Light intensity is crucial if healthy plants and bountiful harvests are what you’re after, especially with indoor grows. All the terms, acronyms, and power metrics can be somewhat confusing, though. Understanding PAR for cannabis plants allows you to choose and set up your grow environment lights, ensuring your plants have all they need to thrive.
Let’s take a deeper look at PAR and how it works.
What is Photosynthetically Active Radiation (PAR)?
To provide the best PAR for cannabis, you need to understand what it is. Photosynthetically active radiation, or PAR for short, is part of the visible light spectrum that allows photosynthesis in plants.
This part of the electromagnetic spectrum occurs between 400 to 700 nanometers (nm) wavelengths. At the bottom of this range, light is colored blue; below 400 nm, you find the ultraviolet region. Above 700 nm, the electromagnetic waves are called infrared, and the colors close to this range are deep reds.
Your cannabis light spectrum needs to include both blue and red ranges of light for healthy, happy plants to grow. Keep in mind the different growth phases, as the PAR for flowering differs from what’s needed in the vegetative or seedling stage. Luckily, there are many ways that growers can measure the amount of PAR their lights are emitting.
Two related phrases that cannabis growers need to know are Photosynthetic Photon Flux Density (PPFD) and Micromoles (μmols). PPFD is measured in units called micromoles (μmols) and is the strength or amount of light photons that reach the surface of your plants. Light loses strength the further it has to travel, so pay attention to light intensity requirements for your crops.
What is the best PAR for cannabis?
Getting the light cycle for weed just right can be tricky, and determining the best PAR for veg, flowering, or seedlings is somewhat complex. There are many influencing factors, but in short, different growth stages have different PAR requirements.
When you add up PPFD numbers for one day, you get the Daily Light Integral (DLI), measured in moles. Crop-producing plants like fruit and vegetables need between 20 and 40 moles of light daily. The same is true for cannabis plants, 20 moles per day is the bare minimum that your plants need. To maximize yields, most growers aim for 40 moles daily.
How much PAR for seedlings
The PAR for cannabis seedlings is relatively low in the first three weeks of life. Seedlings require only 100–300 PPFD per second in an 18 hour light period.
The best PAR for veg
During the vegetative stage of development, plants need a minimum of 20 moles of light for 18 hours straight. So the PAR for veg is generally between 300–600 PPFD.
How much PAR for flowering
To activate the flowering stage, weed plants need to receive 12 hours of light and 12 hours of darkness. During these 12 hours of light, the flowering plants will need light with a minimum of 460 PPFD.
The PAR values can be increased to between 620 and 1000 nm for better yields. The flowering stage of cannabis is photosensitive, so knowing how much PAR for flowering is vital.
How much PAR do you get for autoflowers?
Autoflowers don’t require any adjustments in their light cycles right throughout their development. Unlike photoperiod cannabis that requires a change from 18to 12 hours of light for flowering to begin, autoflowers need 18 hours from seedling to flowering.
As the daily light cycle for autoflowers is longer than that of photoperiod plants, the light doesn’t need to be as intense. The recommended PAR for cannabis autoflowers is between 300 and 620 PPFD and can remain the same for the whole life cycle.
What are the other factors that can influence the PAR?
Plant metabolism and photosynthesis are more complex than you would think. PAR for cannabis can be influenced by many factors, some above soil and some below. You’re about to discover some that are most influential on light levels.
Carbon Dioxide (CO2) levels
CO2 is the main influencer when considering PAR for cannabis levels because photosynthesis mainly needs photons (light) and CO2. One of the primary limiting factors to the rate of photosynthesis is inadequate CO2 levels.
When growing indoors, CO2 levels are generally between 400–600 ppm; with too low levels of CO2, photosynthetic activity can quickly become restricted and growth stunted. Increasing CO2 for cannabis plants to at least 800 ppm greatly boosts photosynthetic activity and allows for higher PAR levels.
Temperature and humidity
Finding and maintaining the optimal temperature for cannabis plants increases growth and yields. Optimal temperature and CO2 concentration go hand in hand, though the higher the CO2 levels, the higher the optimal temperature. Too high temperatures can decrease or halt plant growth even with increased CO2.
