How to Optimize Your PAR For Plants Without Burning Them

grow room

Are your cannabis plants struggling to flourish? It could be that they’re not getting the right amount of light.

When it comes to growing cannabis indoors, understanding how much PAR for plants you need and properly adjusting these levels can make all the difference in achieving healthy and fruitful harvests.

In this blog post, we’ll discuss the importance of Par levels for indoor gardening and provide some helpful tips on how to adjust your grow lights without risking damage or stress to your crops!

So if you’re an indoor gardener looking for ways to optimize your yields while keeping your crops happy and healthy – keep reading!

What is PAR?

Photosynthetically Active Radiation (PAR) is the range of light wavelengths within 400 to 700 nanometers that plants use as fuel for their photosynthesis processes.

Photosynthesis is essential for plants to produce oxygen and construct their food source through sugars.

To complete this cycle, they must absorb CO2 and water while simultaneously taking in photons from a light source.

Typically, indoor cannabis farmers measure the amount of photosynthetically active radiation using a PAR meter.

Photosynthetic Photon Flux Density(PPFD) allows us to accurately measure the amount of PAR (Photosynthetically Active Radiation) and how much light plants can absorb.

light wavelength chart for par for plants
Image sourced from Ceres Cann

What is PPFD(Photosynthetic Photon Flux Density)?

Photosynthetic Photon Flux Density (PPFD) quantifies the amount of photosynthetically active photons that drop down on a given surface area in one second.

PPFD is measured in units of micromoles per square meter per second (μmol/m²/s) and accurately represents the amount of light energy available for photosynthesis to occur in plants.

For growers, PPFD is the ultimate indicator of successful growth—it clearly shows how much light your plants are receiving from your grow light at any given distance.

It is crucial to consider PPFD when designing lighting systems for indoor plant growth, as different plants require different levels of PPFD for optimal growth and yield.

Additionally, measuring PPFD can help us optimize our grow lights’ efficiency and ensure our plants receive adequate light intensity for healthy growth.

Have you ever noticed other acronyms related to light measurements, such as PPF(photosynthetic photon flux) and PFD(photon flux density)?

If so, this video is an excellent resource for deepening your understanding of these topics.

Things You Need to Know for Adjusting PAR Levels for Healthy Plant Growth

It can be daunting to decide how much light intensity you need for flowering cannabis, but knowing your plants’ growth phase can give you a better understanding.

The light intensity required for each stage can differ based on the light source you are using and their heat loads, such as LED grow lights vs HPS light fixtures.

Each strain (and even each phenotype) may have its particular preference for preferred lighting conditions, so experimentation can be beneficial in determining what works best with your plant lighting system.

Some things you need to know:

  • Cannabis can thrive under a wide range of light levels, so it’s not necessary to constantly measure and adjust your grow lights’ intensity.

  • It’s worth noting that increasing light intensity only sometimes leads to higher yields, as there is a saturation point beyond which the law of diminishing returns applies.

  • In other words, excessive light can become a growth limiter instead of an enhancer and cause unnecessary problems in your grow room.

I’d advise novice cultivators to exercise caution when maximizing their PPFD levels and gain some experience with their grow lights at moderate light intensities before pushing the boundaries with too much light.

Let’s not forget that we can always measure light output and amp it up if needed – there is no reason to risk damaging our precious cannabis plants.

ppfd chart

My Process To Adjust Photosynthetic Active Radiation Levels

First, you’ll need to pick up a PAR light meter. While I prefer the Apogee MQ-500, there are many different models to choose.

With a PAR light meter, you can correctly set the height of your grow light above your canopy, adjust the dimming of your light or determine if your lights are aging or need cleaning.

par meter
Image sourced from Amazon

As we progress, let’s review the light specifications your grow lights must meet to achieve optimal results for each growth stage.

  1. Seedling/ Clone Growth Stage

  2. Vegetative Growth Stage

  3. Flowering/ Generative Growth Stage

Once you’ve identified the ideal light intensity range for each growth stage, measure the PPFD levels in your grow room at least once every week.

If necessary, adjust the distance between your grow lights and plants, or consider changing their hanging height if it’s too low or too high.

Doing so will help you reach your desired PPFD levels without increasing or decreasing the lights’ intensity.

