What is the best PAR intensity for plants? PPFD DLI adding CO2

Optimum PAR intensity for plants

Depending on the growth stage and grower experience the best PAR intensity will range from 250 µmols/m²/second for seedlings up to 1,400 µmols/m²/second for flowering at expert level. For home growers we recommend flowering at 900 µmols/m²/second for photoperiod plants and 550 µmols/m²/second for autoflowering plants.

Photosynthetically Active Radiation (PAR)

PAR is the range of light wavelengths (usually measured in the 400 to 700 nanometer range) that plants use for photosynthesis.

PAR intensity vs growth rate

LED grow lights have allowed growers to increase the PAR intensity in their grow rooms. Grow light manufacturers are now recommending about 900µmols/m²/second average PAR. For home growers this is the level with the best return in terms of growth rate per watt.

You can increase PAR intensity up to 1,400 µmols/m²/second without increasing CO2 and still get increases in grow rate and yield. However this is a very high performance level which requires a lot of skill to support this level of plant growth with environment control and feeding systems. This level is most suitable for commercial and expert home growers.

PAR intensity and growth rate with added CO2

Tests prove growth rate can increase by 30% with elevated CO₂ levels. However, it is costly and complicated system to setup and run. It is not suitable for most small home growers. It would be simpler and cheaper to add more grow space,  upgrade LED grow lights or other growing methods such as upgrade to hydro.

Daily Light Integral

The Daily Light Integral (DLI) is a crucial metric used in horticulture and agriculture to quantify the total amount of photosynthetically active radiation (PAR) received by a plant in a 24-hour period. It helps growers understand how much light a plant is exposed to on a daily basis, which is essential for optimizing plant growth, flowering, and fruiting. DLI is typically measured in units such as mol/m²/day or moles of photons per square meter per day.

To calculate the Daily Light Integral (DLI), you need to know two main parameters:

Light Intensity, Photosynthetically Active Radiation Photon Flux Density PPFD

This is the amount of photosynthetically active radiation received by the plant during a specific period, usually measured in micromoles (µmol) of photons per square meter per second (µmol/m²/s). You can measure this using a PAR meter or light sensor at various points throughout the day.

Daily Light integral calculation

The number of hours a day during which the plant is exposed to light is also crucial. You'll need to know the photoperiod (daylength) for your setup, typically 16 hours for Veg, 12 hours for flowering of photoperiod plants and 20 hours for auto flowering plants.

The formula to calculate DLI is as follows:

DLI = (PAR in µmol/m²/s) × (Number of daylight hours) x (60 minutes) x (60 seconds)

Here's how you can calculate DLI for a hypothetical situation:

Let's say you have a grow light setup with an average PAR intensity of 800 µmol/m²/s, and you want to calculate the DLI for flowering with 12 hours of light.

DLI = 800 µmol/m²/s × 12 hours x 60 minutes x 60 seconds = 34,500,000 µmol/m²/day

Divide by 100,000 to convert to mol/day

34,500,000/100,000 = 34.5 mol/Day

So, in this scenario, the plants in your grow room receive a Daily Light Integral of 34.5 mol/day.