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Understanding VPD for Seedlings in Cannabis Cultivation

Cannabis lifecycle is divided into several stages. The seedling stage marks the beginning of its life. During this stage, cannabis needs specific conditions such as light, temperature and humidity to support it in developing its leaf and rhizome structure. Apart from this, VPD for seedlings is one of the factors that should not be ignored.

Understanding the basic concept of VPD, it is natural to focus on how it affects cannabis health at different growth stages. For the seedling stage, proper management of VPD can enhance the plant's resilience. Let's take a look at how VPD changes and develops during the seedling stage of cannabis to help growers better understand and optimize this critical growing condition.

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Characteristics of Cannabis Seedlings

The seed stage of marijuana is the beginning of growth, and although the seedlings may seem fragile, they contain amazing growth potential. So one wonders what potential characteristics marijuana displays in the early stages of growth?

Physical Appearance

Cannabis seedlings exhibit specific physical characteristics during the early stages of development. These physical characteristics are specifically manifested in the plant's leaf development and stem growth.

Leaf Development

Cannabis seeds germinate and grow a pair of simple cotyledons, which are the plant's initial photosynthetic organs. Subsequently, marijuana grows true leaves, which are larger than the cotyledons and more complex in shape, often with serrated edges. As marijuana continues to grow, the number of leaves gradually increases and the shape of the leaves becomes more typical.

Stem Growth

At the beginning of the cannabis seed stage, the stem elongates rapidly, pushing the leaves toward the light source. At the same time, the stem thickens, providing stronger support for the plant. As more leaves are added, the stem segment between the two leaf nodes gradually becomes shorter, making the plant more compact.

Observing the appearance of the plant, we will notice that marijuana seedlings show positive growth in terms of the number of leaves and stem height, which promotes the growth of the plant.

Physiological Characteristics

In addition to the direct observation of the appearance of cannabis seedlings, their inner physiological processes likewise exhibit many unique characteristics.

Root Development

Root development is an important part of marijuana during the seed stage. The primary root grows downward first, absorbing water and nutrients. Lateral roots branch from the main root to increase the area of absorption. Root hairs form on the surface of the root to increase the efficiency of absorption.

Metabolic Activity

The metabolic activity of marijuana begins when the cotyledons unfold and organic matter is synthesized through photosynthesis to provide energy for growth. Respiration supplies energy to the cells, and the root system absorbs water and inorganic salts to support the plant's transition from seed to seedling.

Cannabis at the seed stage exhibits characteristics of leaves, stems, roots and metabolic activity. These characteristics show the growth potential of cannabis and provide a good foundation for healthy future growth.

Primary Tasks of Cannabis Seedlings

Each stage in the growth process of cannabis has a different primary task. The primary tasks of the seed stage may seem simple, but each one is closely related to the healthy growth of the plant.


Establishing Root System

The outer shell of the seed breaks down and from it grows the embryonic root. It grows downward and becomes the plant's primary root system, responsible for anchoring the plant and absorbing water and nutrients from the soil.

As the seedling matures, the root system becomes more complex, developing lateral roots that extend outward to increase surface area and stability for absorption.

Root hairs form on the surface of the roots, increasing the root system's absorptive capacity and improving the efficiency of water and nutrient uptake.

Developing Initial Leaves

After the root system effectively absorbs nutrients, the seed germ grows upward and pushes out of the ground. When the germ breaks through the soil surface, cannabis grows its first cotyledons.

They usually appear in pairs and are round or oval in shape, providing the seedling with the energy to grow new leaves. Cotyledons are rich in stored nutrients. It helps the seedling transition from relying on the seed for nutrition to growing on its own using sunlight and nutrients from the soil.

After the cotyledons, the true leaves begin to grow. These leaves have the serrated edges characteristic of marijuana and have a more complex structure than the cotyledons. Initially there is only one true leaf, but the number of leaves increases over time. They develop in a certain pattern: from one to three, to five, and finally to a fan-shaped blade with multiple leaflets.

Strengthening Stem Structure

The increase in the number of leaves means that the plant's crown becomes denser and heavier, and this is where the development of the stem structure becomes especially important.

Initially, stems are usually light green in color and are relatively slender and weak. As the plant grows, the stems of young seedlings elongate to support the growth of more leaves and lift the leaves toward the light source. The elongation of the stem enhances photosynthesis in the leaves and promotes nutrient uptake by the plant.

Efficient photosynthesis also encourages the plant to synthesize enough lignin and cellulose to enhance the robustness of the cannabis stem structure. The right amount of air movement in the growing room increases mechanical stress, and cannabis responds to the stress by strengthening the stem structure, so the stems become thicker and tougher.

Initiating Metabolic Processes

Metabolic processes are initiated at the seed stage of cannabis. When the seed sprouts and develops cotyledons, the plant begins to photosynthesize. This is the initial metabolic activity of the plant.