Cannabis humidity is another factor that influences plant growth and PAR. The optimal humidity percentage is dependent on the temperature. Generally speaking, higher temperatures and higher humidity increase growth, but like all these factors, it also depends on the growth stage.
The best PAR for cannabis can be achieved when you keep the temperature and humidity within the recommended values. If you factor in climate you can increase PAR levels to the top end of the recommended PPFD values.
Cannabis plants naturally exist in several diverse climates, from rocky mountains to tropical forests—each with its own unique light levels. Genetics vary greatly, making the question of how much PAR for seedlings, veg, and flowering all the more tricky.
Most seed manufacturers should provide you with information on the light needs of your specific weed strain. This info should give you a sense of whether you need to use the lower or higher end of the following recommendations.
|Growth phase||PAR Level (PPFD)||Light cycle duration||CO2 Level (ppm)||Temperature||Humidity level|
|Seedlings/clones||100–300 μMols||18 hr daily||400||Day 77°F Night 69.8°F||65%–80%|
|Vegatative||300–600 μMols||18 hr daily||400–800||Day 69.8 – 82.4°F Night 64.4 – 75.2°F||40%–70%|
|Flowering/Blooming||460–1000 μMols||12 hr daily||800–1400||Early 68 – 78.8°FLate 64.4 – 75.2°F||Early 40%–55%Late 30%–40%|
|Autoflowering (continuous)||300–600 μMols||18 hr daily||400–800||Dependant on growth stage||Dependant on growth stage|
Signs of too much light
With so many factors to consider, it’s easy to overdo it and end up with too much light. Some of the signs to look out for are:
Leaf burn is one of the most common signs that the level of PAR for cannabis is too high. Plants with cannabis light burn or leaf burn tend to have yellow leaves with green veins and stems. Nitrogen deficiency can appear similar as the leaves also turn yellow.
A good indicator is the affected area and how strongly the leaves are attached to the stem. Nitrogen-deficient leaves fall off, whereas light-burned leaves are hard to pluck off. Light burn mostly affects the leaves at the top of the plant, while nitrogen deficiency can appear near the bottom of the plant.
Loose and airy buds
When plants receive excessive light, their metabolisms have to work harder, leading to nutrient deficiencies. Cannabis buds receiving excess light tend to be loose, weak, and airy, often feeling crisp to the touch, encouraging you to ensure you achieve the best PAR for flowering.
Light-bleached “white” buds
When buds receive too much light, they can lose some or all of their color pigments, turning white. Bud bleaching often results in lower potency and might even cause the nugs to lose their cannabinoids. Monitor the PAR for flowering and be careful not to mistake bleached buds with very resinous, trichome-covered ones.
Signs of too little light
As with too much light, there are certain negative effects on plants that receive too little light.
An obvious effect of too little light is slower than expected growth. For the process of photosynthesis, light is a key ingredient, and a lack thereof means that plants can’t grow as they should.
The PAR value for seedlings differs from that of adult crops. If you suspect slow growth,see how your plant’s height matches that of others of the same strain and age.
Plants that aren’t receiving enough light tend to put more growth into their stems as they try to move closer to the light. Visually these crops look stretched or appear taller, with internodes being spaced further apart than normal. Be especially careful to watch your PAR for veg stages.
Light matters: the best PAR for your plants
As you’ve seen, the right light intensity matters, making it an important factor to consider when setting up and running your grow space. Having the correct equipment and maintaining the PAR for cannabis seedlings to adults goes a long way to simplifying your life and achieving that perfect bountiful yield.
Aim for 20 moles of light per light session as the bare minimum, but if you’re an overachiever, aim for 40 moles of light daily. That said, be the ever-watchful grower and regularly adjust to the growth stages of your plants, changing CO2 levels, temperature, and humidity along the way as needed.
Whether you’re just starting your cultivation journey or are an avid grower, why not sign up to create your own grow diary! All you have to do is buy seeds from a huge selection in our store and sign up to start. Record your grow process weekly, like how much PAR for flowering you’re using, and get tips along the way to that bumper yield.
About the author: Derek LaRose
Also known as Kronic from The Cannabis Kronicles, Derek LaRose is a young ambitious cultivator and a staple educator for indoor cultivation.