Finally, ensure not to exceed the maximum recommended PPFD level for each genetic, as this may cause light burn or other issues arising from overexposure.

If this does occur, keep a close eye on your plants so that you can react quickly and return them to a healthy, balanced light environment.

damaged cannabis plant

Seedling Growth Stage

  1. Properly measuring light with your PAR meter requires holding it at the plant level for the most precise reading.

  2. You’ll want to walk around and take several measurements, typically in at least four spots around the grow room, to account for any differences based on location.

For optimal cannabis growth, PPFD levels during the seedling phase should land within the range of 80-150μmol for the first week or so, then slowly increase to the 200-300μmol range.

It’s essential to monitor your seedlings during this stage closely.

Whether it be Metal Halide, T5, or LED grow lights, this technique has proven to work exceptionally well with all of them.

Ultimately the choice boils down to your preference, as I’ve had great success in my cultivation endeavors with each type.

Cannabis seedlings require a minimal amount of light to thrive. The fragile tissues of tiny seedlings can suffer from overexposure to light and become overwhelmed.

When the light intensity is low, seedlings can gradually grow until they become resilient enough to withstand higher illumination levels.

During this stage, seedlings or clones require a minimum of 18 hours of light per day.

Maintaining the appropriate day length during this stage is crucial to ensure healthy growth and development before transitioning into the vegetative stage.

cannabis seedlings

Vegetative Growth Stage

To reiterate, complete steps one and two from the Seedling stage.

Cannabis growers typically administer light at PPFD levels of 250-500μmol during the vegetative stage.

To maximize the outcome of my harvest, I strive to gradually increase the light intensity from 300μmol up to 600μmol over a period of 3 weeks as my cannabis plants arrive at their flowering stage.

Week 1300-400μmol
Week 2400-500μmol
Week 3500-600μmol

To preserve its vegetative state and continue to generate leaves, branches, and roots, a plant requires at least 18 hours of light daily during this stage.

Suppose they receive less than 18 hours of light. In that case, they may begin to transition into the flowering stage prematurely, which can significantly reduce yield.

At this stage we don’t want to see any buds, only leaf and stem growth.

cannabis plant

Flowering/ Generative Growth Stage

Once again, complete steps one and two from the Seedling stage.

When cultivating the cannabis plant, growers often strive for maximum PPFD levels of 1000μmol or higher in the Flowering Growth Stage.

However, experienced cultivators know that different strains can handle varying PPFD levels before exhibiting signs of stress.

Some of my best-yielding strains have only been able to withstand a PPFD level of 600-700μmol when grown under HPS grow lights, while others I’ve grown under LED grow light systems have reached up to 1200μmol.

Hence, there is no universal approach to determining the ideal PPFD levels for cannabis cultivation in the flowering phase.

Here’s an example of an LED PPFD schedule:

Week 1600-650μmol
Week 2650-750μmol
Week 3750-850μmol
Week 4750-900μmol
Week 5850-950μmol
Week 6950-1200μmol
Week 7900-1100μmol
Week 8900-1000μmol
Week 9850-950μmol

The best rule of thumb is to take it slow. Never increase your PPFD by more than 100-200μmol per week to give your plants enough time to adjust and avoid potential damage.

If you’re using LED lights, It can be beneficial to dial back the intensity towards the end of flowering to 950μmol or lower.

Leaving it too high for too long can cause over-stressing and negatively affect total yield, but this depends on your genetics.

In this growth stage, cannabis requires no more than 12 hours of light each day.

Any more than this could trigger them to revert to a vegetative stage. I’ve always advocated for 11.5-hour days in my grow rooms just to be safe.

par for plants

Top Considerations For Successfully Adjusting PAR For Plants

As you may have guessed by now, there is a lot of nuance to adjusting your par levels.

It’s a delicate balancing act to get the most out of your plants while not causing them damage in the process.

In the seedling and vegetative stages, it’s pretty straightforward, but in the flowering stage, you can get more experimental.

Always be careful not to add too much light too quickly; you should be alright.

It’s important to remember that once the plants have sustained damage, your yield will be forever compromised – there is no reversing it.

So, do what you can to finish up and move on to the next harvest!

light burned plant
Damage from light burn/Image source Grow Weed Easy

How To Go To The Next Level: CO2

According to some research, cannabis plants of significant size can tolerate peak light levels up to 1500µmol, as long as adequate additional CO2 is continuously supplied and other environment variables like temperature and humidity are at their ideal ranges.