The cotyledons, as the earliest photosynthetic organs, absorb sunlight and convert the light energy into chemical energy to produce the organic substances necessary for the plant's growth, providing the plant with essential nutrients. With the emergence and unfolding of the true leaves, the efficiency of photosynthesis increases significantly and the plant is able to synthesize more organic matter.

At the same time, respiration becomes active in the seedlings. Through respiration, the plant releases energy stored in organic matter to provide the energy needed for cell division and growth. Respiration occurs in all parts of the plant, including roots, stems, and leaves.

In addition, metabolic processes include the uptake and transportation of nutrients. The root system provides essential minerals to the plant by absorbing water and inorganic salts from the soil. These minerals are transported to various parts of the plant through the vascular bundle system of the stem and participate in various physiological activities of the plant.

The movement of water through the plant promotes transpiration of cannabis and facilitates the transportation of nutrients from the roots to the buds.

Adapting to Environmental Conditions

The seedling stage is the most vulnerable period of the cannabis life cycle. During this period, cannabis seedlings begin to be exposed to and adapt to stresses from temperature, humidity, water, light, etc. to ensure that they will survive.

Well-adapted plants will show greater resistance in the face of pests and diseases, reducing the chances of invasion by pathogens. Healthy, mature plants are also better able to produce high-quality marijuana buds later in their growth.

Cannabis seedlings accomplish the major tasks of the seed stage through interlocking growth processes. Together, these processes contribute to the growth of the seedling and ensure that it can develop healthily in different environments.

Growth Requirements to Achieve Optimal VPD for Seedlings

The ability of cannabis seedlings to successfully accomplish their growth tasks is dependent on the right environmental conditions. Next, we'll take a closer look at these key growth factors.


Light Requirements

Cannabis seedlings grow rapidly and are highly dependent on photosynthesis. In indoor grow rooms, plant lights provide the necessary light to meet the light source for photosynthesis.

Cannabis requires 18 hours of light per day, simulating the long days of summer. This amount of light time increases the efficiency of photosynthesis and ensures that seedlings receive enough energy for rapid growth.

At the seedling stage, the plant needs mainly blue and red light. Blue light aids in leaf development and solid stem growth, while red light promotes photosynthesis and biomass accumulation. Full-spectrum LED lights are ideal for providing these spectra.

Air Circulation Requirements

In indoor grow rooms, air circulation can also affect the growth of marijuana seedlings.

Growers will use oscillating and wobbling fans to create a breeze around the seedlings, encouraging localized airflow for efficient circulation.

The mechanical stress created by the fresh airflow mimics natural wind conditions and encourages cannabis seedlings to grow thicker stems that support the plant canopy.

Watering Requirements

During the seedling stage of marijuana, the goal is to keep the soil consistently moist but not waterlogged.

A rule of thumb is to water seedlings when the top inch of soil feels dry. In the beginning, small amounts of water should be applied frequently to ensure that the soil remains evenly moist but not saturated. This may mean watering daily or every other day, depending on environmental conditions such as humidity and temperature.

Using a spray bottle or watering can with a fine nozzle can help apply water gently, prevent soil disturbance and protect delicate root systems. It is important to water slowly to allow the soil to absorb the water evenly, which helps prevent loss and ensures that the water reaches the root zone.

Nutrient Requirements

During the seedling stage of marijuana, the plant requires specific nutrients to support its rapid growth and development. These nutrients include macronutrients such as nitrogen, phosphorus, and potassium, as well as micronutrients such as calcium, magnesium, and iron.

Nitrogen is essential for plant leaf and stem growth. Phosphorus promotes root development and internal plant energy conversion. Potassium helps regulate a variety of physiological processes, including water uptake and enzyme activation. Micronutrients are required in small amounts, but are equally important for metabolic function and overall plant health.

By understanding and managing these growth requirements, growers can maintain optimal VPD for seedlings, ensuring their healthy growth and setting the stage for successful maturation.

How Growth Requirements of Cannabis Affect VPD for Seedlings

Cannabis seedlings require the right environmental conditions to successfully accomplish their growth tasks. So what exactly do these seedlings need to thrive? Let's start with photosynthesis and the light required for overall growth.

Light Requirements and Temperature

High light intensity generates too much heat, causing the overall temperature of the environment to rise.

If the ambient temperature increases while the relative humidity remains the same, this means that the amount of water vapor that can be held in the air increases, and the saturated water vapor pressure in the air increases.

When the saturated water vapor pressure increases and the actual water vapor pressure does not change much, an elevated VPD can be deduced from the VPD formula.

Higher VPD leads to increased transpiration and plants have to accelerate water uptake and transport in order to maintain water balance. If not managed properly, this can lead to plants facing water loss and growth stress due to excessive transpiration.

