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What’s the best PPFD for seedlings? The cannabis edition

Lucy Starley
January 31, 2024
12 min read
A healthy cannabis seedling under the correct PPFD.
A healthy cannabis seedling under the correct PPFD.
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Getting the lighting conditions right for cannabis seedlings is crucial. PPFD (Photosynthetic Photon Flux Density) plays a significant role in this early stage of growth. But what's the best PPFD for cannabis seedlings? 

Today, we’ll cover the essentials of PPFD. We’ll be shedding light on how it influences the development of your young cannabis plants. 

Whether you're a first-time grower or have been cultivating cannabis for years, understanding the correct PPFD levels can make a massive difference in the health and yield of your plants.

Let's explore the optimal PPFD for cannabis seedlings, ensuring you're equipped to give your plants the best start.

Disclaimer: Any information given on this site is for educational purposes only. Please ensure if you’re growing cannabis, you’re doing so in accordance with the law and subject to appropriate permissions and licenses of the applicable country.

A tray of cannabis seedling that need more PPFD.

Understanding PPFD

What is PPFD?

PPFD stands for Photosynthetic Photon Flux Density. It measures the amount of light, in photons, that reaches your plant's surface every second. 

This measurement is crucial. It directly impacts your plants’ ability to photosynthesise and convert light into energy to grow.

But why is PPFD so important for your seedlings? 

In the early stages of growth, seedlings are particularly sensitive to their environment. Too little light, and they won't have the energy they need to develop strong roots and leaves. 

Too much light, however, can overwhelm them, leading to stress or even damage. 

There’s another factor at play, too - if your lights are too far away, your plants will stretch. This can lead to leggy, weak and unstable plants that struggle to recover to healthy vegetative growth. Longer term lower light levels will also have a negative effect on yield quality and quantity.

Finding that PPFD sweet spot is key to giving your seedlings the best and fastest possible start in life.


An infographic showing that your cannabis seeds will stretch if the PPFD is not high enough.

The right PPFD levels for each stage of growth

Understanding the different growth stages is crucial for optimising conditions like PPFD. These stages include the seedling, vegetative, and flowering phases, each with its own unique light requirements.

During the seedling stage, cannabis plants are just beginning their journey. They have sprouted from the seed and are developing their first true leaves. The seedlings are not yet ready to absorb high light levels at this delicate phase. 

The right PPFD at this stage ensures that plants develop strong roots and healthy foliage. Setting a solid foundation for the rest of their growth cycle.

As the plants transition to the vegetative stage, they require more intense light to support their rapid growth and leaf development. This stage prepares the plants for flowering, where light requirements will shift again to promote bud formation.


An infographic explaining the correct PPFD for cannabis at different stages of it's life cycle.

The ideal PPFD for seedlings

For cannabis seedlings, the optimal PPFD levels are generally between 100 to 300 μmol/m²/s. This range provides enough light energy to support the initial growth without overwhelming the young plants. 

At levels below 100 μmol/m²/s, seedlings may not receive enough light, leading to slow growth and weak development. 

On the other hand, exceeding 300 μmol/m²/s can cause light stress, which might stunt growth or even damage the delicate seedlings.

Seedlings rely on the energy stored in the seed until their leaves can perform photosynthesis. They require a gentle environment with moderate PPFD levels (100-300 μmol/m²/s). Avoiding stress and encouraging strong root and foliage development. 

Striking the right balance in PPFD during this stage is key. Especially when striving for vigorous, healthy seedlings that are well-prepared for the vegetative stage.

Seedlings benefit from a stable environment with consistent light, humidity, and temperature. Often provided under a light schedule of 18 hours on (or more) and 6 hours off​​​​. 

Adjusting your grow lights to maintain PPFD within this range can significantly impact your seedlings' health. 

The right PPFD encourages a robust root system, strong stem growth, and the development of the first true leaves. Which are critical for the plant's overall vitality.


A man holding a healthy clone.

The ideal PPFD for clones

On the other hand, clones are cuttings taken from a mature plant, meaning they start with a more developed structure, including pre-formed leaves. 

Clones skip the germination stage but need to develop roots from the cutting site, which is a critical phase. 

Clones generally require a similar environment and feeding as the mother plant to root quickly. But it’s a safe bet to reduce the PPFD (to around 100 μmol/m²/s) to reduce stress, especially in the first 3 to 5 days.  