This combination promotes more robust growth and larger yields than increasing light intensity without additional CO2.

By pushing my CO2 levels to a staggering 1,900 parts per million with specific cultivars, I could see remarkable results in the size of both the calyxes and yield.

Still, I’ve never achieved 1500µmol without scorching my plants.

So, you’re serious about maximizing your yields. In that case, it’s worth researching CO2 supplementation and seeing what works for your particular strain.

CO2 infographic

Frequently Asked Questions

What is Daily Light Integral(DLI)?

PPFD(Photosynthetic Photon Flux Density) is a calculation of how much light hits an area within one second (μmol/m²/s).

It can then be multiplied by 86,400 – the number of seconds per day – to obtain the Daily Light Integral (DLI), generally calculated in Moles. A Mole equals one million micro-Moles for reference.

For successful growth of fruit or vegetable-bearing crops, including cannabis, 20-40 moles of light is required daily.

The minimum amount of light should be a mere 20 moles per day, as mentioned above, yet to obtain the most significant yield; many cannabis growers shoot for 40 moles – which is equivalent to an astounding 40 million μmol daily.

To summarize, the daily light integral is the total of PPFD over an entire day. This explains why plants must be exposed to natural light or grow lights for a certain amount each day (photoperiod).

Can PPFD be too high?

Absolutely! Exceeding the maximum Photosynthetic Photon Flux Density for your cannabis crop can be detrimental, resulting in leaf burn and an overall decline in your plant’s health.

To avoid this, adhere to the recommended range of light measures for the best growth and development.

Does PPFD change with distance?

Yes. Moving the light closer or farther away from your plant canopy, the PPFD/PAR will change accordingly.

You can see this as you move your PAR meter closer to the light source.

More distance will result in lower readings, while more proximity equals higher readings.

Can I adjust PPFD during flowering?

Yes, you’ll need to adjust PPFD levels during the flowering phase.

Still, you should do so slowly and cautiously, as most plants respond sensitively to light changes.

Sudden increases or decreases of PPFD during the flowering phase can harm plant health and overall yield. 

When should I switch to a 12-12 light cycle?

On average, plants tend to reach twice their height when flowering occurs; in some instances of genetic variance, they may even triple their size.

Therefore, if the height of the space you are working with is limited, it’s best to switch your plant to 12/12 lighting when it reaches half its available height.

Remember that a minimum distance between your grow light and foliage should also be considered for the best growth!

My experience has taught me that the most abundant harvests come from plants standing between 3 1/2 and 4 feet.

Anything taller, unfortunately, will start to diminish in quality due to a lack of light penetration deep into the canopy.

When should I increase light intensity in flowering?

At the start of flowering, slowly raise your PPFD each week to optimize growth.

Weeks 3-6 are the most crucial, as this is when buds will start to form.

During these weeks, you should provide them with maximum light intensity and ensure optimal maturity development for a successful harvest.

Be sure to observe your plants closely and adjust your grow lights accordingly.

Too much PPFD can result in leaf burn and other damage, while too little won’t be enough to reach the desired yield.

It’s all about striking the right balance!

cannabis plant

Bottom Line: My Experience With Adjusting PAR Levels

As you can see, there are many aspects to consider when adjusting PPFD/PAR with your grow lights.

Every strain has unique requirements and preferences, so you can research beforehand to understand the best approach for your plant development.

From my personal experience, minor tweaks to PPFD levels have made the most significant difference in my yields.

Combining CO2 supplementation and changing PPFD throughout the flowering phase allowed me to maximize my growing space and achieve superior results.

As a grower, understanding this concept is vital to maximizing yields and producing high-quality cannabis.

It may take some trial and error at first, but you’ll eventually find the sweet spot for your own particular strains and reap the rewards of your efforts.

Now is the perfect time to benefit from a quality PAR light meter, so don’t hesitate to get yours and start measuring today!

Good luck and happy growing!

Jesh

Jesh is the founder of jeshuascheumack.com. He was born in Stockholm, Sweden, and raised in the American South and the foothills of Northern California. He loves to grow all things, including as much of his own food as possible. He also loves to read and geek out on board games.

Similar Posts