What is PAR and PPFD?

PAR Definition

Photosynthetically Active Radiation (PAR) is the part of the electromagnetic spectrum that drives photosynthesis. It has traditionally been defined as being around the same wavelengths as visible light, from 400-700nm.

In reality, plants use light beyond this photosynthetic range, absorbing Far Red (700-800nm) and UV (280-400nm) that can trigger responses in growth structure, fruiting/flowering, and secondary metabolite production.

While photosynthesis converts light, carbon dioxide and water into compounds essential for plant growth and reproduction – known as primary metabolites – secondary metabolites give plants their distinctive smells, tastes, colours and medicinal effects (such as cannabinoids in medicinal cannabis).

UV light around 400nm causes mild stress responses in plants that can increase secondary metabolite production. Stronger UV light below this wavelength – especially in the low UVA (315-400nm) and high UVB (280-315nm) ranges – elicits the same response, but not only risks rapid DNA damage, it also breaks down the same secondary metabolites the plant is trying to produce to protect itself (from DNA damage).

At the other end of the plant spectrum, Far Red has a synergistic effect with Red light (600-700nm) that compounds photosynthesis (known as the Emerson Effect), as well as triggering and accelerating fruiting and flowering.

Gronet light GA |GC | GD series is one of the few “true” full-spectrum LED grow lights on the market with photosynthetic violet (400-430nm), UVA (1%) and high levels of Far Red (10%). Plant scientists now recognise the importance of this extended PAR range, and some have coined the term “ePAR” to include photosynthetic light from 400-750nm.*

Measuring light intensity

Spectrum is one of the most important elements in growing. But light intensity (all else being equal) is more important. Too much light can be just as detrimental as too little.

To ensure your plants get the correct amount of light, you need to work out how many photons an LED grow light emits over a given area in a given time, and how its intensity varies over the same area. An even light canopy is the ultimate goal (although there can be some exceptions).

What is PPFD?

The unit of measurement for PAR maps is PPFD, which stands for Photosynthetic Photon Flux Density. PPFD is a measure of how many photosynthetically active photons (counted in micromoles, or umol/μmol) hit a 1m2 area in one second. One unit of PPFD is equal to 1 μmol/m2/s.

As a general rule, tropical short-day plants need higher PPFD levels around 800-1000 PPFD during fruiting and flowering (12-hour light period), and around 550-650 PPFD during the vegetative stage (18-hour light period). This can increase to up to 1200-1500 PPFD if using supplemental CO2 to accelerate photosynthesis.

Leafy greens need lower levels of light around 200-400 PPFD, while shade-loving ornamentals such as aroids can thrive under as little as 100-200 PPFD.

Clones and some seedlings need lower light conditions until they harden off, so generally need to start at around 100-150 PPFD before increasing to normal levels.