Optimum PAR intensity for plants
The optimal PAR (Photosynthetically Active Radiation) intensity for plants can vary depending on the specific plant species and its growth stage.
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
Medicinal herbs can utilise high PAR intensity and growth rate increases at a high rate up until about 800 µmols/m²/second, after which the rate of growth reduces and the system is less efficient. This is why most grow light manufacturers design grow lights to deliver an average PAR of about this level.
PAR intensity and growth rate with added CO2
Making more CO2 available to the plant by artificially raising the CO2 parts per million in a sealed grow room increases the photosynthesis rate by up to 30% and also allows plants to absorb higher PAR intensity. However this is expensive and complex to setup and is not viable for most small home growers.
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 × 16 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.