The light intensity for clones should be on the lower end to prevent dehydration and stress. Clones initially lack a developed root system to uptake water efficiently​​​​.

Both seedlings and clones thrive in high-humidity environments, which helps prevent water loss through transpiration. However, clones might need higher humidity levels and a lower VPD environment initially because they lack roots. 

Adjusting the light intensity and ensuring proper humidity and temperature are key to the successful growth of both seedlings and clones. 

The ideal PPFD for the vegetative stage

As plants grow and require more energy for leaf and stem development, a higher PPFD range of 300-600 μmol/m²/s is suitable. This ensures robust growth and prepares plants for the flowering phase​​​​.

The ideal PPFD for the flowering stage

During flowering, cannabis plants can benefit from even higher PPFD levels. 

Ranging from 600-900 μmol/m²/s and potentially up to 1000 μmol/m²/s under ideal conditions (with adequate nutrients, CO2, temperature, and humidity). 

For beginners, aiming for 800 μmol/m²/s is a good starting point​​​​.

Monitor plants for signs of light stress, such as leaf burn or discolouration, and adjust PPFD levels accordingly. 

Additionally, maintaining the appropriate distance of grow lights from the plant canopy is vital. Ensuring even light distribution and preventing hotspots that could harm the plants.


Some tomato seedlings under a grow light.

How does the height of grow lights affect PPFD?

The height of grow lights plays a significant role in determining the PPFD that reaches your cannabis plants. 

When grow lights are placed closer to the plants, the PPFD increases, meaning more light photons are available for photosynthesis. 

However, too close can lead to light burn, where the intense light causes damage to plant tissues, particularly the leaves.

Conversely, raising the height of the grow lights reduces PPFD as light spreads out and becomes less intense over a larger area. 

However, if the lights are too high, the light your plants receive may not be sufficient. Leading to stretching as they reach towards the light source, resulting in weak and elongated stems.

Therefore, balancing the height of your grow lights is a delicate task. It requires regular adjustments based on the growth stage of your plants and their response to the light intensity. 

How to control stretching in cannabis seedlings

You’ll often see advice suggesting keeping lights at a lower PPFD by increasing the height. Especially for clones and seedlings.

You may also read that vegetative and flowering stages benefit from less distance between the lights and your plants (and higher PPFD levels). 

Every strain or pheno is different, though. With some, you’ll be better off hanging your LED light close at the beginning to prevent stretching.

This is an option if you’ve got adjustable lights. You can reduce the PPF and keep the light close. Just make sure to measure the PPF/PPFD using a sensor like ours and adjust accordingly. 

If you’ve not got the option to turn the brightness of your lights up and down, keeping them on full power and hanging them further away can work. But this comes with a risk of stretching during the seedling stage.

Regularly measuring the PPFD at the canopy level with a PAR meter can also help. Enabling you to make precise adjustments to ensure optimal light conditions across all stages of growth.

If you hang your grow lights closer during the seedling stage, you must be careful to prevent high leaf surface temperatures (LST). 

You’ll also need to monitor your humidity when there’s less distance between your plants and your lights. Particularly as seedlings require higher humidities.

Ensure you pay super close attention; as plants grow, you’ll need to move light higher to prevent stress.

For practical applications, starting with manufacturer recommendations for light height is advisable. Then, adjust based on your plants' response, observing for signs of light stress or inadequate light exposure. 


A collection of cannabis seedlings that have stretched

Measuring PPFD in your grow space

To ensure your cannabis seedlings receive the optimal PPFD, measuring the light intensity accurately is essential. 

This is where a PAR (Photosynthetically Active Radiation) meter comes into play. You need a  PAR meter to accurately measure the light within the spectrum that plants can use for photosynthesis. Giving you a clear picture of the PPFD your plants are exposed to.

Are PPFD and PAR the same?

PPFD and PAR are related concepts in plant lighting, but they are not the same.

  • PAR (Photosynthetically Active Radiation) refers to the range of light wavelengths that plants can use for photosynthesis, typically spanning from 400 to 700 nanometers. It encompasses the light that plants need for growth and development.
  • PPFD (Photosynthetic Photon Flux Density) measures the amount of PAR that actually reaches the plant surface per second, measured in micromoles of photons per square meter per second (μmol/m²/s). It quantifies the intensity of PAR available to plants for photosynthesis at a specific location and time.

