Plant Light Science — The 2 Minute Version
Finding the right grow light for your needs can be a bewildering and frustrating experience. It doesn’t have to be that way.
We start from a few simple facts about how all plants use light:
- All light is made up of photons. Photons are the fundamental “packets” of light energy. This is true whether we’re talking about sunlight or light from a grow lamp.
- Plants convert photons of light into the energy they need to grow. That’s photosynthesis — photons land on leaves and are converted to sugars that feed the plant.
- Different plants need more or less photons each day for strong growth. A tomato plant needs a lot of photons to grow and fruit. Mint needs many fewer photons. That’s why one needs full sun outdoors and the other will grow in part-shade.
The Magic Number for Selecting a Grow Light: DLI
DLI (Daily Light Integral) is the standard measurement of light for professional indoor growers. Yet it’s quite poorly understood in the amateur community. We’re stuck with vague terms like “full sun” or “partial shade” — which mean nothing very much for indoor growing.
DLI lets us understand the light requirements of our plants and match them to the right grow light.
DLI literally represents a quantity of photons. DLI tells us the the ideal number of photons a specific plants needs each day so it can thrive.
A lettuce plant will happily grow with a DLI of 12. While hot peppers prefer something closer to 30.
Knowing a plant’s DLI is half of the equation…
Matching a Plant to the Right Grow Light
Now we know that each variety of plant has its own DLI. The next step is to select the light that will deliver that amount of photons over the course of the day.
There are three things that determine the DLI that a grow light delivers to your plants:
- The strength of the grow light.
- The distance from lamp to leaves. Photons spread out as they travel from their source. So a plant placed close to a grow light will receive more photons than one that’s further away.
- The number of hours per day the grow light is switched on for. More hours of light means more photons for the plant.
Distance and hours are easy to measure. Measuring light strength is the tricky bit. Professionals use a device called a PAR meter (aka a quantum meter) to accurately measure plant-useful light. Unfortunately reliable PAR meters aren’t cheap — starting at several hundred dollars.