As we well know, the secret to good growing is good light. As photosynthesis is perhaps the most important process in a plant to help it grow and flower, improving the light will help a lot. Previously in our why use growlights over sunlight article we talked about the merits of using growlights, but how many and what strength lights should you use? In this article, we will explore this very question and relate it to different types of plant. 

First of all we will look at the PPFD requirements of plants. PPFD stands of photosynthetic photon flux density; this means the amount of light that plants use hitting an area per second. If the PPFD is too low then not enough sugar is produced through photosynthesis for plants to produce ATP (Adenosine Triphosphate) to maintain cellular respiration, so a plant would die. If there is too much PPFD, the plant is saturated and even damaged by the intensity, growth slows or the leaves burn. Plants are therefore like Goldilocks, they want a comfortable range of light suited to their needs. What is this range? Roughly it is 80-800 µmol/m2s, but of course there will be many outliers. Different plants have different light requirements, therefore a rough guide has been produced to adapt your plants general light requirements for grow light use via PPFD calculation. These measurements hold worldwide, as peak intense sunlight experienced in temperate summers is not dissimilar to tropical daily sun, so plants adapt to PPFD similarly. 

• Full Sun: 1600-2000 µmol/m2/s – this is where the plant gets full sunlight all day at least 6 months in a year. Many plants can fall into this category such as trees, cacti, and many plants and flowers. 
• Partial Sun: 800-1600 µmol/m2/s – this is where direct sunlight is experienced for a few hours a day, with prolonged exposure perhaps weakening the plant slightly or not showing net benefits. Leaves may be smaller and very dark if grown in full sun. 
• Indirect sun: 400-800 µmol/m2/s – this is where there is no direct sunlight but full sky exposure, typically you want low PPFD for these plants or they burn or show no faster growth. 
• Partially shaded: 100-400 µmol/m2/s – this low light requirement is suited for forest plants that grow under trees and perhaps indirect sunlight diffused through canopy a day. They will grow slow and may burn under full sun or high PPFD. 
• Shade: 100 µmol/m2/s and below – these plants are in the darkest forests or jungles and grow very slowly but may be especially damaged by high PPFD. 

All these ranges depend entirely on the plant, some plants may be able to tolerate all intensities, with 500-800 µmol/m2s showing the fastest growth, some plants can tolerate high PPFD but would just not absorb any extra PPFD beyond 400 µmol/m2s, and some would be bleached and burnt by high intensity. However in pure indoor environments, due to the conditions given (temperature, humidity, carbon dioxide, water and nutrients), a high light concentration like in full sun cannot be fully absorbed by the light-hungriest plant. By our experience, we recommend a maximum of 600-800 PPFD per m2 for a hobbyist growing high light plants. Read about your plant and its susceptibility to sunburn, and if it grows in high light areas such as forest clearings or open fields, you can always move grow lights upwards to reduce and downwards to increase PPFD. 

Putting It All Together

Now you know about the light needs of your plant, you can choose the grow light design. Typically, you want a certain PPFD lamp output benchmarked at 20cm and you could raise it higher to 50cm and above to reduce PPFD to suit the plant. 

Infographic of PPFD levels vs height of lights above plant

Alternatively, if you are perhaps growing shade loving plants or plants you don’t particularly want to grow much, you can use less grow lights over an area to maintain a minimal effective PPFD concentration. For example, someone growing tomatoes indoors would want 600 µmol/m2/s effective PPFD with 4 iSpectrums per m2 area at a 30-50cm height. However, someone lighting a green wall indoors could get away with perhaps 1 iSpectrum per m2 area at 1m or more just to keep the plants alive and healthy, not growing.