In essence, while PAR describes the type of light useful for plants, PPFD measures how much of that light actually reaches the plant. Making both crucial parameters if you’re looking to optimise plant growth conditions.

How to use PAR and PPFD meters to optimise your grow space

Take measurements at canopy level

You’ll need a light meter to take measurements. This is why we’ve built a high quality top-mounted PPF meter and PPFD sensor into our state-of-the-art grow room monitor



An infographic explaining that the grow sensor should be hung at the seedling canopy level when growing cannabis.

Position the sensor at the same height as the top of your seedlings. This gives you the most accurate representation of the light they're receiving. Our PAR sensor measures light intensity across the full spectrum and is designed to stand next to a seedling or young plant.

The Grow Sensor ensures continuous data collection for many key growing factors. Allowing you to drill down into the data with super usable graphs.


An infographic explaining that the user needs to measure the PPF under the LED grow light.

Check for uniformity

Move the PAR meter across the canopy to check for light distribution. PPFD levels should be consistent across the entire grow area to ensure uniform growth.

Adjust your lights

Based on your readings, adjust the height or intensity of your lighting system to achieve the recommended PPFD range for seedlings. Remember, the goal is to hit that sweet spot of 100 to 300 μmol/m²/s.

Achieving uniform PPFD distribution is key. Uneven light can lead to inconsistent growth, with some plants thriving while others struggle. 

Regularly monitoring and adjusting your setup can significantly impact your seedlings' overall health, ensuring optimal growth.

Adjusting PPFD Levels

Once you've measured the PPFD in your grow space, the next step is fine-tuning your setup. You do this to maintain optimal light conditions for your cannabis seedlings. 

Here's how you can adjust PPFD levels effectively:

An infographic explaining that the user needs to adjust the light hight to get the correct PPFD to grow healthy cannabis seedlings.

Adjust light height

Moving your grow lights closer to or further from the plant canopy can increase or decrease PPFD. Lowering the lights increases PPFD while raising them decreases PPFD. However, be mindful of the heat produced by the lights to avoid overheating your plants.


An infographic explaining that the user can also dim the lights do get the correct PPFD for growing cannabis.

Change light intensity

Some grow lights come with adjustable intensity settings. Use these settings to dial in the perfect PPFD levels without altering the light's position.

Use reflective materials

Placing reflective materials around your grow space can help distribute light more evenly. This can increase PPFD in areas that might be receiving less light.

Implement light schedules

Cannabis seedlings benefit from a consistent light schedule, typically 18 hours of light followed by 6 hours of darkness. This schedule supports their growth while giving them time to rest and respire.

Regular monitoring and adjusting are key. As your seedlings grow, their PPFD requirements may change, and your setup should evolve accordingly. 

Keeping a close eye on plant health and growth patterns will guide your adjustments. The aim of the game is to ensure your seedlings receive just the right amount of light, always.

Case studies

Scientific studies support the importance of PPFD in influencing plant health and yield, particularly in cannabis cultivation. 

Research indicates that cannabis has a high capacity to convert PAR into biomass. This highlights the need for growers to use the optimum light intensity to balance growing costs and productivity. 

This balance is crucial for maximising profits while minimising energy use.

A study aimed to help indoor cannabis producers determine the optimal light intensity to maximise profits while minimising energy use​​.

The study demonstrated that cannabis has a significant ability to convert light into biomass. This makes the choice of light intensity crucial for balancing production costs and yield. 

It explored how different PPFD levels, ranging from 120 to 1,800 μmol·m^−2·s^−1, affected cannabis growth, morphology, yield, and quality. 

Another study focused on the effects of light intensity and ultraviolet radiation on the yield and quality of cannabis. 

The research aimed to understand how UVA and UVB exposure and light intensity impact the quality and yield of indoor cannabis​​.

It found that increasing light intensity proportionally increased cannabis yield. However, UV radiation did not significantly affect yield or cannabinoid content. 

Common mistakes to avoid

When managing PPFD for seedlings, it's easy to fall into some common traps. Being aware of these can help you navigate the early stages of growth more smoothly:

Overexposure to light

It's tempting to give seedlings as much light as possible to boost growth. However, too much light can overwhelm young plants, leading to stress, leaf burn, or stunted growth. Stick to the recommended PPFD range of 100 to 300 μmol/m²/s.

