What's The Right PPFD For Weed Grow?

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When cannabis cultivators begin evaluating an LED grow light’s specs, they gravitate towards the PPFD count. But what is PPFD, and just how much do you need for your cannabis plants?

What’s PPFD?

First off, PPFD stands for “Photosynthetic Photon Flux Density,” and it’s measured in μmol/m2/s. This measurement is crucial for cultivators because it tells you how much light is available as “food” for photosynthesis. To get more technical, PPFD measures the available Photosynthetic Active Radiation (PAR) at different locations in your grow space.

Unlike other light intensity measures, PAR focuses on photons we can’t see with our physical eyes. Plants like cannabis absorb a wide range of light wavelengths to convert water and CO2 into oxygen and carbs. Hence, more refined measures like PAR and PPFD give cultivators a better understanding of how much “photon food” their plants are getting.

Natural sunlight has a PPFD value of around 900 - 2,000 μmol/m2/s throughout the year.

How Much PPFD in Seedling, Vegetation, Flowering?
Here what we are going to talk about is how much “photon food” your plants should be getting at different growth stages in different environments.

Normal Home-Scale Cultivation


Under normal home-scale operations, cannabis typically needs a PPFD of between 100 - 300 μmol/m2/s during the seedling, 400 - 600 μmol/m2/s during vegetation, and 800 - 1,000 μmol/m2/s during flowering.

These are the basic required food photons to grow a healthy and strong weed.

While many growers assume that the higher the PPFD, the higher the yields. However, more PPFD doesn’t always mean “more pot.” In fact, every plant has a “saturation point” where the benefits of PPFD gradually diminish. Too much light at certain phases of growth could increase the risk of issues like light burn, nutrient burn, or water deprivation. This is where the supplemental CO2 comes in.

Supplemental CO2 System

One step towards increasing yield in an indoor garden is to supplement with CO2, which does aid in packing weight onto buds. The principle is to make plants take advantage of CO2 to absorb more PPFD, grow faster, and have bigger and stronger buds.

Please also note that excessive CO2 levels also require a corresponding level of PPFD levels to provide sufficient energy for the plants, otherwise you will see your plants damaged by excess CO2. So, what’s the required PPFD value for cannabis in a supplemental CO2 system?

Under supplemental CO2 operations, cannabis typically needs a PPFD of between 800 - 1,000 μmol/m2/s during vegetation and 1,000 - 1,500 μmol/m2/s during flowering. Generally, growers will not add supplemental CO2 in seedlings.

Beware that a CO2 level of 1,200 - 2,000ppm during flower will help increase your plant growth while the temperature is rising at the same time. A comfortable CO2 range is better not to exceed 1,500ppm.



One More Tip
Careful research on PPFD maps before you invest in any LED grow lights.

Prospective LED buyers should also know that some manufacturers only list their central PPFD figures. While these numbers may seem impressive, they often mask the company’s weak peripheral PPFD. An unbalanced light flow increases the chance of bud burn, and it could diminish your return. For general indoor growing scenarios, it is important that the PPFD is evenly distributed. Just make sure that the temperature of the leaves' surface areas does not exceed 30 ℃, then all plants can get greater PPFD as much as feasible.