Watering Requirements and Humidity

When overwatering results in too much water in the soil, the excess water is evaporated into the air, increasing the humidity in the air.

The rise in humidity in the air causes the water vapor content of the air to become closer to saturation and the actual water vapor pressure increases significantly. Based on the formula for VPD it can be deduced that the difference between saturated water vapor pressure and actual water vapor pressure decreases, resulting in a lower VPD.

Lower VPD leads to reduced plant transpiration. This means that less water evaporates and less water and nutrients are absorbed by the plant, which can lead to slower growth and an increased risk of marijuana mold or mildew.

Air Circulation and Temperature & Humidity Regulation

In indoor grow rooms, heat from lights and other equipment can cause the ambient temperature to rise. The use of oscillating fans creates gentle air movement, helping to distribute heat evenly and preventing it from building up in certain areas and creating hot spots.

Watering and plant transpiration increase humidity in the air. The right amount of air movement can accelerate evaporation and diffusion of water to evenly distribute humidity, thus avoiding excessive localized humidity.

In short, air movement can effectively regulate temperature and humidity, prevent the formation of microclimates around cannabis leaves, and reduce the chance of pest and disease infestation.

The VPD for seedlings is influenced by a variety of growth needs, including light, watering, and air circulation, which together shape the environment in which the plant grows. Growers need to understand how these needs affect VPD in order to adjust environmental conditions to ensure seedlings can grow at their best.

Optimal Cannabis VPD for Seedlings

Cannabis' growth needs at the seed stage directly affect temperature and humidity, which indirectly leads to changes in VPD. We know that abnormal fluctuations in VPD can jeopardize plant health. So how can you tell if cannabis is affected by VPD fluctuations? And what is the ideal VPD range for the seed stage of cannabis?

Identifying Signs of Incorrect VPD

Monitoring a seedling's VPD can help determine if the growing environment is within the optimal range. Deviations from this range can manifest themselves in a variety of ways that affect seedling health and growth. By recognizing these signs early, adjustments can be made in time to maintain a good growing environment.

High VPD for Seedlings

High VPD means that the air is dry, causing rapid water loss in seedlings, which may result in wilting and leaf curling. Leaves may be dry and brittle, growth may slow, and nutrient uptake may be impeded, as evidenced by leaf yellowing or other nutrient deficiencies.

Low VPD for Seedlings

Low VPD means that the air is humid, which may result in excess moisture in seedlings. High humidity tends to breed mold and mildew, which can damage plants. Slowed transpiration may affect nutrient uptake, resulting in poor plant growth.

Correctly identifying and adjusting VPD at the right time ensures that seedlings thrive in the right environment.

Optimal VPD for Seedlings

Keep the VPD of seedlings within the optimal VPD value range of 0.8 to 1.0 kPa. To balance the optimal VPD values for seedlings, growers need to control the ambient temperature at 20-25°C and humidity between 65-70%. Good air movement and proper ventilation will help maintain a stable VPD and ensure healthy seedling growth.

By recognizing the signs of incorrect VPD and keeping seed stage VPD within the ideal range, growers can create an environment that supports thriving and prepares cannabis seedlings for successful maturation.

How to Adjust Cannabis VPD for Seedlings by Traditional Climate Control Methods

Cannabis indoor growers can achieve optimal VPD values at the seed stage by adjusting temperature and humidity. Next, we will learn about traditional temperature and humidity control schemes and their shortcomings.


Using Air Conditioners and Dehumidifiers to Control Temperature and Humidity

Traditional climate control methods, such as the use of air conditioners and dehumidifiers, are effective tools for regulating the temperature and humidity of the growing environment.

Most small to medium sized marijuana indoor growers will opt for a climate control solution that combines air conditioning and dehumidifiers. They will use an air conditioner to regulate the temperature of the grow room and a dehumidifier to reduce the humidity. If the grow room is too dry, the grower will also have a humidifier to increase the humidity of the growing environment.

Drawbacks of Traditional Methods

While using air conditioners and dehumidifiers can help manage temperature and humidity, they come with their own set of challenges. One of the main problems is high energy consumption and high costs.

High Energy Consumption and Costs

The cost of running air conditioners and dehumidifiers as energy-intensive equipment poses a challenge for growers seeking economies of scale. The two devices are controlled by separate systems that may interfere with each other when operating independently.

For example, heat generated by plant lights and other equipment can raise the temperature in the room. The grower will activate the air conditioner to cool the room, but the air conditioner will lower the air humidity while lowering the temperature, causing the actual humidity to fall below the target. At this point, the grower will need to use a humidifier to raise the humidity to reach the target value. This conflicting effect reduces cooling and dehumidification efficiency and increases energy consumption and operating costs.

