What kind of light do plants need to grow?

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If you have been researching for any type of grow light for your indoor gardening project your, chances are that you have been bombarded with too many metrics, leaving you confused about what to look for in order to find the best possible grow light fixture for your project. Lighting manufacturers use terms and acronyms including watts, lumens, LUX, foot candles, PAR, PPF, PPFD, and photon efficiency. It is true that all these terms have relations to light. However, only a select few really tell you how efficient the light fixture is when it comes to supporting plants. The purpose of this article is to correct some common misunderstandings and help growers understand which metrics to focus on and which to neglect.


What is PAR?

Light consists of photons. Photons are packages of energy carried along in electromagnetic waves. Photosynthetic Active Radiation or PAR is light that travels between 400-700 nm. This is the band of radiation both the human eye can see, and plants can absorb to fix carbon dioxide from the air and bind it with water to produce glucose and oxygen. The process is known as photosynthesis.

What’s the difference between Lumens/LUX and PAR?

Other light measuring units include Lumens or/and Lux. These units are used to measure light in terms of brightness. What distinguishes PAR from LUX is that PAR tells you about light in terms of its usefulness to grow a plant. Lux or lumens tells you how bright the light is.

How do you measure PAR?

Too high1000-1500
No growth0-250

When measuring PAR, what you do is that you asses the lights ability to provide a life for a plant. You can do this by using a quantum sensor or a PAR-meter. A PAR-meter is a photon counter. It counts photons as they arrive at a certain point. Photosynthetically photon flux density (PPFD) is the measuring unit for photons arriving a certain point. The different values are from a grow light hanging at 25 cm. Based on these measurements we can tell that the center point of the grow area, receives too much light whereas corner points are in risk of not getting enough light. In this case, it would be a good idea to either adjust the height or supply the grow area with side lights.

How much light do your plants need?

The amount of light a plant needs depends on plant type and the stage of its current life. Is it in the seedling, vegging or flowering? The graph above is based on medicinal herbs in the flowering stage. But as a rule of thumb, you should aim to reach 750 moles/m2/sec (PPFD) for 18 hours a day. That is a good basis to assure good overall growth.

Why is measuring PAR or PPFD important?

It is important to measure PAR or PPFD to determine the amount of energy a leaf is potentially absorbing. The purpose of mearing PAR is to secure a decent light outreach from different points of the plant canopy. This way you can control that the light reaches corners and is not concentrated in the middle. So when measuring PAR, remember to place the measuring tool on different areas of the growing area and not only from the middle. You do this by dividing the grow area into a grid and measure center points of each and one of the squares in the grid. The picture above gives you an idea as to how to divide your grow area into grids.

What happens to the light when you adjust the height?

By hanging the grow light at different height levels, you achieve different PAR-values. The lower you have the light to the plant canopy the higher the PAR-value you get. But also, the light will be quite concentrated in the middle. On the contrary, when you have the light positioned higher you can cover a broader area and reach a better spread of light, but the amount of light reaching the grow area is going to be less because the light spreads in different areas and travels further distance, so it loses intensity

What happens when plants get too much light?

Use the information provided in this article to avoid bad lighting habits for your plants. If your plants get exposed to too much light or light that is too intense it can get burned and damaged affecting your allover harvest.

Here are some signs that a plant is getting too much sun:

  • Flower petals dry out.
  • Leaf edges look burnt or dried.
  • Flower color looks faded or washed out.
  • The entire plant starts to weaken and droop.
  • And here are signs that a plant isn’t getting enough light:
  • Growth is sparse.
  • Stems are lanky and spindly.
  • The distance between leaves, where they’re attached to the stems, is especially wide.
  • You see fewer flower buds and, thus, fewer flowers.
  • The entire plant leans toward the light source

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