Ignoring light uniformity

Focusing light too intensely in one area can lead to uneven growth. Some seedlings may thrive, while others remain underdeveloped. Ensure light is evenly distributed across the entire canopy.

Neglecting daily light integral (DLI)

While PPFD measures the amount of light delivered per second, DLI accounts for the total light received over a day. 

Even if your PPFD is in the optimal range, an incorrect light schedule can result in insufficient or excessive daily light.

Forgetting to adjust as plants grow

As seedlings develop, their light needs change. What works during the first few weeks might not be suitable as they get bigger. Regularly reassess and adjust your lighting setup to keep up with their growth.


An infographic explaining that the user needs to move the sensor up as their cannabis plants grow.

Poor environment control

PPFD is just one piece of the puzzle. Temperature, humidity, and CO2 levels also significantly affect plant health. Neglecting these factors can undermine even the most optimal lighting conditions.

Avoiding these mistakes can significantly enhance the health and vitality of your seedlings. Setting a solid foundation for the subsequent stages of growth.

The benefits of the Grow Sensor

The Grow sensor is a game-changer for growers looking to up their game. Get real-time insights into your grow room's environment. 

Ensure you're always aware of crucial factors like light intensity (PPFD), humidity, temperature, and CO2 levels. 

With this data, you can fine-tune your grow and deliver the exact amount of light your plants need at each stage of growth. You can optimise their health and boost your yields from seedlings to flowering. 

Plus, by closely monitoring humidity and temperature, you can create the perfect conditions for your plants to thrive. 

Grow Sensor equips you with the knowledge to control your grow environment precisely, leading to healthier plants and higher yields.

The Grow Sensor and app

Takeaways

Mastering PPFD levels for your seedlings is a crucial step towards cultivating a healthy and bountiful crop. 

Understanding and maintaining the optimal PPFD range ensures that your seedlings develop strong roots, sturdy stems, and lush leaves. Laying the foundation for a successful grow cycle. 

Remember, the goal is to provide your plants with the right amount of light. Not too much and not too little. All while keeping an eye on uniformity and adjusting as needed.

Avoid common pitfalls by monitoring your plants regularly. Adjust light intensity and height, and pay attention to the overall environment. 

By doing so, you'll be nurturing robust cannabis plants ready to flourish in the vegetative and flowering stages.

As you apply these insights, remember that every grow space is unique. What works best may require some experimentation and adjustment. 

Here's to your growing success!

FAQs

What is the proper PPFD for seedlings?

The ideal PPFD for cannabis seedlings is between 100 to 300 μmol/m²/s. This range provides enough light for healthy growth without causing stress or damage to the young plants.

What is the best light PAR for seedlings?

The best PAR (Photosynthetically Active Radiation) for seedlings is the light that contributes to the PPFD range of 100 to 300 μmol/m²/s. 

PAR refers to the spectrum of light that plants can use for photosynthesis, which is effectively measured by PPFD.

Can PPFD be too high?

Yes, PPFD can be too high for seedlings. Exceeding 300 μmol/m²/s can lead to light stress, causing issues like leaf burn, stunted growth, or other forms of damage to seedlings.

Can light be too strong for seedlings?

Absolutely. If the light intensity is too high, it can overwhelm seedlings, leading to stress and potential damage. It's crucial to maintain the recommended PPFD levels.

Are LED lights too strong for seedlings?

LED lights can be too strong for seedlings if not used properly. However, many LED grow lights are adjustable, allowing you to control the intensity to suit the needs of seedlings.

How do I know if my seedlings have too much light?

Signs that your seedlings are getting too much light include yellowing or browning of the leaves, leaf curling, or a general appearance of being burnt or stressed.

How long should LED lights be on for seedlings?

A typical light schedule for cannabis seedlings under LEDs is 18 hours on and 6 hours off. This provides ample light for growth while allowing a rest period that mimics natural light cycles.

Should I dim my LED grow lights for seedlings?

Yes, dimming your LED grow lights can be beneficial for seedlings if the lights are too intense. Adjusting the light to fall within the 100 to 300 μmol/m²/s PPFD range is ideal.

How high should LED grow lights be above seedlings?

The height can vary depending on the light's intensity, but a general rule is to start with LED lights about 60-75 cm (24-30 inches) above the seedlings.

Adjust based on the plant's response and the PPFD measurements.

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Lucy Starley
January 31, 2024
12 min read

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