your crop.

Happy shopping and hey, happy growing.