Inconsistent Control Issues

Air conditioners and dehumidifiers may not always maintain precise temperature and humidity levels, leading to fluctuations that can affect the VPD of marijuana seedlings.These fluctuations create an unstable environment that can stress seedlings and negatively impact their growth and development.

When an air conditioner reaches its target temperature and stops cooling, the dehumidification effect that follows stops. This cessation results in a gradual rise in humidity as active dehumidification is no longer taking place. Similarly, when a dehumidifier works to reduce humidity, additional heat is generated, which can cause the air conditioner to kick in more frequently to re-cool the space.

While traditional climate control options are effective in regulating VPD at the seed stage of cannabis, growers need to be aware of the limitations of these methods. And consider possible strategies to optimize energy use and improve control accuracy to ensure seedlings can grow in the best possible environment.

Getting Optimal Cannabis VPD for Seedlings with Altaqua's Grow Room HVAC System

Altaqua's Grow Room HVAC System provides an advanced solution for maintaining the precise environmental conditions required for cannabis seedlings compared to traditional solutions. The system incorporates several key features that address the challenges of traditional methods.


Precise Temperature Control

Fluctuating temperatures can cause significant stress to cannabis seedlings, often resulting in yellowing of the leaves, which Altaqua's Grow Room HVAC System effectively addresses through a precise control mechanism.

With a deadband setting of just 0.5°C, the system ensures that the temperature remains stable within a very narrow range. For example, if the target temperature is set at 25°C, the system will initiate heating if the temperature falls below 24.5°C and cooling if the temperature exceeds 25.5°C. This precision in maintaining a constant temperature prevents heat stress on cannabis seedlings, thus supporting stable VPD levels.

Dew Point Control

Traditional relative humidity control methods are subject to temperature fluctuations and can be misleading. Instead, Altaqua's Grow Room HVAC Systems focus on managing dew point temperatures.

By controlling the dew point, the system can consistently measure the moisture content of the air regardless of temperature changes. This accuracy ensures that humidity levels remain consistent, preventing conditions that can lead to plant stress and susceptibility to diseases such as mold. Stable humidity levels support a balanced VPD, which improves effective transpiration and nutrient uptake by seedlings.

Energy Saving

Altaqua's Grow Room HVAC System integrates air conditioning and dehumidification in a single unit, providing significant energy savings. Conventional equipment that uses separate air conditioning and dehumidifiers often faces efficiency problems. This is because these units interact with each other, such as the air conditioner cooling the air and the dehumidifier generating heat. This conflict increases energy consumption and reduces overall efficiency.

In contrast, Altaqua's unitary system ensures that the cooling and dehumidification functions work in harmony, eliminating the inefficiencies associated with separate systems. In the long run, this integrated approach improves efficiency and reduces energy costs, making it a more sustainable and cost-effective solution for growers.

Altaqua's Grow Room HVAC System provides an efficient solution for maintaining optimal VPD for seedlings. With precise temperature and dew point control, the system overcomes the limitations of traditional climate control by realizing significant energy savings and reducing installation costs. It creates a stable and efficient growing environment that encourages cannabis seedlings to thrive, providing a solid foundation for successful cultivation.


Maintaining optimal VPD for cannabis seedlings requires effective management of light, air circulation, watering and nutrient requirements. These factors affect temperature and humidity, which have a direct impact on VPD.By understanding and controlling these factors, growers can create a stable environment that promotes healthy seedling growth and sets the stage for successful plant development.


1. What is the ideal VPD for cannabis seedlings?

The ideal VPD (Vapor Pressure Deficit) for cannabis seedlings typically ranges between 0.8 to 1.0 kPa. Maintaining this range ensures balanced transpiration and nutrient uptake.

2. How does light intensity affect VPD for cannabis seedlings?

High-intensity light increases the temperature in the grow environment, which can raise the VPD if relative humidity remains constant. This increase in VPD enhances transpiration rates, requiring seedlings to absorb and transport more water to maintain balance, thus preventing stress.

3. How does overwatering affect VPD for cannabis seedlings?

Overwatering increases soil moisture, leading to higher humidity levels as excess water evaporates. This rise in humidity lowers the VPD, reducing transpiration rates. Lower transpiration can slow down seedling growth and increase the risk of mold and mildew.

4. Why is air circulation important for maintaining VPD for seedlings?

Air circulation helps distribute heat and humidity evenly throughout the grow space, preventing hotspots and areas of high humidity. Proper airflow ensures a stable VPD, supporting efficient transpiration and reducing the risk of diseases caused by excessive moisture around the plants.

5. How do traditional climate control methods affect VPD for cannabis seedlings?

Traditional methods using separate air conditioners and dehumidifiers can lead to high energy consumption and inconsistent control. These systems may work against each other, causing fluctuations in temperature and humidity that disrupt the optimal VPD, stressing the seedlings and affecting their growth.

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